Single-zone Synchrotron Research Articles

Very High-energy Gamma-Ray Episodic Activity of Radio Galaxy NGC 1275 in 2022–2023 Measured with MACE

The radio galaxy NGC 1275, located at the central region of Perseus cluster, is a well-known very high-energy (VHE) gamma-ray emitter. The Major Atmospheric Cherenkov Experiment Telescope has detected two distinct episodes of VHE (E > 80 GeV) gamma-ray emission from NGC 1275 during 2022 December and 2023 January. The second outburst, observed on 2023 January 10, was the more intense of the two, with flux reaching 58% of the Crab Nebula flux above 80 GeV. The differential energy spectrum measured between 80 GeV and 1.5ṪeV can be described by a power law with a spectral index of Γ = −2.90 ± 0.16stat for both flaring events. The broadband spectral energy distribution derived from these flares, along with quasisimultaneous low-energy counterparts, suggests that the observed gamma-ray emission can be explained using a homogeneous single-zone synchrotron self-Compton model. The physical parameters derived from this model for both flaring states are similar. The intermediate state observed between two flaring episodes is explained by a lower Doppler factor or magnetic field, which subsequently returned to its previous value during the high-activity state observed on 2023 January 10.

TeV flaring activity of the AGN PKS 0625–354 in November 2018

Most γ-ray detected active galactic nuclei are blazars with one of their relativistic jets pointing towards the Earth. Only a few objects belong to the class of radio galaxies or misaligned blazars. Here, we investigate the nature of the object PKS 0625−354, its γ-ray flux and spectral variability and its broad-band spectral emission with observations from H.E.S.S., Fermi-LAT, Swift-XRT, and UVOT taken in November 2018. The H.E.S.S. light curve above 200 GeV shows an outburst in the first night of observations followed by a declining flux with a halving time scale of 5.9 h. The γγ-opacity constrains the upper limit of the angle between the jet and the line of sight to ∼10°. The broad-band spectral energy distribution shows two humps and can be well fitted with a single-zone synchrotron self Compton emission model. We conclude that PKS 0625−354, as an object showing clear features of both blazars and radio galaxies, can be classified as an intermediate active galactic nuclei. Multi-wavelength studies of such intermediate objects exhibiting features of both blazars and radio galaxies are sparse but crucial for the understanding of the broad-band emission of γ-ray detected active galactic nuclei in general.

GRB Afterglow Parameters in the Era of TeV Observations: The Case of GRB 190114C

Abstract The afterglow of GRB 190114C has been observed at 60–1200 s after the burst in the sub-TeV range by the MAGIC Cerenkov telescope. The simultaneous observations in the X-ray range, which is presumed to be of synchrotron origin, and in the sub-TeV range, where the emission is presumed to be inverse Compton, provide new stringent constraints on the conditions within the emitting regions and their evolution in time. While the additional data contain a lot of new information, it turns out that fitting both the X-ray and the TeV emission is much more complicated than what was originally anticipated. We find that optical flux measurements provide important complementary information that, in combination with TeV measurements, breaks degeneracy in the parameter space. We present here a numerical fit to the multiwavelength observed spectrum using a new code that calculates the single-zone synchrotron including self-Compton emission, taking into account the exact Klein–Nishina cross sections, as well as pair production via absorption of the high-energy photons inside the emitting zone and the emission from the resulting secondary pairs. We also present a revised set of single-zone parameters and a method for fitting the data to the observations. Our model for GRB 190114C that fits all the observations, from the optical data point to the sub-TeV range, suggests that it is in the fast-cooling regime. The inferred parameters for observations at two separate moments of time show significant deviations from some of the common expectations in afterglow modeling but are all consistent with the predictions of the pair-balance model.

