研究者業績
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2022年2月
論文
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\mnras 529(4) 3894-3911 2024年3月PG 1553 + 113 is one of the few blazars with a convincing quasi-periodic emission in the gamma-ray band. The source is also a very high energy (VHE; >100 GeV) gamma-ray emitter. To better understand its properties and identify the underlying physical processes driving its variability, the MAGIC Collaboration initiated a multiyear, multiwavelength monitoring campaign in 2015 involving the OVRO 40-m and Medicina radio telescopes, REM, KVA, and the MAGIC telescopes, Swift and Fermi satellites, and the WEBT network. The analysis presented in this paper uses data until 2017 and focuses on the characterization of the variability. The gamma-ray data show a (hint of a) periodic signal compatible with literature, but the X-ray and VHE gamma-ray data do not show statistical evidence for a periodic signal. In other bands, the data are compatible with the gamma-ray period, but with a relatively high p-value. The complex connection between the low- and high-energy emission and the non-monochromatic modulation and changes in flux suggests that a simple one-zone model is unable to explain all the variability. Instead, a model including a periodic component along with multiple emission zones is required.
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åp 682 2024年2月Aims. The BL Lac 1ES 2344+514 is known for temporary extreme properties characterised by a shift of the synchrotron spectral energy distribution (SED) peak energy νsynch;p above 1 keV. While those extreme states have only been observed during high flux levels thus far, additional multi-year observing campaigns are required to achieve a coherent picture. Here, we report the longest investigation of the source from radio to very high energy (VHE) performed so far, focussing on a systematic characterisation of the intermittent extreme states. Methods.We organised a monitoring campaign covering a 3-year period from 2019 to 2021.Morethan ten instruments participated in the observations in order to cover the emission from radio to VHE. In particular, sensitive X-ray measurements by XMM-Newton, NuSTAR, and AstroSat took place simultaneously with multi-hour MAGIC observations, providing an unprecedented constraint of the two SED components for this blazar. Results. While our results confirm that 1ES 2344+514 typically exhibits νsynch;p > 1 keV during elevated flux periods, we also find periods where the extreme state coincides with low flux activity. A strong spectral variability thus happens in the quiescent state, and is likely caused by an increase in the electron acceleration efficiency without a change in the electron injection luminosity. On the other hand, we also report a strong X-ray flare (among the brightest for 1ES 2344+514) without a significant shift of νsynch;p. During this particular flare, the X-ray spectrum is among the softest of the campaign. It unveils complexity in the spectral evolution, where the common harder-when-brighter trend observed in BL Lacs is violated. By combining Swift-XRT and Swift-UVOT measurements during a low and hard X-ray state, we find an excess of the UV flux with respect to an extrapolation of the X-ray spectrum to lower energies. This UV excess implies that at least two regions significantly contribute to the infrared/optical/ultraviolet/X-ray emission. Using the simultaneous MAGIC, XMM-Newton, NuSTAR, and AstroSat observations, we argue that a region possibly associated with the 10 GHz radio core may explain such an excess. Finally, we investigate a VHE flare, showing an absence of simultaneous variability in the 0.3-2 keV band. Using time-dependent leptonic modelling, we show that this behaviour, in contradiction to single-zone scenarios, can instead be explained by a two-component model.
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Physics of the Dark Universe 35 100912-100912 2022年3月 査読有り
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Monthly Notices of the Royal Astronomical Society 507(1) 1528-1545 2021年10月1日We report here on the first multiwavelength (MWL) campaign on the blazar TXS 1515-273, undertaken in 2019 and extending from radio to very-high-energy gamma-rays (VHE). Up until now, this blazar had not been the subject of any detailed MWL observations. It has a rather hard photon index at GeV energies and was considered a candidate extreme high-synchrotron-peaked source. MAGIC observations resulted in the first-time detection of the source in VHE with a statistical significance of 7.6σ. The average integral VHE flux of the source is 6 ± 1 per cent of the Crab nebula flux above 400 GeV. X-ray coverage was provided by Swift-XRT, XMM-Newton, and NuSTAR. The long continuous X-ray observations were separated by ∼9 h, both showing clear hour scale flares. In the XMM-Newton data, both the rise and decay time-scales are longer in the soft X-ray than in the hard X-ray band, indicating the presence of a particle cooling regime. The X-ray variability time-scales were used to constrain the size of the emission region and the strength of the magnetic field. The data allowed us to determine the synchrotron peak frequency and classify the source as a flaring high, but not extreme synchrotron-peaked object. Considering the constraints and variability patterns from the X-ray data, we model the broad-band spectral energy distribution. We applied a simple one-zone model, which could not reproduce the radio emission and the shape of the optical emission, and a two-component leptonic model with two interacting components, enabling us to reproduce the emission from radio to VHE band.
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ASTRONOMY & ASTROPHYSICS 647 2021年3月 査読有りContext. QSO B1420+326 is a blazar classified as a flat-spectrum radio quasar (FSRQ). At the beginning of the year 2020, it was found to be in an enhanced flux state and an extensive multiwavelength campaign allowed us to trace the evolution of the flare.Aims. We search for very high-energy (VHE) gamma-ray emission from QSO B1420+326 during this flaring state. We aim to characterize and model the broadband emission of the source over different phases of the flare.Methods. The source was observed with a number of instruments in radio, near-infrared, optical (including polarimetry and spectroscopy), ultraviolet, X-ray, and gamma-ray bands. We use dedicated optical spectroscopy results to estimate the accretion disk and the dust torus luminosity. We performed spectral energy distribution modeling in the framework of combined synchrotron-self-Compton and external Compton scenario in which the electron energy distribution is partially determined from acceleration and cooling processes.Results. During the enhanced state, the flux of both SED components of QSO B1420+326 drastically increased and the peaks were shifted to higher energies. Follow-up observations with the MAGIC telescopes led to the detection of VHE gamma-ray emission from this source, making it one of only a handful of FSRQs known in this energy range. Modeling allows us to constrain the evolution of the magnetic field and electron energy distribution in the emission region. The gamma-ray flare was accompanied by a rotation of the optical polarization vector during a low -polarization state. Also, a new superluminal radio knot contemporaneously appeared in the radio image of the jet. The optical spectroscopy shows a prominent FeII bump with flux evolving together with the continuum emission and a MgII line with varying equivalent width.
