Synchrotron self-Compton Model Research Articles

Synchrotron Self-Compton as a Likely Mechanism of Photons beyond the Synchrotron Limit in GRB 190114C

Abstract GRB 190114C, a long and luminous burst, was detected by several satellites and ground-based telescopes from radio wavelengths to GeV gamma-rays. In the GeV gamma-rays, the Fermi Large Area Telescope detected 48 photons above 1 GeV during the first 100 s after the trigger time, and the MAGIC telescopes observed for more than 1000 s very high-energy (VHE) emission above 300 GeV. Previous analysis of the multi-wavelength observations showed that, although these are consistent with the synchrotron forward-shock model that evolves from a stratified stellar-wind to a homogeneous ISM-like medium, photons above a few GeV can hardly be interpreted in the synchrotron framework. In the context of the synchrotron forward-shock model, we derive the light curves and spectra of the synchrotron self-Compton (SSC) model in a stratified and homogeneous medium. In particular, we study the evolution of these light curves during the stratified-to-homogeneous afterglow transition. Using the best-fit parameters reported for GRB 190114C we interpret the photons beyond the synchrotron limit in the SSC framework and model its spectral energy distribution. We conclude that low-redshift gamma-ray bursts described under a favorable set of parameters as found in the early afterglow of GRB 190114C could be detected at hundreds of GeV, and also afterglow transitions would allow that VHE emission could be observed for longer periods.

Inverse Compton Scattering of Starlight in the Kiloparsec-scale Jet in Centaurus A: The Origin of Excess TeV γ-Ray Emission

Abstract Centaurus A (Cen A) is the nearest active radio galaxy, which has kiloparsec-scale jets and giant lobes detected by various instruments in radio and X-ray frequency ranges. The Fermi-Large Area Telescope and High Energy Stereoscopic System (HESS) confirmed that Cen A is a very high-energy (VHE; >0.1 TeV) γ-ray emitter with a known spectral softening in the energy range from a few GeV to TeV. In this work, we consider a synchrotron self-Compton model in the nucleus for the broadband spectrum below the break energy and an external Compton model in kiloparsec-scale jets for the γ-ray excess. Our results show that the observed γ-ray excess can be suitably described by the inverse Compton scattering of the starlight photons in the kiloparsec-scale jets, which is consistent with the recent tentative report by HESS on the spatial extension of the TeV emission along the jets. Considering the spectral fitting results, the excess can only be seen in Cen A, which is probably due to two factors: (1) the host galaxy is approximately 50 times more luminous than other typical radio galaxies and (2) the core γ-ray spectrum quickly decays above a few MeV due to the low maximum electron Lorentz factor of γ c = 2.8 × 103 resulting from the large magnetic field of 3.8 G in the core. By the comparison with other γ-ray detected radio galaxies, we found that the magnetic field strength of relativistic jets scales with the distance from the central black holes d with B(d) ∝ d −0.88 ± 0.14.

Reconcilement of VHE γ -ray/X-ray correlation studies in Mrk 421 and break-down at high fluxes

Multi-wavelength campaigns have been carried out to study the correlation between the very high energy (VHE) $\gamma$-ray and the X-ray emissions in blazars but, no conclusive results have been achieved yet. In this paper, we add Milagro data to the existing VHE $\gamma$-ray data from HEGRA-CT1 and Whipple and test the consistency and robustness of the reported correlation between VHE $\gamma$-ray and X-ray fluxes in Mrk 421. We found that at monthly time scale the correlation is robust, consistent between instruments and described as a linear function. Furthermore, most of the fluxes on shorter time scales are consistent with the correlation within 3 $\sigma_A$ even, where $\sigma_A$ is an estimated intrinsic scatter. However, a break-down of the correlation becomes clearly evident at high states of activity with fluxes $\rm \gtrsim 2.5\times 10^{-10}\, cm^{-2}s^{-1}$ at energies above 400 GeV independently of the time scale, observational period or instrument, even for single flares, the X-ray and VHE $\gamma$-ray emissions lie on the correlation until the VHE $\gamma$-ray flux reaches values higher than the one mentioned above. We have interpreted our results within the one-zone synchrotron self-Compton model. We found that describing a single and unique $\gamma$-ray/X-ray correlation strongly narrows the range of possible values of the magnetic field $B$ when a constant value of the spectral index along the correlation is assumed.