Multiwavelength analysis and modeling of OJ 287 during 2017–2020

Context. The blazar OJ 287 has been proposed as binary black hole system based on its periodic optical outburst. Black hole binary systems are scarce among blazars with parsec scale jets, and hence this source is exciting to study Aims. The BL Lac OJ 287 is an interesting object for multiwavelength study due to its periodic outbursts. We analysed the optical, X-ray and γ-ray data of OJ 287 for the period of 2017–2020. There are several high states in optical–UV and X-ray frequencies during this period. Based on the observed variability in optical and X-ray frequencies, the entire period 2017–2020 is divided in five segments, in this paper referred as A, B, C, D, and E. A detailed temporal and spectral analysis is performed to understand the nature of the flaring activities of OJ 287. Methods. To understand the temporal variability in this source we studied the intraday and fractional variability for all the various states. In additon, fast variability time was also estimated in order to understand the nature of variability. Furthermore, the multiwavelength spectral energy distribution (SED) modeling was performed to know more about the physical processes responsible for the simultaneous broadband emission and the fast variability. Results. The Fermi-LAT observations show a moderate flux level of this source in γ-ray frequency throughout this period, though flux variability has been observed. The source has shown a strong flux variability in X-ray, optical, and UV during early 2017 and mid 2020 when the source was in very high state. A single-zone synchrotron self-Compton emission model is considered to model the SED, and this helps us to explore the nature of this BL Lac with binary supermassive black holes.

The Great Markarian 421 Flare of 2010 February: Multiwavelength Variability and Correlation Studies

Abstract We report on variability and correlation studies using multiwavelength observations of the blazar Mrk 421 during the month of 2010 February, when an extraordinary flare reaching a level of ∼27 Crab Units above 1 TeV was measured in very high energy (VHE) γ-rays with the Very Energetic Radiation Imaging Telescope Array System (VERITAS) observatory. This is the highest flux state for Mrk 421 ever observed in VHE γ-rays. Data are analyzed from a coordinated campaign across multiple instruments, including VHE γ-ray (VERITAS, Major Atmospheric Gamma-ray Imaging Cherenkov), high-energy γ-ray (Fermi-LAT), X-ray (Swift, Rossi X-ray Timing Experiment, MAXI), optical (including the GASP-WEBT collaboration and polarization data), and radio (Metsähovi, Owens Valley Radio Observatory, University of Michigan Radio Astronomy Observatory). Light curves are produced spanning multiple days before and after the peak of the VHE flare, including over several flare “decline” epochs. The main flare statistics allow 2 minute time bins to be constructed in both the VHE and optical bands enabling a cross-correlation analysis that shows evidence for an optical lag of ∼25–55 minutes, the first time-lagged correlation between these bands reported on such short timescales. Limits on the Doppler factor (δ ≳ 33) and the size of the emission region ( ) are obtained from the fast variability observed by VERITAS during the main flare. Analysis of 10 minute binned VHE and X-ray data over the decline epochs shows an extraordinary range of behavior in the flux–flux relationship, from linear to quadratic to lack of correlation to anticorrelation. Taken together, these detailed observations of an unprecedented flare seen in Mrk 421 are difficult to explain with the classic single-zone synchrotron self-Compton model.

Fermi/LAT observations of lobe-dominant radio galaxy 3C 207 and possible radiation region of γ-rays

3C 207 is a lobe-dominant radio galaxy with a one sided jet and bright knots, spanning a kpc-Mpc scale, which have been resolved in the radio, optical and X-ray bands. This target was confirmed as a γ-ray emitter with Fermi/LAT, but it is uncertain whether the γ-ray emission region is the core or knots due to the low spatial resolution of Fermi/LAT. We present an analysis of its Fermi/LAT data acquired during the past 9 years. Different from the radio and optical emission from the core, it is found that the γ-ray emission is steady without detection of flux variation at over a 2σ confidence level. This likely implies that the γ-ray emission is from its knots. We collect the radio, optical and X-ray data of knot-A, the closest knot from the core at 1.4″, and compile its spectral energy distribution (SED). Although the single-zone synchrotron+SSC+IC/CMB model that assumes knot-A is at rest can reproduce the SED in the radio-optical-X-ray band, the predicted γ-ray flux is lower than the LAT observations and the derived magnetic field strength deviates from the equipartition condition by 3 orders of magnitude. Assuming that knot-A is moving relativistically, its SED from radio to γ-ray bands would be represented well with the single-zone synchrotron+SSC+IC/CMB model under the equipartition condition. These results likely suggest that the γ-ray emission may be from knot-A via the IC/CMB process and the knot should have relativistical motion. The jet power derived from our model parameters is also roughly consistent with the kinetic power estimated with radio data.