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ASTROPHYSICAL JOURNAL 908(1) 2021年2月 査読有りThe coincident detection of GW170817 in gravitational waves and electromagnetic radiation spanning the radio to MeV gamma-ray bands provided the first direct evidence that short gamma-ray bursts (GRBs) can originate from binary neutron star (BNS) mergers. On the other hand, the properties of short GRBs in high-energy gamma-rays are still poorly constrained, with only similar to 20 events detected in the GeV band, and none in the TeV band. GRB 160821B is one of the nearest short GRBs known at z = 0.162. Recent analyses of the multiwavelength observational data of its afterglow emission revealed an optical-infrared kilonova component, characteristic of heavy-element nucleosynthesis in a BNS merger. Aiming to better clarify the nature of short GRBs, this burst was automatically followed up with the MAGIC telescopes, starting from 24 s after the burst trigger. Evidence of a gamma-ray signal is found above similar to 0.5 TeV at a significance of similar to 3 sigma during observations that lasted until 4 hr after the burst. Assuming that the observed excess events correspond to gamma-ray emission from GRB 160821B, in conjunction with data at other wavelengths, we investigate its origin in the framework of GRB afterglow models. The simplest interpretation with one-zone models of synchrotron-self-Compton emission from the external forward shock has difficulty accounting for the putative TeV flux. Alternative scenarios are discussed where the TeV emission can be relatively enhanced. The role of future GeV-TeV observations of short GRBs in advancing our understanding of BNS mergers and related topics is briefly addressed.
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Proceedings Volume 11451, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation IV 114510G 2020年12月13日
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Monthly Notices of the Royal Astronomical Society 504(1) 1427-1451 2020年12月8日 査読有りWe report a characterization of the multi-band flux variability and correlations of the nearby (z=0.031) blazar Markarian 421 (Mrk 421) using data from Metsähovi, Swift, Fermi-LAT, MAGIC, FACT and other collaborations and instruments from November 2014 till June 2016. Mrk 421 did not show any prominent flaring activity, but exhibited periods of historically low activity above 1 TeV (F>1TeV < 1.7× 10−12 ph cm−2 s−1) and in the 2-10 keV (X-ray) band (F2 − 10 keV < 3.6 × 10−11 erg cm−2 s−1), during which the Swift-BAT data suggests an additional spectral component beyond the regular synchrotron emission.The highest flux variability occurs in X-rays and very-high-energy (E>0.1 TeV) γ-rays, which, despite the low activity, show a significant positive correlation with no time lag. The HRkeV and HRTeV show the harder-when-brighter trend observed in many blazars, but the trend flattens at the highest fluxes, which suggests a change in the processes dominating the blazar variability. Enlarging our data set with data from years 2007 to 2014, we measured a positive correlation between the optical and the GeV emission over a range of about 60 days centered at time lag zero, and a positive correlation between the optical/GeV and the radio emission over a range of about 60 days centered at a time lag of $43^{+9}_{-6}$ days.This observation is consistent with the radio-bright zone being located about 0.2 parsec downstream from the optical/GeV emission regions of the jet. The flux distributions are better described with a LogNormal function in most of the energy bands probed, indicating that the variability in Mrk 421 is likely produced by a multiplicative process.
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Astronomy & Astrophysics 643 L14-L14 2020年11月 査読有りWe report the detection of pulsed gamma-ray emission from the Geminga pulsar (PSR J0633+1746) between 15 GeV and 75 GeV. This is the first time a middle-aged pulsar has been detected up to these energies. Observations were carried out with the MAGIC telescopes between 2017 and 2019 using the low-energy threshold Sum-Trigger-II system. After quality selection cuts, ∼80 h of observational data were used for this analysis. To compare with the emission at lower energies below the sensitivity range of MAGIC, 11 years of <italic>Fermi</italic>-LAT data above 100 MeV were also analysed. From the two pulses per rotation seen by <italic>Fermi</italic>-LAT, only the second one, <italic>P</italic>2, is detected in the MAGIC energy range, with a significance of 6.3<italic>σ</italic>. The spectrum measured by MAGIC is well-represented by a simple power law of spectral index Γ = 5.62 ± 0.54, which smoothly extends the <italic>Fermi</italic>-LAT spectrum. A joint fit to MAGIC and <italic>Fermi</italic>-LAT data rules out the existence of a sub-exponential cut-off in the combined energy range at the 3.6<italic>σ</italic> significance level. The power-law tail emission detected by MAGIC is interpreted as the transition from curvature radiation to Inverse Compton Scattering of particles accelerated in the northern outer gap.
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Astronomy & Astrophysics 642 A190-A190 2020年10月<italic>Aims.</italic> In the presence of a sufficient amount of target material, <italic>γ</italic>-rays can be used as a tracer in the search for sources of Galactic cosmic rays (CRs). Here we present deep observations of the Galactic center (GC) region with the MAGIC telescopes and use them to infer the underlying CR distribution and to study the alleged PeV proton accelerator at the center of our Galaxy. <italic>Methods.</italic> We used data from ≈100 h observations of the GC region conducted with the MAGIC telescopes over five years (from 2012 to 2017). Those were collected at high zenith angles (58−70 deg), leading to a larger energy threshold, but also an increased effective collection area compared to low zenith observations. Using recently developed software tools, we derived the instrument response and background models required for extracting the diffuse emission in the region. We used existing measurements of the gas distribution in the GC region to derive the underlying distribution of CRs. We present a discussion of the associated biases and limitations of such an approach. <italic>Results.</italic> We obtain a significant detection for all four model components used to fit our data (Sgr A*, “Arc”, G0.9+0.1, and an extended component for the Galactic Ridge). We observe no significant difference between the <italic>γ</italic>-ray spectra of the immediate GC surroundings, which we model as a point source (Sgr A*) and the Galactic Ridge. The latter can be described as a power-law with index 2 and an exponential cut-off at around 20 TeV with the significance of the cut-off being only 2<italic>σ</italic>. The derived cosmic-ray profile hints to a peak at the GC position and with a measured profile index of 1.2 ± 0.3 is consistent with the 1/<italic>r</italic> radial distance scaling law, which supports the hypothesis of a CR accelerator at the GC. We argue that the measurements of this profile are presently limited by our knowledge of the gas distribution in the GC vicinity.