Variability studies and modelling of the blazar PKS 2155−304 in the light of a decade of multi-wavelength observations

The variability of the high-frequency peaked BL Lac object PKS 2155-304 is studied using almost 10 years of optical, X-ray and gamma-rays data. Publicly available data have been gathered and analyzed with the aim to characterize the variability and to search for log-normality or periodic behavior. The optical and X-ray range follow a log-normal process; a hint for a periodicity of about 700 days is found in optical and in the high energy (100 MeV <E<300 GeV) range. A one zone, time-dependent, synchrotron self-Compton model is successfully used to reproduce the evolution with energy of the variability and the tentatively reported periodicity.

BL Lacertae Objects: A Short Review

BL Lacertae objects (BLLs) constitute a class of active galactic nuclei (AGNs) with extreme observational features explained by non-thermal radiation from a relativistic jet nearly pointed along the observer’s line-of-sight. Their spectral energy distribution (SED), extending over 17-19 orders of the frequency, is of non-thermal origin and shows a typical two-humped structure. The lower-energy component, ranging from the radio to X-rays, is explained via synchrotron radiation emitted by ultra-relativistic electrons/positrons/protons, to be initially accelerated via the Blandford-Znajek mechanism or magneto-hydrodynamic processes in the vicinity of a central supermassive black hole. Afterwards, the particles should undergo further acceleration to ultra-relativistic energies by means of different mechanisms (first and second-order Fermi processes, relativistic magnetic reconnection, shear acceleration, jet-star interaction etc.) locally, in the jet emission zone. Our intensive X-ray spectral study of TeV-detected, high-energy-peaked BLLs (HBLs) often show the signatures of an effective second-order Fermi (stochastic) acceleration close to the shock front, while the processes related to the first-order Fermi acceleration are relatively rarely presented. The TeV-undetected HBLs and low-energy-peaked BLLs (LBLs) mostly do not show the signatures of efficient stochastic acceleration in their jets. Concerning the higher-energy component, the most frequently considered scenario incorporates an inverse Compton (IC) scattering of synchrotron photons by their ”parent” electron-positron population (synchrotron self-Compton model, SSC). However, this simple scenario sometimes is challenged by uncorrelated X-ray and TeV variability, more easily explained by multizone SSC, external Compton (EC) and hadronic scenarios.

Extreme HBL behavior of Markarian 501 during 2012

Aims. We aim to characterize the multiwavelength emission from Markarian 501 (Mrk 501), quantify the energy-dependent variability, study the potential multiband correlations, and describe the temporal evolution of the broadband emission within leptonic theoretical scenarios. Methods. We organized a multiwavelength campaign to take place between March and July of 2012. Excellent temporal coverage was obtained with more than 25 instruments, including the MAGIC, FACT and VERITAS Cherenkov telescopes, the instruments on board the Swift and Fermi spacecraft, and the telescopes operated by the GASP-WEBT collaboration. Results. Mrk 501 showed a very high energy (VHE) gamma-ray flux above 0.2 TeV of ∼0.5 times the Crab Nebula flux (CU) for most of the campaign. The highest activity occurred on 2012 June 9, when the VHE flux was ∼3 CU, and the peak of the high-energy spectral component was found to be at ∼2 TeV. Both the X-ray and VHE gamma-ray spectral slopes were measured to be extremely hard, with spectral indices &lt; 2 during most of the observing campaign, regardless of the X-ray and VHE flux. This study reports the hardest Mrk 501 VHE spectra measured to date. The fractional variability was found to increase with energy, with the highest variability occurring at VHE. Using the complete data set, we found correlation between the X-ray and VHE bands; however, if the June 9 flare is excluded, the correlation disappears (significance &lt; 3σ) despite the existence of substantial variability in the X-ray and VHE bands throughout the campaign. Conclusions. The unprecedentedly hard X-ray and VHE spectra measured imply that their low- and high-energy components peaked above 5 keV and 0.5 TeV, respectively, during a large fraction of the observing campaign, and hence that Mrk 501 behaved like an extreme high-frequency-peaked blazar (EHBL) throughout the 2012 observing season. This suggests that being an EHBL may not be a permanent characteristic of a blazar, but rather a state which may change over time. The data set acquired shows that the broadband spectral energy distribution (SED) of Mrk 501, and its transient evolution, is very complex, requiring, within the framework of synchrotron self-Compton (SSC) models, various emission regions for a satisfactory description. Nevertheless the one-zone SSC scenario can successfully describe the segments of the SED where most energy is emitted, with a significant correlation between the electron energy density and the VHE gamma-ray activity, suggesting that most of the variability may be explained by the injection of high-energy electrons. The one-zone SSC scenario used reproduces the behavior seen between the measured X-ray and VHE gamma-ray fluxes, and predicts that the correlation becomes stronger with increasing energy of the X-rays.