Modeling the spectral energy distribution of the radio galaxy IC310

The radio galaxy IC310 located in the Perseus Cluster is one of the brightest objects in the radio and X-ray bands, and one of the closest active galactic nuclei observed in very-high energies. In GeV - TeV γ-rays, IC310 was detected in low and high flux states by the MAGIC telescopes from October 2009 to February 2010. Taking into account that the spectral energy distribution (SED) up to a few GeV seems to exhibit a double-peak feature and that a single-zone synchrotron self-Compton (SSC) model can explain all of the multiwavelength emission except for the non-simultaneous MAGIC emission, we interpret, in this work, the multifrequency data set of the radio galaxy IC310 in the context of homogeneous hadronic and leptonic models. In the leptonic framework, we present a multi-zone SSC model with two electron populations to explain the whole SED whereas for the hadronic model, we propose that a single-zone SSC model describes the SED up to a few GeVs and neutral pion decay products resulting from pγ interactions could describe the TeV - GeV γ-ray spectra. These interactions occur when Fermi-accelerated protons interact with the seed photons around the SSC peaks. We show that, in the leptonic model the minimum Lorentz factor of second electron population is exceedingly high γe ∼ 105 disfavoring this model, and in the hadronic model the required proton luminosity is not extremely high ∼1044 erg/s, provided that charge neutrality between the number of electrons and protons is given. Correlating the TeV γ-ray and neutrino spectra through photo-hadronic interactions, we find that the contribution of the emitting region of IC310 to the observed neutrino and ultra-high-energy cosmic ray fluxes are negligible.

The TeV emission of Ap Librae: a hadronic interpretation and prospects for CTA

Ap Librae is one out of a handful of low-frequency peaked blazars to be detected at TeV $\gamma$-rays and the only one with an identified X-ray jet. Combined observations of Fermi-LAT at high energies (HE) and of H.E.S.S at very high energies (VHE) revealed a striking spectral property of Ap Librae; the presence of a broad high-energy component that extends more than nine orders of magnitude in energy and is, therefore, hard to be explained by the usual single-zone synchrotron self-Compton model. We show that the superposition of different emission components related to photohadronic interactions can explain the $\gamma$-ray emission of Ap Librae without invoking external radiation fields. We present two indicative model fits to the spectral energy distribution of Ap Librae where the VHE emission is assumed to originate from a compact, sub-pc scale region of the jet. A robust prediction of our model is VHE flux variability on timescales similar to those observed at X-rays and HE $\gamma$-rays, which can be further used to distinguish between a sub-pc or kpc scale origin of the TeV emission. We thus calculate the expected variability signatures at X-rays, HE and VHE $\gamma$-rays and show that quasi-simultaneous flares are expected, with larger amplitude flares appearing at $\gamma$-rays. We assess the detectability of VHE variability from Ap Librae with CTA, next generation of IACTs. We show that $\sim$hr timescale variability at $E_{\gamma}>0.1$ TeV could be detectable at high significance with shorter exposure times than current Cherenkov telescopes.

NO EVIDENCE OF INTRINSIC OPTICAL/NEAR-INFRARED LINEAR POLARIZATION FOR V404 CYGNI DURING ITS BRIGHT OUTBURST IN 2015: BROADBAND MODELING AND CONSTRAINT ON JET PARAMETERS