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Monthly Notices of the Royal Astronomical Society 497(3) 3734-3745 2020年9月21日 査読有り<title>ABSTRACT</title> We investigate the physical nature and origin of the gamma-ray emission from the extended source HESS J1841−055 observed at TeV and GeV energies. We observed HESS J1841−055 at TeV energies for a total effective time of 43 h with the MAGIC telescopes, in 2012 and 2013. Additionally, we analysed the GeV counterpart making use of about 10 yr of Fermi-LAT data. Using both Fermi-LAT and MAGIC, we study both the spectral and energy-dependent morphology of the source for almost four decades of energy. The origin of the gamma-ray emission from this region is investigated using multiwaveband information on sources present in this region, suggested to be associated with this unidentified gamma-ray source. We find that the extended emission at GeV–TeV energies is best described by more than one source model. We also perform the first energy-dependent analysis of the HESS J1841−055 region at GeV–TeV. We find that the emission at lower energies comes from a diffuse or extended component, while the major contribution of gamma rays above 1 TeV arises from the southern part of the source. Moreover, we find that a significant curvature is present in the combined observed spectrum of MAGIC and Fermi-LAT. The first multiwavelength spectral energy distribution of this unidentified source shows that the emission at GeV–TeV energies can be well explained with both leptonic and hadronic models. For the leptonic scenario, bremsstrahlung is the dominant emission compared to inverse Compton. On the other hand, for the hadronic model, gamma-ray resulting from the decay of neutral pions (π0) can explain the observed spectrum. The presence of dense molecular clouds overlapping with HESS J1841−055 makes both bremsstrahlung and π0-decay processes the dominant emission mechanisms for the source.
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Astronomy & Astrophysics 640 A132-A132 2020年8月 査読有りContext. It has become evident that one-zone synchrotron self-Compton models are not always adequate for very high-energy (VHE) gamma-ray-emitting blazars. While two-component models perform better, they are difficult to constrain due to the large number of free parameters. Aims. In this work, we make a first attempt at taking into account the observational constraints from very long baseline interferometry (VLBI) data, long-term light curves (radio, optical, and X-rays), and optical polarisation to limit the parameter space for a two-component model and test whether or not it can still reproduce the observed spectral energy distribution (SED) of the blazars. Methods. We selected five TeV BL Lac objects based on the availability of VHE gamma-ray and optical polarisation data. We collected constraints for the jet parameters from VLBI observations. We evaluated the contributions of the two components to the optical flux by means of decomposition of long-term radio and optical light curves as well as modelling of the optical polarisation variability of the objects. We selected eight epochs for these five objects based on the variability observed at VHE gamma rays, for which we constructed the SEDs that we then modelled with a two-component model. Results. We found parameter sets which can reproduce the broadband SED of the sources in the framework of two-component models considering all available observational constraints from VLBI observations. Moreover, the constraints obtained from the long-term behaviour of the sources in the lower energy bands could be used to determine the region where the emission in each band originates. Finally, we attempt to use optical polarisation data to shed new light on the behaviour of the two components in the optical band. Our observationally constrained two-component model allows explanation of the entire SED from radio to VHE with two co-located emission regions.
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The Astrophysical Journal Supplement Series 248(2) 29-29 2020年6月10日
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Astronomy & Astrophysics 638 A14-A14 2020年6月1ES 1959+650 is a bright TeV high-frequency-peaked BL Lac object exhibiting interesting features like “orphan” TeV flares and broad emission in the high-energy regime that are difficult to interpret using conventional one-zone Synchrotron Self-Compton (SSC) scenarios. We report the results from the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) observations in 2016 along with the multi-wavelength data from the<italic>Fermi</italic>Large Area Telescope (LAT) and<italic>Swift</italic>instruments. MAGIC observed 1ES 1959+650 with different emission levels in the very-high-energy (VHE,<italic>E</italic> > 100 GeV)<italic>γ</italic>-ray band during 2016. In the long-term data, the X-ray spectrum becomes harder with increasing flux and a hint of a similar trend is also visible in the VHE band. An exceptionally high VHE flux reaching ∼3 times the Crab Nebula flux was measured by MAGIC on the 13 and 14 of June, and 1 July 2016 (the highest flux observed since 2002). During these flares, the high-energy peak of the spectral energy distribution (SED) lies in the VHE domain and extends up to several TeV. The spectrum in the<italic>γ</italic>-ray (both<italic>Fermi</italic>-LAT and VHE bands) and the X-ray bands are quite hard. On 13 June and 1 July 2016, the source showed rapid variations in the VHE flux within timescales of less than an hour. A simple one-zone SSC model can describe the data during the flares requiring moderate to large values of the Doppler factors (<italic>δ</italic> ≥ 30−60). Alternatively, the high-energy peak of the SED can be explained by a purely hadronic model attributed to proton-synchrotron radiation with jet power<italic>L</italic>jet ∼ 1046erg s−1and under high values of the magnetic field strength (∼100 G) and maximum proton energy (∼few EeV). Mixed lepto-hadronic models require super-Eddington values of the jet power. We conclude that it is difficult to get detectable neutrino emission from the source during the extreme VHE flaring period of 2016.