Multi-wavelength characterization of the blazar S5 0716+714 during an unprecedented outburst phase

Context. The BL Lac object S5 0716+714, a highly variable blazar, underwent an impressive outburst in January 2015 (Phase A), followed by minor activity in February (Phase B). The MAGIC observations were triggered by the optical flux observed in Phase A, corresponding to the brightest ever reported state of the source in the R-band. Aims.The comprehensive dataset collected is investigated in order to shed light on the mechanism of the broadband emission. Methods. Multi-wavelength light curves have been studied together with the broadband spectral energy distributions (SEDs). The sample includes data from Effelsberg, OVRO, Metsähovi, VLBI, CARMA, IRAM, SMA, Swift-UVOT, KVA, Tuorla, Steward, RINGO3, KANATA, AZT-8+ST7, Perkins, LX-200, Swift-XRT, NuSTAR, Fermi-LAT and MAGIC. Results. The flaring state of Phase A was detected in all the energy bands, providing for the first time a multi-wavelength sample of simultaneous data from the radio band to the very-high-energy (VHE, E &gt; 100 GeV). In the constructed SED, the Swift-XRT+NuSTAR data constrain the transition between the synchrotron and inverse Compton components very accurately, while the second peak is constrained from 0.1 GeV to 600 GeV by Fermi+MAGIC data. The broadband SED cannot be described with a one-zone synchrotron self-Compton model as it severely underestimates the optical flux in order to reproduce the X-ray to γ-ray data. Instead we use a two-zone model. The electric vector position angle (EVPA) shows an unprecedented fast rotation. An estimation of the redshift of the source by combined high-energy (HE, 0.1 GeV &lt; E &lt; 100 GeV) and VHE data provides a value of z = 0.31 ± 0.02stats ± 0.05sys, confirming the literature value. Conclusions. The data show the VHE emission originating in the entrance and exit of a superluminal knot in and out of a recollimation shock in the inner jet. A shock–shock interaction in the jet seems responsible for the observed flares and EVPA swing. This scenario is also consistent with the SED modeling.

HESS J1943+213: An Extreme Blazar Shining through the Galactic Plane

Abstract HESS J1943+213 is a very high energy (VHE; &gt;100 GeV) γ-ray source in the direction of the Galactic plane. Studies exploring the classification of the source are converging toward its identification as an extreme synchrotron BL Lac object. Here we present 38 hr of VERITAS observations of HESS J1943+213 taken over 2 yr. The source is detected with a significance of ∼20 standard deviations, showing a remarkably stable flux and spectrum in VHE γ-rays. Multifrequency Very Long Baseline Array (VLBA) observations of the source confirm the extended, jet-like structure previously found in the 1.6 GHz band with the European VLBI Network and detect this component in the 4.6 and 7.3 GHz bands. The radio spectral indices of the core and the jet and the level of polarization derived from the VLBA observations are in a range typical for blazars. Data from VERITAS, Fermi-LAT, Swift-XRT, the FLWO 48″ telescope, and archival infrared and hard X-ray observations are used to construct and model the spectral energy distribution (SED) of the source with a synchrotron self-Compton model. The well-measured γ-ray peak of the SED with VERITAS and Fermi-LAT provides constraining upper limits on the source redshift. Possible contribution of secondary γ-rays from ultra-high-energy cosmic-ray-initiated electromagnetic cascades to the γ-ray emission is explored, finding that only a segment of the VHE spectrum can be accommodated with this process. A variability search is performed across X-ray and γ-ray bands. No statistically significant flux or spectral variability is detected.