ABSTRACT We present simultaneous optical and near-infrared (NIR) polarimetric results for the black hole binary V404 Cyg spanning the duration of its seven-day-long optically brightest phase of its 2015 June outburst. The simultaneous R- and K s -band light curves showed almost the same temporal variation except for the isolated (∼30-minute duration) orphan K s -band flare observed at MJD 57193.54. We did not find any significant temporal variation of polarization degree (PD) and position angle (PA) in both R and K s bands throughout our observations, including the duration of the orphan NIR flare. We show that the observed PD and PA are predominantly interstellar in origin by comparing the V404 Cyg polarimetric results with those of the surrounding sources within the 7′ × 7′ field of view. The low intrinsic PD (less than a few percent) implies that the optical and NIR emissions are dominated by either disk or optically thick synchrotron emission, or both. We also present the broadband spectra of V404 Cyg during the orphan NIR flare and a relatively faint and steady state by including quasi-simultaneous Swift/XRT and INTEGRAL fluxes. By adopting a single-zone synchrotron plus inverse-Compton model as widely used in modeling of blazars, we constrained the parameters of a putative jet. Because the jet synchrotron component cannot exceed the Swift/XRT disk/corona flux, the cutoff Lorentz factor in the electron energy distribution is constrained to be <102, suggesting that particle acceleration is less efficient in this microquasar jet outburst compared to active galactic nucleus jets. We also suggest that the loading of the baryon component inside the jet is inevitable based on energetic arguments.

Radiation mechanisms and physical properties of the γ-ray narrow-line Seyfert 1 galaxies

Abstract We investigate the physical properties and radiation mechanisms of 11 states of five narrow-line Seyfert 1 (NLS1) galaxies detected by the Large Area Telescope on board Fermi through modeling the quasi-simultaneous multi-band observations. We obtain the best-fitting model parameters and their uncertainties for each state with the χ2-minimization procedure and discuss their implications on the characteristics of jet. Similar to blazars, their spectral energy distributions (SEDs) have a two-humped structure and their non-thermal emission can be modelled with the single-zone synchrotron + inverse Compton (IC) model. For all states, the GeV γ-rays may be contributed by the external Compton (EC) emission components. The observations of Fermi are mostly located at the declining stage of the EC humps. Text < 0.5 eV in all cases (Text is the characteristic temperature of external soft photons), suggesting that their radiation zones may be usually located outside of the broad line region (BLR) and the soft photons of Compton scattering mainly come from the dust torus. Compared with the bright Fermi blazars studied by Ghisellini et al. (2014, Nature, 515, 376), the Pjet (the power of the jets) of NLS1 galaxies detected by Fermi is similar to that of the flat spectrum radio quasars (FSRQs) but a little larger than that of the BL Lac objects (BL Lacs). However, a comparison of Pr (the powers of radiations) with the FSRQs and BL Lac objects shows that NLS1 galaxies’ Pr has values comparable to BL Lac objects but lower than FSRQs in spite of having similar Pjet values and the same energy carrier (the cold protons) as the FSRQs. Observations indicate that γ-NLS1 galaxies might have lower η (efficiency of gravitational energy release) values than GeV blazars.

High-energy emission processes in M87

We study the contribution of thermal and non-thermal processes to the inverse Compton emission of the radio galaxy M 87 by modelling its broad-band emission. Through this we aim to derive insight into where within the AGN the X-ray, gamma-ray, and VHE emission is produced. We have analysed all available INTEGRAL IBIS/ISGRI data on M 87, spanning almost 10 years, to set an upper limit to the average hard X-ray flux of $f(20 - 60 \rm \, keV) < 3\times 10^{-12}$ $\rm \, erg \, cm^{-2} \, s^{-1}$, using several techniques beyond the standard analysis which are also presented here. We also analysed hard X-ray data from Suzaku/PIN taken late November 2006, and we report the first hard X-ray detection of M 87 with a flux of $f(20 - 60 \rm \, keV) = 10^{-11}\rm \, erg \, cm^{-2} \, s^{-1} $. In addition we analyse data from Fermi/LAT, INTEGRAL/JEM-X, and Suzaku/XIS. We collected historical radio/IR/optical and VHE data and combined them with the X-ray and gamma-ray data, to create broad-band spectral energy distributions for the average low-flux state and the flaring state. The resulting spectral energy distributions are modelled by applying a single-zone SSC model with a jet angle of theta = 15 degrees. We also show that modelling the core emission of M 87 using a single-zone synchrotron self-Compton model does represent the SED, suggesting that the core emission is dominated by a BL Lac type AGN core. Using SED modelling we also show that the hard X-ray emission detected in 2006 is likely due to a flare of the jet knot HST-1, rather than being related to the core.