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Astronomy & Astrophysics 637 A86-A86 2020年5月Context. Markarian 501 (Mrk 501) is a very high-energy (VHE) gamma-ray blazar located at z = 0.034, which is regularly monitored by a wide range of multi-wavelength instruments, from radio to VHE gamma rays. During a period of almost two weeks in July 2014, the highest X-ray activity of Mrk 501 was observed in ∼14 years of operation of the Neil Gehrels Swift Gamma-ray Burst Observatory. Aims. We characterize the broadband variability of Mrk 501 from radio to VHE gamma rays during the most extreme X-ray activity measured in the last 14 years, and evaluate whether it can be interpreted within theoretical scenarios widely used to explain the broadband emission from blazars. Methods. The emission of Mrk 501 was measured at radio with Metsähovi, at optical–UV with KVA and Swift/UVOT, at X-ray with Swift/XRT and Swift/BAT, at gamma ray with Fermi-LAT, and at VHE gamma rays with the FACT and MAGIC telescopes. The multi-band variability and correlations were quantified, and the broadband spectral energy distributions (SEDs) were compared with predictions from theoretical models. Results. The VHE emission of Mrk 501 was found to be elevated during the X-ray outburst, with a gamma-ray flux above 0.15 TeV varying from ∼0.5 to ∼2 times the Crab nebula flux. The X-ray and VHE emission both varied on timescales of 1 day and were found to be correlated. We measured a general increase in the fractional variability with energy, with the VHE variability being twice as large as the X-ray variability. The temporal evolution of the most prominent and variable segments of the SED, characterized on a day-by-day basis from 2014 July 16 to 2014 July 31, is described with a one-zone synchrotron self-Compton model with variations in the break energy of the electron energy distribution (EED), and with some adjustments in the magnetic field strength and spectral shape of the EED. These results suggest that the main flux variations during this extreme X-ray outburst are produced by the acceleration and the cooling of the high-energy electrons. A narrow feature at ∼3 TeV was observed in the VHE spectrum measured on 2014 July 19 (MJD 56857.98), which is the day with the highest X-ray flux (>0.3 keV) measured during the entire Swift mission. This feature is inconsistent with the classical analytic functions to describe the measured VHE spectra (power law, log-parabola, and log-parabola with exponential cutoff) at more than 3σ. A fit with a log-parabola plus a narrow component is preferred over the fit with a single log-parabola at more than 4σ, and a dedicated Monte Carlo simulation estimated the significance of this extra component to be larger than 3σ. Under the assumption that this VHE spectral feature is real, we show that it can be reproduced with three distinct theoretical scenarios: (a) a pileup in the EED due to stochastic acceleration; (b) a structured jet with two-SSC emitting regions, with one region dominated by an extremely narrow EED; and (c) an emission from an IC pair cascade induced by electrons accelerated in a magnetospheric vacuum gap, in addition to the SSC emission from a more conventional region along the jet of Mrk 501.
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Physics of the Dark Universe 28 100529-100529 2020年5月
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Monthly Notices of the Royal Astronomical Society 492(4) 5354-5365 2020年3月11日 査読有り<title>ABSTRACT</title> M 87 is one of the closest (z = 0.004 36) extragalactic sources emitting at very high energies (VHE, E &gt; 100 GeV). The aim of this work is to locate the region of the VHE gamma-ray emission and to describe the observed broad-band spectral energy distribution (SED) during the low VHE gamma-ray state. The data from M 87 collected between 2012 and 2015 as part of a MAGIC monitoring programme are analysed and combined with multiwavelength data from Fermi-LAT, Chandra, HST, EVN, VLBA, and the Liverpool Telescope. The averaged VHE gamma-ray spectrum can be fitted from ∼100 GeV to ∼10 TeV with a simple power law with a photon index of (−2.41 ± 0.07), while the integral flux above 300 GeV is $(1.44\pm 0.13)\times 10^{-12}\, \mathrm{cm}^{-2}\, \mathrm{s}^{-1}$. During the campaign between 2012 and 2015, M 87 is generally found in a low-emission state at all observed wavelengths. The VHE gamma-ray flux from the present 2012–2015M 87 campaign is consistent with a constant flux with some hint of variability ($\sim 3\, \sigma$) on a daily time-scale in 2013. The low-state gamma-ray emission likely originates from the same region as the flare-state emission. Given the broad-band SED, both a leptonic synchrotron self-Compton and a hybrid photohadronic model reproduce the available data well, even if the latter is preferred. We note, however, that the energy stored in the magnetic field in the leptonic scenario is very low, suggesting a matter-dominated emission region.
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Astronomy & Astrophysics 635 A158-A158 2020年3月<italic>Aims.</italic> We measure the Crab Nebula <italic>γ</italic>-ray spectral energy distribution in the ~100 TeV energy domain and test the validity of existing leptonic emission models at these high energies. <italic>Methods.</italic> We used the novel very large zenith angle observations with the MAGIC telescope system to increase the collection area above 10 TeV. We also developed an auxiliary procedure of monitoring atmospheric transmission in order to assure proper calibration of the accumulated data. This employs recording optical images of the stellar field next to the source position, which provides a better than 10% accuracy for the transmission measurements. <italic>Results.</italic> We demonstrate that MAGIC very large zenith angle observations yield a collection area larger than a square kilometer. In only ~ 56 h of observations, we detect the <italic>γ</italic>-ray emission from the Crab Nebula up to 100 TeV, thus providing the highest energy measurement of this source to date with Imaging Atmospheric Cherenkov Telescopes. Comparing accumulated and archival MAGIC and <italic>Fermi</italic>/LAT data with some of the existing emission models, we find that none of them provides an accurate description of the 1 GeV to 100 TeV <italic>γ</italic>-ray signal.
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The Astrophysical Journal Supplement Series 247(1) 16-16 2020年2月20日 査読有りExtreme high-frequency-peaked BL Lac objects (EHBLs) are blazars that exhibit extremely energetic synchrotron emission. They also feature nonthermal gamma-ray emission whose peak lies in the very high-energy (VHE, E > 100 GeV) range, and in some sources exceeds 1 TeV: this is the case for hard-TeV EHBLs such as 1ES 0229+200. With the aim of increasing the EHBL population, 10 targets were observed with the MAGIC telescopes from 2010 to 2017, for a total of 265 hr of good-quality data. The data were complemented by coordinated Swift observations. The X-ray data analysis confirms that all but two sources are EHBLs. The sources show only a modest variability and a harder-when-brighter behavior, typical for this class of objects. At VHE gamma-rays, three new sources were detected and a hint of a signal was found for another new source. In each case, the intrinsic spectrum is compatible with the hypothesis of a hard-TeV nature of these EHBLs. The broadband spectral energy distributions (SEDs) of all sources are built and modeled in the framework of a single-zone, purely leptonic model. The VHE gamma-ray-detected sources were also interpreted with a spine-layer model and a proton synchrotron model. The three models provide a good description of the SEDs. However, the resulting parameters differ substantially in the three scenarios, in particular the magnetization parameter. This work presents the first mini catalog of VHE gamma-ray and multiwavelength observations of EHBLs.