The broad-band properties of the intermediate synchrotron peaked BL Lac S2 0109+22 from radio to VHE gamma-rays

The MAGIC telescopes observed S2 0109+22 in 2015 July during its flaring activity in high energy gamma rays observed by Fermi-LAT. We analyse the MAGIC data to characterise the very high energy (VHE) gamma-ray emission of S2 0109+22, which belongs to the subclass of intermediate synchrotron peak (ISP) BL Lac objects. We study the multi-frequency emission in order to investigate the source classification. Finally, we compare the source long-term behaviour to other VHE gamma-ray emitting (TeV) blazars. We performed a temporal and spectral analysis of the data centred around the MAGIC interval of observation (MJD 57225-57231). Long-term radio and optical data have also been investigated using the discrete correlation function. The redshift of the source is estimated through optical host-galaxy imaging and also using the amount of VHE gamma-ray absorption. The quasi-simultaneous multi-frequency spectral energy distribution (SED) is modelled with the conventional one-zone synchrotron self-Compton (SSC) model. MAGIC observations resulted in the detection of the source at a significance level of $5.3\,\sigma$. The VHE gamma-ray emission of S2 0109+22 is variable on a daily time scale. VHE gamma-ray luminosity of the source is lower than the average of TeV BL Lacs. The optical polarization, and long-term optical/radio behaviour of the source are different from the general population of TeV blazars. All these findings agree with the classification of the source as an ISP BL Lac object. We estimate the source redshift as $z = 0.36 \pm 0.07$. The SSC parameters describing the SED are rather typical for blazars.

The Origins of the Gamma-Ray Flux Variations of NGC 1275 Based on Eight Years of Fermi-LAT Observations

Abstract We present an analysis of eight years of Fermi-LAT (&gt;0.1 GeV) γ-ray data obtained for the radio galaxy NGC 1275. The γ-ray flux from NGC 1275 is highly variable on short (∼days to weeks) timescales, and has steadily increased over this eight year timespan. By examining the changes in its flux and spectral shape in the LAT energy band over the entire data set, we found that its spectral behavior changed around 2011 February (∼MJD 55600). The γ-ray spectra at early times evolved largely at high energies, while the photon indices were unchanged at later times despite rather large flux variations. To explain these observations, we suggest that the flux changes at the early times were caused by injection of high-energy electrons into the jet while, later, the γ-ray flares were caused by a changing Doppler factor owing to variations in the jet Lorentz factor and/or changes in the angle to our line of sight. To demonstrate the viability of these scenarios, we fit the broad band spectral energy distribution data with a one-zone synchrotron self-Compton (SSC) model for flaring and quiescent intervals before and after 2011 February. To explain the γ-ray spectral behavior in the context of the SSC model, the maximum electron Lorentz factor would have changed at the early times, while a modest change in the Doppler factor adequately fits the quiescent and flaring state γ-ray spectra at the later times.

The NuSTAR view on hard-TeV BL Lacs

Hard-TeV BL Lacs are a new type of blazars characterized by a hard intrinsic TeV spectrum, locating the peak of their gamma-ray emission in the spectral energy distribution (SED) above 2-10 TeV. Such high energies are problematic for the Compton emission, using a standard one-zone leptonic model. We study six examples of this new type of BL Lacs in the hard X-ray band with the NuSTAR satellite. Together with simultaneous observations with the SWIFT satellite, we fully constrain the peak of the synchrotron emission in their SED, and test the leptonic synchrotron self-Compton (SSC) model. We confirm the extreme nature of 5 objects also in the synchrotron emission. We do not find evidence of additional emission components in the hard X-ray band. We find that a one-zone SSC model can in principle reproduce the extreme properties of both peaks in the SED, from X-ray up to TeV energies, but at the cost of i) extreme electron energies with very low radiative efficiency, ii) conditions heavily out of equipartition (by 3 to 5 orders of magnitude), and iii) not accounting for the simultaneous UV data, which then should belong to a different emission component, possibly the same as the far-IR (WISE) data. We find evidence of this separation of the UV and X-ray emission in at least two objects. In any case, the TeV electrons must not "see" the UV or lower-energy photons, even if coming from different zones/populations, or the increased radiative cooling would steepen the VHE spectrum.

Exploring the origin of broad-band emissions of Mrk 501 with a two-zone model

Abstract We propose a two-zone synchrotron self-Compton (SSC) model, including an inner gamma-ray emitting region with spherical shape and a conical radio emitting region located at the extended jet, to alleviate the long-standing “bulk Lorentz factor crisis” in blazars. In this model, the spectral energy distributions (SEDs) of blazars are produced by considering the gamma-ray emitting region inverse Compton scattering of both the synchrotron photons itself and the ambient photons from the radio emitting region. Applying the model to Mrk 501, we obtain that the radio emitting region has a comoving length of ∼0.15 pc and is located at sub-parsec scale from the central engine by modeling the radio data; the flux of the Compton scattering of the ambient photons is so low that it can be neglected safely. The characteristic hard gamma-ray spectrum can be explained by the superposition of two SSC processes, and the model can approximately explain the very high energy (VHE) data. The insights into the spectral shape and the inter-band correlations under the flaring state will provide us with a diagnostic for the bulk Lorentz factor of radio emitting region, where the low and upper limits of 8 and 15 are preferred, and for the two-zone SSC model itself. In addition, our two-zone SSC model shows that the gamma-ray emitting region creates flare on the timescale of merely a few hours, and the long time outbursts more likely originate from the extended radio emitting region.