VERITAS OBSERVATIONS OF SIX BRIGHT, HARD-SPECTRUMFERMI-LAT BLAZARS

We report on VERITAS very-high-energy (VHE; E>100 GeV) observations of six blazars selected from the Fermi Large Area Telescope First Source Catalog (1FGL). The gamma-ray emission from 1FGL sources was extrapolated up to the VHE band, taking gamma-ray absorption by the extragalactic background light into account. This allowed the selection of six bright, hard-spectrum blazars that were good candidate TeV emitters. Spectroscopic redshift measurements were attempted with the Keck Telescope for the targets without Sloan Digital Sky Survey (SDSS) spectroscopic data. No VHE emission is detected during the observations of the six sources described here. Corresponding TeV upper limits are presented, along with contemporaneous Fermi observations and non-concurrent Swift UVOT and XRT data. The blazar broadband spectral energy distributions (SEDs) are assembled and modeled with a single-zone synchrotron self-Compton model. The SED built for each of the six blazars show a synchrotron peak bordering between the intermediate- and high-spectrum-peak classifications, with four of the six resulting in particle-dominated emission regions.

A VARIABLE MID-INFRARED SYNCHROTRON BREAK ASSOCIATED WITH THE COMPACT JET IN GX 339-4

Many X-ray binaries remain undetected in the mid-infrared, a regime where emission from their compact jets is likely to dominate. Here, we report the detection of the black hole binary GX 339-4 with the Wide-field Infrared Survey Explorer (WISE) during a very bright, hard accretion state in 2010. Combined with a rich contemporaneous multiwavelength dataset, clear spectral curvature is found in the infrared, associated with the peak flux density expected from the compact jet. An optically-thin slope of ~-0.7 and a jet radiative power of >6x10^{35} erg/s (d/8 kpc)^2 are measured. A ~24 h WISE light curve shows dramatic variations in mid-infrared spectral slope on timescales at least as short as the satellite orbital period ~95 mins. There is also significant change during one pair of observations spaced by only 11 s. These variations imply that the spectral break associated with the transition from self-absorbed to optically-thin jet synchrotron radiation must be varying across the full wavelength range of ~3-22 microns that WISE is sensitive to, and more. Based on four-band simultaneous mid-infrared detections, the break lies at ~5x10^{13} Hz in at least two epochs of observation, consistent with a magnetic field B~1.5x10^4 G assuming a single-zone synchrotron emission region. The observed variability implies that either B, or the size of the acceleration zone above the jet base, are being modulated by factors of ~10 on relatively-short timescales.

A synchrotron self-Compton scenario for the very high energy γ-ray emission of the intermediate BL Lacertae object W Comae

W Comae has significant variability in multi-wavelengthes, from radio to gamma-ray bands. A bright outburst in optical and X-ray bands was observed in 1998, and most recently, a strong TeV flare was detected by VERITAS in 2008. It is the first TeV intermediate-frequency-peaked BL Lacertae source. I find that both the broadband spectral energy distributions (SEDs) which were quasi-simultaneously obtained during the TeV flare and during the optical/X-ray outburst are well fit by using a single-zone synchrotron + synchrotron-self-Compton model. The satisfactory fitting requires a large beaming factor, i.e., δ ∼ 25 and δ ∼ 20 for the TeV flare and the optical/X-ray outburst, respectively, suggesting that both the optical/X-ray outburst and the TeV flare are from a relativistic jet. The size of the emission region of the TeV flare is three times larger than that of the optical/X-ray outburst, and the strength of the magnetic field for the TeV flare is ∼ 14 times smaller than that of the X-ray/optical outburst, likely indicating that the region of the TeV flare is more distant from the core than that of the X-ray/optical outburst. The inverse Compton component of the TeV flare peaks around 1.3 GeV, but it is around 20 MeV for the X-ray/optical outburst, lower than that for the TeV flare by two orders of magnitude. The model predicts that the optical/X-ray outburst might be accompanied by a strong MeV/GeV emission, but the TeV flare may be not associated with the X-ray/optical outburst. The GeV emission is critical for characterizing the SEDs of the optical/X-ray outburst and the TeV flare. The predicted GeV flux is above the sensitivity of Fermi/LAT, and it could be verified with the observations by Fermi/LAT in the near future.