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ASTRONOMY & ASTROPHYSICS 634 2020年1月 査読有りAims. The aim of this study is to search for evidence of a common emission engine between radio giant pulses (GPs) and very-high-energy (VHE, E& x2004;> & x2004;100 GeV) gamma-rays from the Crab pulsar. Methods. We performed 16 h of simultaneous observations of the Crab pulsar at 1.4 GHz with the Effelsberg radio telescope and the Westerbork Synthesis Radio Telescope (WSRT), and at energies above 60 GeV we used the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescopes. We searched for a statistical correlation between the radio and VHE gamma-ray emission with search windows of different lengths and different time lags to the arrival times of a radio GP. A dedicated search for an enhancement in the number of VHE gamma-rays correlated with the occurrence of radio GPs was carried out separately for the P1 and P2 phase ranges, respectively. Results. In the radio data sample, 99444 radio GPs were detected. We find no significant correlation between the GPs and VHE photons in any of the search windows. Depending on phase cuts and the chosen search windows, we find upper limits at a 95% confidence level on an increase in VHE gamma-ray events correlated with radio GPs between 7% and 61% of the average Crab pulsar VHE flux for the P1 and P2 phase ranges, respectively. This puts upper limits on the flux increase during a radio GP between 12% and 2900% of the pulsed VHE flux, depending on the search window duration and phase cuts. This is the most stringent upper limit on a correlation between gamma-ray emission and radio GPs reported so far.
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Monthly Notices of the Royal Astronomical Society 490(2) 2284-2299 2019年12月1日<title>ABSTRACT</title> Extreme high-energy-peaked BL Lac objects (EHBLs) are an emerging class of blazars. Their typical two-hump-structured spectral energy distribution (SED) peaks at higher energies with respect to conventional blazars. Multiwavelength (MWL) observations constrain their synchrotron peak in the medium to hard X-ray band. Their gamma-ray SED peaks above the GeV band, and in some objects it extends up to several TeV. Up to now, only a few EHBLs have been detected in the TeV gamma-ray range. In this paper, we report the detection of the EHBL 2WHSP J073326.7+515354, observed and detected during 2018 in TeV gamma rays with the MAGIC telescopes. The broad-band SED is studied within an MWL context, including an analysis of the Fermi-LAT data over 10 yr of observation and with simultaneous Swift-XRT, Swift-UVOT, and KVA data. Our analysis results in a set of spectral parameters that confirms the classification of the source as an EHBL. In order to investigate the physical nature of this extreme emission, different theoretical frameworks were tested to model the broad-band SED. The hard TeV spectrum of 2WHSP J073326.7+515354 sets the SED far from the energy equipartition regime in the standard one-zone leptonic scenario of blazar emission. Conversely, more complex models of the jet, represented by either a two-zone spine-layer model or a hadronic emission model, better represent the broad-band SED.
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Nature 575(7783) 455-458 2019年11月21日 査読有りLong-duration γ-ray bursts (GRBs) are the most luminous sources of electromagnetic radiation known in the Universe. They arise from outflows of plasma with velocities near the speed of light that are ejected by newly formed neutron stars or black holes (of stellar mass) at cosmological distances. Prompt flashes of megaelectronvolt-energy γ-rays are followed by a longer-lasting afterglow emission in a wide range of energies (from radio waves to gigaelectronvolt γ-rays), which originates from synchrotron radiation generated by energetic electrons in the accompanying shock waves. Although emission of γ-rays at even higher (teraelectronvolt) energies by other radiation mechanisms has been theoretically predicted, it has not been previously detected. Here we report observations of teraelectronvolt emission from the γ-ray burst GRB 190114C. γ-rays were observed in the energy range 0.2-1 teraelectronvolt from about one minute after the burst (at more than 50 standard deviations in the first 20 minutes), revealing a distinct emission component of the afterglow with power comparable to that of the synchrotron component. The observed similarity in the radiated power and temporal behaviour of the teraelectronvolt and X-ray bands points to processes such as inverse Compton upscattering as the mechanism of the teraelectronvolt emission. By contrast, processes such as synchrotron emission by ultrahigh-energy protons are not favoured because of their low radiative efficiency. These results are anticipated to be a step towards a deeper understanding of the physics of GRBs and relativistic shock waves.
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Nature 575(7783) 459-463 2019年11月21日 査読有り
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The Astrophysical Journal 883(2) 135-135 2019年9月27日
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Monthly Notices of the Royal Astronomical Society 486(3) 4233-4251 2019年7月1日
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MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 485(1) 356-366 2019年5月 査読有り
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MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 484(2) 2876-2885 2019年4月 査読有り
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Astronomy & Astrophysics 623 A175-A175 2019年3月 査読有りThe mechanisms producing fast variability of the γ-ray emission in active galactic nuclei (AGNs) are under debate. The MAGIC telescopes detected a fast, very-high-energy (VHE, E > 100 GeV) γ-ray flare from BL Lacertae on 2015 June 15. The flare had a maximum flux of (1.5 ± 0.3) × 10−10 photons cm−2 s−1 and halving time of 26 ± 8 min. The MAGIC observations were triggered by a high state in the optical and high-energy (HE, E > 100 MeV) γ-ray bands. In this paper we present the MAGIC VHE γ-ray data together with multi-wavelength data from radio, optical, X-rays, and HE γ rays from 2015 May 1 to July 31. Well-sampled multi-wavelength data allow us to study the variability in detail and compare it to the other epochs when fast, VHE γ-ray flares have been detected from this source. Interestingly, we find that the behaviour in radio, optical, X-rays, and HE γ-rays is very similar to two other observed VHE γ-ray flares. In particular, also during this flare there was an indication of rotation of the optical polarization angle and of activity at the 43 GHz core. These repeating patterns indicate a connection between the three events. We also test modelling of the spectral energy distribution based on constraints from the light curves and VLBA observations, with two different geometrical setups of two-zone inverse Compton models. In addition we model the γ-ray data with the star-jet interaction model. We find that all of the tested emission models are compatible with the fast VHE γ-ray flare, but all have some tension with the multi-wavelength observations.