Broadband study of blazar 1ES 1959+650 during flaring state in 2016

Aims. The nearby TeV blazar 1ES 1959+650 (z = 0.047) was reported to be in flaring state during June–July 2016 by Fermi-LAT, FACT, MAGIC and VERITAS collaborations. We studied the spectral energy distributions (SEDs) in different states of the flare during MJD 57530–57589 using simultaneous multiwaveband data with the aim of understanding the possible broadband emission scenario during the flare. Methods. The UV-optical and X-ray data from UVOT and XRT respectively on board Swift and high energy γ-ray data from Fermi-LAT were used to generate multiwaveband lightcurves as well as to obtain high flux states and quiescent state SEDs. The correlation and lag between different energy bands was quantified using discrete correlation function. The synchrotron self-Compton (SSC) model was used to reproduce the observed SEDs during flaring and quiescent states of the source. Results. A good correlation is seen between X-ray and high energy γ-ray fluxes. The spectral hardening with increase in the flux is seen in X-ray band. The power law index vs. flux plot in γ-ray band indicates the different emission regions for 0.1–3 GeV and 3–300 GeV energy photons. Two zone SSC model satisfactorily fits the observed broadband SEDs. The inner zone is mainly responsible for producing synchrotron peak and high energy γ-ray part of the SED in all states. The second zone is mainly required to produce less variable optical-UV and low energy γ-ray emission. Conclusions. Conventional single zone SSC model does not satisfactorily explain broadband emission during observation period considered. There is an indication of two emission zones in the jet which are responsible for producing broadband emission from optical to high energy γ-rays.

The Cocoon Shocks of Cygnus A: Pressures and Their Implications for the Jets and Lobes

Abstract We use 2.0 Msec of Chandra observations to investigate the cocoon shocks of Cygnus A and some implications for its lobes and jet. Measured shock Mach numbers vary in the range 1.18–1.66 around the cocoon. We estimate a total outburst energy of , with an age of . The average postshock pressure is found to be , which agrees with the average pressure of the thin rim of compressed gas between the radio lobes and shocks, as determined from X-ray spectra. However, average rim pressures are found to be lower in the western lobe than in the eastern lobe by ≃20%. Pressure estimates for hotspots A and D from synchrotron self-Compton models imply that each jet exerts a ram pressure ≳3 times its static pressure, consistent with the positions of the hotspots moving about on the cocoon shock over time. A steady, one-dimensional flow model is used to estimate jet properties, finding mildly relativistic flow speeds within the allowed parameter range. Models in which the jet carries a negligible flux of rest mass are consistent with the observed properties of the jets and hotspots. This favors the jets being light, implying that the kinetic power and momentum flux are carried primarily by the internal energy of the jet plasma rather than by its rest mass.