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483(4) 4578-4585 2019年3月 査読有り
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Proceedings of Science 358 2019年The first two LIGO and Virgo observation runs have been important milestones in the gravitational wave (GW) field, thanks to the detection of GW signals from ten binary black hole systems and a binary neutron star system [1, 2]. In order to fully characterize the emitting source, the remnant object and its environment, electromagnetic follow-up observations at different wavelengths are essential, as learned from the GW170817/GRB170817A case. Given the quite large localization uncertainties provided by interferometers, the main challenge faced by facilities with a narrow field of view (e.g. Imaging Atmospheric Cherenkov Telescopes, IACTs) is to setup a suitable follow-up strategy in order to observe sky regions with the highest probability to host the electromagnetic (EM) counterpart of the GW signal. As member of the EM follow-up community, the MAGIC collaboration joined this effort in 2014. As the third observation run (O3) is currently ongoing, where both LIGO and Virgo are expected to have much better sensitivities, MAGIC is refining its follow-up strategy to maximize the chances of observing the EM counterparts as soon as possible. In this contribution we will describe this strategy, focusing on the different observation cases, which mainly depends on the information available from both GW and EM partner facilities.
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Proceedings of Science 358 2019年In January 2015, an impressive outburst of the BL Lac object S5 0716+714 was registered in all energy bands, from low-frequency radio to very-high-energy gamma-rays (VHE, E > 100 GeV). For the first time for this object, simultaneous Fermi-LAT and MAGIC spectra were obtained, allowing a close investigation of the higher-energy peak of the spectral energy distribution (SED). The VLBA analysis of the parsec-scale jet, together with two-zone modeling of the simultaneous broadband SED, lead us to propose a scenario in which VHE emission arises during the entrance and exit of a superluminal knot through a recollimation shock in the inner jet. The gamma-ray emission in the high-energy (0.1 GeV < E < 100 GeV) and VHE bands is attributed to a shock-shock interaction in the helical jet downstream of the mm-wave VLBI core, closely followed by optical and X-ray outbursts that appear to occur in the core. An interpretation that the VHE gamma-ray emission is associated with a jet component entering and exiting the core region is consistent with the timing of the two VHE gamma-ray emission peaks, optical polarization angle (EVPA) behavior, and jet kinematics: the first peak took place ~2 days after a very fast EVPA rotation, and the second ~18 days after the new knot emerged from the 43 GHz VLBI core. In December 2017 the source was detected by MAGIC at a flux level ~6 times higher than that of the 2015 outburst. We present the modeling of the 2015 flare and more recent results from the 2017 event.
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Proceedings of Science 358 2019年MAGIC is a system of two Cherenkov telescopes located in the Canary island of La Palma. A key part of MAGIC Fundamental Physics program is the search for indirect signals of Dark Matter (DM) from different sources. In the Milky Way, DM forms an almost spherically symmetric halo, with a density peaked towards the center of the Galaxy and decreasing toward the outer region. We search for DM decay signals from the Galactic Halo, with a special methodology developed for this work. Our strategy is to compare pairs of observations performed at different angular distances from the Galactic Center, selected in such a way that all the diffuse components cancel out, except for those coming from the DM. In order to keep the systematic uncertainty of this novel background estimation method down to a minimum, the observation pairs have been acquired during the same nights and follow exactly the same azimuth and zenith paths. We collected 20 hours of data during 2018. Using half of them to determine the systematic uncertainty in the background estimation of our analysis, we obtain a value of 4.8% with no dependence on energy. Accounting for this systematic uncertainty in the likelihood analysis based on the 10 remaining hours of data collected so far, we present the limit to TeV DM particle with a lifetime of 1026 s in the bb¯ decay channel.
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Monthly Notices of the Royal Astronomical Society 476(3) 2874-2875 2018年12月1日 査読有りThe paper 'A cut-off in the TeV gamma-ray spectrum of the SNR Cassiopeia A' was originally published in 472(3), 2956-2962.
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Astroparticle Physics 102 77-88 2018年11月1日A search for tau neutrino induced showers with the MAGIC telescopes is presented. The MAGIC telescopes located at an altitude of 2200 m a.s.l. in the Canary Island of La Palma, can point towards the horizon or a few degrees below across an azimuthal range of about 80°. This provides a possibility to search for air showers induced by tau leptons arising from interactions of tau neutrinos in the Earth crust or the surrounding ocean. In this paper we show how such air showers can be discriminated from the background of very inclined hadronic showers by using Monte Carlo simulations. Taking into account the orography of the site, the point source acceptance and the event rates expected have been calculated for a sample of generic neutrino fluxes from photo-hadronic interactions in AGNs. The analysis of about 30 h of data taken towards the sea leads to a 90% C.L. point source limit for tau neutrinos in the energy range from 1.0 × 1015 eV to 3.0 × 1018 eV of about Eντ 2×ϕ(Eντ )< 2.0×10−4 GeV cm−2 s−1 for an assumed power-law neutrino spectrum with spectral index γ=−2. However, with 300 h and in case of an optimistic neutrino flare model, limits of the level down to Eντ 2×ϕ(Eντ )< 8.4×10−6 GeV cm−2 s−1 can be expected.