Effective spectral index properties for Fermi blazars

Blazars are a special subclass of active galactic nuclei withextreme observation properties. This subclass can be divided into two furthersubclasses of flat spectrum radio quasars (FSRQs) and BL Lacertae objects (BL Lacs) according to their emissionline features. To compare the spectral properties of FSRQs andBL Lacs, the 1.4 GHz radio, opticalR-band, 1 keV X-ray, and 1 GeV $\gamma$-ray flux densities for 1108 Fermiblazars are calculated to discuss the properties of the sixeffective spectral indices of radio to optical ($\alpha_{\rmRO}$), radio to X-ray ($\alpha_{\rm~RX}$), radio to $\gamma$ ray($\alpha_{\rm~R\gamma}$), optical to X-ray ($\alpha_{\rm~OX}$),optical to $\gamma$ ray ($\alpha_{\rm~O\gamma}$), and X-ray to$\gamma$ ray ($\alpha_{\rm~X\gamma}$). The main results are asfollows: For the averaged effective spectral indices, $\overline~{\alpha_{\rm~OX}}>~\overline~{\alpha_{\rm~O\gamma}}>~\overline~{\alpha_{\rmX\gamma}}>~\overline~{\alpha_{\rm~R\gamma}}>~\overline{\alpha_{\rm~RX}}>~\overline~{\alpha_{\rm~RO}}$ for samples ofwhole blazars and BL Lacs; $\overline~{\alpha_{\rm~X\gamma}}\approx~\overline~{\alpha_{\rm~R\gamma}}\approx~\overline{\alpha_{\rm~RX}}$ for FSRQs and low-frequency-peaked BL Lacs(LBLs); and $\overline~{\alpha_{\rm~OX}}\approx~\overline{\alpha_{\rm~O\gamma}}\approx~\overline~{\alpha_{\rm~X\gamma}}$for high-synchrotron-frequency-peaked BL Lacs (HBLs). Thedistributions of the effective spectral indices involving opticalemission ($\alpha_{\rm~RO}$, $\alpha_{\rm~OX}$, and $\alpha_{\rmO\gamma}$) for LBLs are different from those for FSRQs, but if theeffective spectral index does not involve optical emission($\alpha_{\rm~RX}$, $\alpha_{\rm~R\gamma}$, and $\alpha_{\rmX\gamma}$), the distributions for LBLs and FSRQs almost come fromthe same parent population. X-ray emissions from blazarsinclude both synchrotron and inverse Compton (IC) components; theIC component for FSRQs and LBLs accounts for a larger proportionthan that for HBLs; and the radiation mechanism for LBLs issimilar to that for FSRQs, but the radiation mechanism for HBLs isdifferent from that for both FSRQs and LBLs in X-ray bands.The tendency of $\alpha_{\rm~R\gamma}$ decreasing from LBLs toHBLs suggests that the synchrotron self-Compton model explains themain process for highly energetic $\gamma$ rays in BL Lacs.

Using the Markov chain Monte Carlo method to study the physical properties of GeV–TeV BL Lac objects

Abstract We fit the spectral energy distributions (SEDs) of 46 GeV–TeV BL Lac objects in the frame of leptonic one-zone synchrotron self-Compton (SSC) model and investigate the physical properties of these objects. We use the Markov chain Monte Carlo (MCMC) method to obtain the basic parameters, such as magnetic field (B), the break energy of the relativistic electron distribution ($\gamma ^{\prime }_{\rm {b}}$), and the electron energy spectral index. Based on the modeling results, we support the following scenarios for GeV–TeV BL Lac objects. (1) Some sources have large Doppler factors, implying other radiation mechanism should be considered. (2) Compared with flat spectrum quasars (FSRQs), GeV–TeV BL Lac objects have weaker magnetic fields and larger Doppler factors, which cause the ineffective cooling and shift the SEDs to higher bands. Their jet powers are around 4.0 × 1045 erg s−1, compared with radiation power, 5.0 × 1042 erg s−1, indicating that only a small fraction of jet power is transformed into the emission power. (3) For some BL Lacs with large Doppler factors, their jet components could have two substructures, e.g., the fast core and the slow sheath. For most GeV–TeV BL Lacs, Kelvin–Helmholtz instabilities are suppressed by their higher magnetic fields, leading to micro-variability or intro-day variability in the optical bands. (4) Combined with a sample of FSRQs, an anti-correlation between the peak luminosity, Lpk, and the peak frequency, νpk, is obtained, favoring the blazar sequence scenario. In addition, an anti-correlation between the jet power, Pjet, and the break Lorentz factor, γb, also supports the blazar sequence.

Constraining the red shifts of TeV BL Lac objects

We present a model-dependent method to estimate the redshifts of three TeV BL Lac objects (BL Lacs) through fltting their (quasi-) simultaneous multi-waveband spectral energy distributions (SEDs) by one-zone leptonic synchrotron self-Compton (SSC) model. Considering the impact of electron energy distributions (EEDs) on the results, we use three types of EEDs, such as the power-law EED with exponential cut-ofi (PLC), the log-parabola (PLLP) and the broken power-law (BPL) EEDs, to flt the SEDs. We also use a parameter fi to describe the uncertainties of the extragalactic background light (EBL) models, as in (Abdo et al. 2010b). We then use Markov Chain Monte Carlo (MCMC) method to explore multi-dimensional parameter space and obtain the uncertainties of the model parameters based on the observational data. We apply our method to obtain the redshifts of three TeV BL Lac objects in the marginalized 68% confldence, and flnd that the PLC EED does not flt the SEDs. For 3C66A, the redshift is 0.14 - 0.31 and 0.16 - 0.32 in the BPL and PLLP EEDs; for PKS1424+240, the redshift is 0.55 - 0.68 and 0.55 - 0.67 in the BPL and PLLP EEDs; for PG1553+113, the redshift is 0.22 - 0.48 and 0.22 - 0.39 in the BPL and PLLP EEDs. We also estimate the redshift of PKS1424+240 in the high stage to be 0.46 - 0.67 in the PLLP EED, roughly consistent with that in the low stage.