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481(2) 2479-2486 2018年9月4日Fast radio bursts (FRBs) are bright flashes observed typically at GHz<br /> frequencies with millisecond duration, whose origin is likely extragalactic.<br /> Their nature remains mysterious, motivating searches for counterparts at other<br /> wavelengths. FRB 121102 is so far the only source known to repeatedly emit FRBs<br /> and is associated with a host galaxy at redshift $z \simeq 0.193$. We conducted<br /> simultaneous observations of FRB 121102 with the Arecibo and MAGIC telescopes<br /> during several epochs in 2016--2017. This allowed searches for<br /> millisecond-timescale burst emission in very-high-energy (VHE) gamma rays as<br /> well as the optical band. While a total of five FRBs were detected during these<br /> observations, no VHE emission was detected, neither of a persistent nature nor<br /> burst-like associated with the FRBs. The average integral flux upper limits<br /> above 100 GeV at 95% confidence level are $6.6 \times 10^{-12}~\mathrm{photons\<br /> cm^{-2}\ s^{-1 } }$ (corresponding to luminosity $L_{\rm VHE} \lesssim<br /> 10^{45}~\mathrm{erg\ s^{-1 } }$) over the entire observation period, and $1.2<br /> \times 10^{-7}~ \mathrm{photons\ cm^{-2}\ s^{-1 } }$ ($L_{\rm VHE} \lesssim<br /> 10^{49}~\mathrm{erg\ s^{-1 } }$) over the total duration of the five FRBs. We<br /> constrain the optical U-band flux to be below 8.6 mJy at 5-$\sigma$ level for<br /> 1-ms intervals around the FRB arrival times. A bright burst with U-band flux<br /> $29~\mathrm{mJy}$ and duration $\sim 12$ ms was detected 4.3 s before the<br /> arrival of one FRB. However, the probability of spuriously detecting such a<br /> signal within the sampled time space is 1.5% (2.2 $\sigma$, post-trial), i.e.<br /> consistent with the expected background. We discuss the implications of the<br /> obtained upper limits for constraining FRB models.
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2018年8月14日A multiwavelength campaign was organized to take place between March and July<br /> of 2012. Excellent temporal coverage was obtained with more than 25<br /> instruments, including the MAGIC, FACT and VERITAS Cherenkov telescopes, the<br /> instruments on board the Swift and Fermi spacecraft, and the telescopes<br /> operated by the GASP-WEBT collaboration.<br /> Mrk 501 showed a very high energy (VHE) gamma-ray flux above 0.2 TeV of<br /> $\sim$0.5 times the Crab Nebula flux (CU) for most of the campaign. The highest<br /> activity occurred on 2012 June 9, when the VHE flux was $\sim$3 CU, and the<br /> peak of the high-energy spectral component was found to be at $\sim$2 TeV. This<br /> study reports very hard X-ray spectra, and the hardest VHE spectra measured to<br /> date for Mrk 501. The fractional variability was found to increase with energy,<br /> with the highest variability occurring at VHE, and a significant correlation<br /> between the X-ray and VHE bands.<br /> The unprecedentedly hard X-ray and VHE spectra measured imply that their low-<br /> and high-energy components peaked above 5 keV and 0.5 TeV, respectively, during<br /> a large fraction of the observing campaign, and hence that Mrk 501 behaved like<br /> an extreme high-frequency- peaked blazar (EHBL) throughout the 2012 observing<br /> season. This suggests that being an EHBL may not be a permanent characteristic<br /> of a blazar, but rather a state which may change over time. The one-zone<br /> synchrotron self-Compton (SSC) scenario can successfully describe the segments<br /> of the SED where most energy is emitted, with a significant correlation between<br /> the electron energy density and the VHE gamma-ray activity, suggesting that<br /> most of the variability may be explained by the injection of high-energy<br /> electrons. The one-zone SSC scenario used reproduces the behaviour seen between<br /> the measured X-ray and VHE gamma-ray fluxes, and predicts that the correlation<br /> becomes stronger with increasing energy of the X-rays.
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Science 361, eaat1378 (2018) 361(6398) eaat1378-eaat1378 2018年7月24日Individual astrophysical sources previously detected in neutrinos are limited<br /> to the Sun and the supernova 1987A, whereas the origins of the diffuse flux of<br /> high-energy cosmic neutrinos remain unidentified. On 22 September 2017 we<br /> detected a high-energy neutrino, IceCube-170922A, with an energy of<br /> approximately 290 TeV. Its arrival direction was consistent with the location<br /> of a known gamma-ray blazar TXS 0506+056, observed to be in a flaring state. An<br /> extensive multi-wavelength campaign followed, ranging from radio frequencies to<br /> gamma-rays. These observations characterize the variability and energetics of<br /> the blazar and include the first detection of TXS 0506+056 in very-high-energy<br /> gamma-rays. This observation of a neutrino in spatial coincidence with a<br /> gamma-ray emitting blazar during an active phase suggests that blazars may be a<br /> source of high-energy neutrinos.
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The Astrophysical Journal Letters, 863 (2018) L10 863(1) L10-L10 2018年7月12日A neutrino with energy of $\sim$290 TeV, IceCube-170922A, was detected in<br /> coincidence with the BL Lac object TXS~0506+056 during enhanced gamma-ray<br /> activity, with chance coincidence being rejected at $\sim 3\sigma$ level. We<br /> monitored the object in the very-high-energy (VHE) band with the MAGIC<br /> telescopes for $\sim$41 hours from 1.3 to 40.4 days after the neutrino<br /> detection. Day-timescale variability is clearly resolved. We interpret the<br /> quasi-simultaneous neutrino and broadband electromagnetic observations with a<br /> novel one-zone lepto-hadronic model, based on interactions of electrons and<br /> protons co-accelerated in the jet with external photons originating from a<br /> slow-moving plasma sheath surrounding the faster jet spine. We can reproduce<br /> the multiwavelength spectra of TXS 0506+056 with neutrino rate and energy<br /> compatible with IceCube-170922A, and with plausible values for the jet power of<br /> $\sim 10^{45} - 4 \times 10^{46} {\rm erg \ s^{-1 } }$. The steep spectrum<br /> observed by MAGIC is concordant with internal $\gamma\gamma$ absorption above a<br /> few tens of GeV entailed by photohadronic production of a $\sim$290 TeV<br /> neutrino, corroborating a genuine connection between the multi-messenger<br /> signals. In contrast to previous predictions of predominantly hadronic emission<br /> from neutrino sources, the gamma-rays can be mostly ascribed to inverse Compton<br /> up-scattering of external photons by accelerated electrons. The X-ray and VHE<br /> bands provide crucial constraints on the emission from both accelerated<br /> electrons and protons. We infer that the maximum energy of protons in the jet<br /> co-moving frame can be in the range $\sim 10^{14}$ to $10^{18}$ eV.
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Monthly Notices of the Royal Astronomical Society, Volume 480, Issue 1, 11 October 2018, Pages 879-892 480(1) 879-892 2018年7月6日The MAGIC telescopes observed S2 0109+22 in 2015 July during its flaring<br /> activity in high energy gamma rays observed by Fermi-LAT. We analyse the MAGIC<br /> data to characterise the very high energy (VHE) gamma-ray emission of S2<br /> 0109+22, which belongs to the subclass of intermediate synchrotron peak (ISP)<br /> BL Lac objects. We study the multi-frequency emission in order to investigate<br /> the source classification. Finally, we compare the source long-term behaviour<br /> to other VHE gamma-ray emitting (TeV) blazars. We performed a temporal and<br /> spectral analysis of the data centred around the MAGIC interval of observation<br /> (MJD 57225-57231). Long-term radio and optical data have also been investigated<br /> using the discrete correlation function. The redshift of the source is<br /> estimated through optical host-galaxy imaging and also using the amount of VHE<br /> gamma-ray absorption. The quasi-simultaneous multi-frequency spectral energy<br /> distribution (SED) is modelled with the conventional one-zone synchrotron<br /> self-Compton (SSC) model. MAGIC observations resulted in the detection of the<br /> source at a significance level of $5.3\,\sigma$. The VHE gamma-ray emission of<br /> S2 0109+22 is variable on a daily time scale. VHE gamma-ray luminosity of the<br /> source is lower than the average of TeV BL Lacs. The optical polarization, and<br /> long-term optical/radio behaviour of the source are different from the general<br /> population of TeV blazars. All these findings agree with the classification of<br /> the source as an ISP BL Lac object. We estimate the source redshift as $z =<br /> 0.36 \pm 0.07$. The SSC parameters describing the SED are rather typical for<br /> blazars.
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2018年7月2日The BL Lac object S5~0716+714, a highly variable blazar, underwent an<br /> impressive outburst in January 2015 (Phase A), followed by minor activity in<br /> February (Phase B). The MAGIC observations were triggered by the optical flux<br /> observed in Phase A, corresponding to the brightest ever reported state of the<br /> source in the R-band. The comprehensive dataset collected is investigated in<br /> order to shed light on the mechanism of the broadband emission.<br /> Multi-wavelength light curves have been studied together with the broadband<br /> Spectral Energy Distributions (SEDs). The data set collected spans from radio,<br /> optical photometry and polarimetry, X-ray, high-energy (HE, 0.1 GeV < E < 100<br /> GeV) with \textit{Fermi}-LAT to the very-high-energy (VHE, E>100 GeV) with<br /> MAGIC. The flaring state of Phase A was detected in all the energy bands,<br /> providing for the first time a multi-wavelength sample of simultaneous data<br /> from the radio band to the VHE. In the constructed SED the<br /> \textit{Swift}-XRT+\textit{NuSTAR} data constrain the transition between the<br /> synchrotron and inverse Compton components very accurately, while the second<br /> peak is constrained from 0.1~GeV to 600~GeV by \textit{Fermi}+MAGIC data. The<br /> broadband SED cannot be described with a one-zone synchrotron self-Compton<br /> model as it severely underestimates the optical flux in order to reproduce the<br /> X-ray to $\gamma$-ray data. Instead we use a two-zone model. The EVPA shows an<br /> unprecedented fast rotation. An estimation of the redshift of the source by<br /> combined HE and VHE data provides a value of $z = 0.31 \pm 0.02_{stats} \pm<br /> 0.05_{sys}$, confirming the literature value. The data show the VHE emission<br /> originating in the entrance and exit of a superluminal knot in and out a<br /> recollimation shock in the inner jet. A shock-shock interaction in the jet<br /> seems responsible for the observed flares and EVPA swing. This scenario is also<br /> consistent with the SED modelling.
MISC
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Proceedings of Science 395 787 2022年3月 査読有り
講演・口頭発表等
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HIGH ENERGY GAMMA-RAY ASTRONOMY 2017年 AMER INST PHYSICSThe MAGIC telescopes are strongly involved in several multimessenger programs. One of them are follow-up observations of astrophysical neutrino events, to search for hadronic gamma-ray emission from the neutrino direction and identify potential neutrino sources. In 2013 the IceCube collaboration reported the first detection of astrophysical neutrinos and up to now 55 events, which interact inside the IceCube detector active volume, dubbed High Energy Starting Events (HESE), have been reported. These include 13 track-like events, caused usually by muon neutrino charge current interactions. The track-like events ( as opposed to cascade-like neutral current interactions) are characterised by a very good angular resolution of < 1 deg, which facilitates follow-up observations and potential source identification. The MAGIC telescopes performed follow-up observations of 4 HESE track-like events from the Northern hemisphere: HESE-37, HESE-38, HET (ATel #7856) and the GCN HESE alert from 27/04/2016 (GCN #19363). No signal was detected. Here we present the results of those observations and discuss the implications for the constraints on the density of the astrophysical neutrino sources.
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日本物理学会講演概要集 2014年8月22日 一般社団法人日本物理学会
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日本物理学会講演概要集 2014年8月22日 一般社団法人日本物理学会
共同研究・競争的資金等の研究課題
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日本学術振興会 科学研究費助成事業 2023年4月 - 2028年3月
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日本学術振興会 科学研究費助成事業 国際共同研究加速基金(国際共同研究強化(B)) 2020年10月 - 2025年3月
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日本学術振興会 科学研究費助成事業 研究活動スタート支援 2019年4月 - 2020年3月
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日本学術振興会 科学研究費助成事業 特別研究員奨励費 2006年 - 2008年