Hard X-Ray Emission from the M87 AGN Detected with NuSTAR

Abstract M87 hosts a 3–6 billion solar mass black hole with a remarkable relativistic jet that has been regularly monitored in radio to TeV bands. However, hard X-ray emission ≳10 keV, which would be expected to primarily come from the jet or the accretion flow, had never been detected from its unresolved X-ray core. We report NuSTAR detection up to 40 keV from the the central regions of M87. Together with simultaneous Chandra observations, we have constrained the dominant hard X-ray emission to be from its unresolved X-ray core, presumably in its quiescent state. The core spectrum is well fitted by a power law with photon index . The measured flux density at 40 keV is consistent with a jet origin, although emission from the advection-dominated accretion flow cannot be completely ruled out. The detected hard X-ray emission is significantly lower than that predicted by synchrotron self-Compton models introduced to explain emission above a GeV.

Rapid Gamma-Ray Variability of NGC 1275

Abstract We report on a detailed analysis of the γ-ray light curve of NGC 1275 using the Fermi Large Area Telescope data accumulated during 2008–2017. Major γ-ray flares were observed in 2015 October and 2016 December/2017 January when the source reached a daily peak flux of , achieving a flux of within 3 hr, which corresponds to an apparent isotropic γ-ray luminosity of . The most rapid flare had an e-folding time as short as hr, which had never been previously observed for any radio galaxy in γ-ray band. Also, γ-ray spectral changes were observed during these flares: in the flux versus photon index plane, the spectral evolution follows correspondingly a counterclockwise and a clockwise loop inferred from the light curve generated by an adaptive binning method. On 2016 December 30 and 2017 January 1 the X-ray photon index softened ( ) and the flux increased nearly ∼3 times as compared with the quiet state. The observed hour-scale variability suggests a very compact emission region ( ), implying that the observed emission is most likely produced in the subparsec-scale jet if the entire jet width is responsible for the emission. During the active periods, the γ-ray photon index hardened, shifting the peak of the high-energy spectral component to , making it difficult to explain the observed X-ray and γ-ray data in the standard one-zone synchrotron self-Compton model.

Multiband variability studies and novel broadband SED modeling of Mrk 501 in 2009

We present an extensive study of the BL Lac object Mrk 501 based on a data set collected during the multi-instrument campaign spanning from 2009 March 15 to 2009 August 1 which includes, among other instruments, MAGIC, VERITAS, Whipple 10-m, Fermi-LAT, RXTE, Swift, GASP-WEBT and VLBA. We find an increase in the fractional variability with energy, while no significant interband correlations of flux changes are found in the acquired data set. The higher variability in the very high energy (>100 GeV, VHE) gamma-ray emission and the lack of correlation with the X-ray emission indicate that the highest-energy electrons that are responsible for the VHE gamma-rays do not make a dominant contribution to the ~1 keV emission. Alternatively, there could be a very variable component contributing to the VHE gamma-ray emission in addition to that coming from the synchrotron self-Compton (SSC) scenarios. The space of SSC model parameters is probed following a dedicated grid-scan strategy, allowing for a wide range of models to be tested and offering a study of the degeneracy of model-to-data agreement in the individual model parameters. We find that there is some degeneracy in both the one-zone and the two-zone SSC scenarios that were probed, with several combinations of model parameters yielding a similar model-to-data agreement, and some parameters better constrained than others. The SSC model grid-scan shows that the flaring activity around 2009 May 22 cannot be modeled adequately with a one-zone SSC scenario, while it can be suitably described within a two-independent-zone SSC scenario. The observation of an electric vector polarization angle rotation coincident with the gamma-ray flare from 2009 May 1 resembles those reported previously for low frequency peaked blazars, hence suggesting that there are many similarities in the flaring mechanisms of blazars with different jet properties.