Gamma-ray Emission Region Research Articles

Localization of the Gamma-Ray Emission Region in the 1 September 2014 Behind-the-Limb Solar Flare According to the Fermi/LAT Data

Localization of the Gamma-Ray Emission Region in the 1 September 2014 Behind-the-Limb Solar Flare According to the Fermi/LAT Data

Multiwavelength Analysis and the C iv λ1549 Å Emission Line Behavior From 2008 to 2020 of FSRQ B2 1633+382

Abstract The flat-spectrum radio quasar B2 1633+382 (4C 38.41) has been monitored for several years and has presented correlated variability in multiple wavelengths. In this article, we are performing different analyses for multiple frequencies, from gamma rays to radio, as well as the C iv λ1549 Å emission line and the λ1350 Å continuum. Using the nonthermal dominance parameter, we separated the C iv and the continuum light curves for when the dominant source of continuum is the accretion disk or the jet. We found a correlation at a delay consistent with zero between the line and the continuum dominated by disk emission indicating a very small broad-line region (BLR). From the resulting delay between the 15 GHz and gamma rays, we estimated the distance of the gamma-ray emission region from the jet apex to be ∼37 pc. The C iv flux decreases when the continuum and gamma rays increase at some of the high-activity periods. The C iv profile presents a larger variable component in its blue wing. The relation between the luminosities of C iv and the continuum does not completely follow the relation for a quasar sample. Our results lead us to propose an outflow of BLR material in the jet flow direction, a gamma-ray production through magnetic reconnection for the flaring event of mid-2011, and that there is not enough BLR material close to the radio core to be easily ionized by the nonthermal continuum.

Variability and Spectral Characteristics of Three Flaring Gamma-Ray Quasars Observed by VERITAS and Fermi-LAT

Abstract Flat-spectrum radio quasars (FSRQs) are the most luminous blazars at GeV energies but only rarely emit detectable fluxes of TeV gamma rays, typically during bright GeV flares. We explore the gamma-ray variability and spectral characteristics of three FSRQs that have been observed at GeV and TeV energies by Fermi-LAT and VERITAS, making use of almost 100 hr of VERITAS observations spread over 10 yr: 3C 279, PKS 1222+216, and Ton 599. We explain the GeV flux distributions of the sources in terms of a model derived from a stochastic differential equation describing fluctuations in the magnetic field in the accretion disk and estimate the timescales of magnetic flux accumulation and stochastic instabilities in their accretion disks. We identify distinct flares using a procedure based on Bayesian blocks and analyze their daily and subdaily variability and gamma-ray energy spectra. Using observations from VERITAS, as well as Fermi, Swift, and the Steward Observatory, we model the broadband spectral energy distributions of PKS 1222+216 and Ton 599 during very high energy (VHE)–detected flares in 2014 and 2017, respectively, strongly constraining the jet Doppler factors and gamma-ray emission region locations during these events. Finally, we place theoretical constraints on the potential production of PeV-scale neutrinos during these VHE flares.

A decade of joint MOJAVE–Fermi AGN monitoring: localization of the gamma-ray emission region

ABSTRACT Within the MOJAVE VLBA programme (Monitoring of Jets in AGN with VLBA Experiments), we have accumulated observational data at 15 GHz for hundreds of jets in gamma-ray bright active galactic nuclei since the beginning of the Fermi scientific observations in 2008 August. We investigated a time delay between the flux density of AGN parsec-scale radio emission at 15 GHz and 0.1–300 GeV Fermi LAT photon flux, taken from constructed light curves using weekly and adaptive binning. The correlation analysis shows that radio is lagging gamma-ray radiation by up to 8 months in the observer’s frame, while in the source frame, the typical delay is about 2–3 months. If the jet radio emission, excluding the opaque core, is considered, significant correlation is found at greater time lags. We supplement these results with VLBI kinematics and core shift data to conclude that the dominant high-energy production zone is typically located at a distance of several parsecs from the central nucleus. We also found that quasars have on average more significant correlation peak, more distant gamma-ray emission region from the central engine and shorter variability time-scale compared to those of BL Lacertae objects.

Explaining the extended GeV gamma-ray emission adjacent to HESS J1825-137

ABSTRACT HESS J1825-137 is one of the most powerful and luminous TeV gamma-ray pulsar wind nebulae (PWN). To the south of HESS J1825-137, Fermi-LAT observation revealed a new region of GeV gamma-ray emission with three apparent peaks (termed here, GeV-ABC). This study presents interstellar medium (ISM) data and spectral energy distribution (SED) modelling towards the GeV emission to understand the underlying particle acceleration. We considered several particle accelerator scenarios – the PWN associated with HESS J1825-137, the progenitor SNR also associated with HESS J1825-137, plus the gamma-ray binary system LS 5039. It was found that the progenitor SNR of HESS J1825-137 has insufficient energetics to account for all GeV emission. GeV-ABC may be a reflection of an earlier epoch in the history of the PWN associated with HESS 1825-137, assuming fast diffusion perhaps including advection. LS 5039 cannot meet the required energetics to be the source of particle acceleration. A combination of HESS J1825-137 and LS 5039 could be plausible sources.

Excluding possible sites of high-energy emission in 3C 84

ABSTRACT The FR-I galaxy 3C 84, that is identified with the misaligned blazar NGC 1275, is well known as one of the very few radio galaxies emitting gamma-rays in the TeV range. Yet, the gamma-ray emission region cannot be pinpointed and the responsible mechanisms are still unclear. We calculate the optical absorption depth of high-energy photons in the broad-line region of 3C 84 depending on their energy and distance to the central black hole. Based on these calculations, a lower limit on the distance of the emission region from the central black hole can be derived. These lower limits are estimated for two broad-line region geometries (shell and ring) and two states of the source, the low state in 2016 October–December and a flare state in 2017 January. For the shell geometry, we can place the emission region outside the Ly α radius. For the ring geometry and the low flux activity, the minimal distance between the black hole, and the gamma-ray emission region is close to the Ly α radius. In the case of the flaring state (ring geometry), the results are not conclusive. Our results exclude the region near the central black hole as the origin of the gamma-rays detected by Fermi–LAT and Major Atmospheric Gamma-Ray Imaging Cherenkov. With these findings, we can constrain the theoretical models of acceleration mechanisms and compare the possible emission region to the source’s morphology resolved by radio images from the Very Long Baseline Array.

Monitoring of the radio galaxy M 87 during a low-emission state from 2012 to 2015 with MAGIC

ABSTRACT M 87 is one of the closest (z = 0.004 36) extragalactic sources emitting at very high energies (VHE, E > 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.

Resolving the Crab pulsar wind nebula at teraelectronvolt energies

The Crab nebula is one of the most studied cosmic particle accelerators, shining brightly across the entire electromagnetic spectrum up to very high-energy gamma rays. It is known from radio to gamma-ray observations that the nebula is powered by a pulsar, which converts most of its rotational energy losses into a highly relativistic outflow. This outflow powers a pulsar wind nebula (PWN), a region of up to 10~light-years across, filled with relativistic electrons and positrons. These particles emit synchrotron photons in the ambient magnetic field and produce very high-energy gamma rays by Compton up-scattering of ambient low-energy photons. While the synchrotron morphology of the nebula is well established, it was up to now not known in which region the very high-energy gamma rays are emitted. Here we report that the Crab nebula has an angular extension at gamma-ray energies of 52 arcseconds (assuming a Gaussian source width), significantly larger than at X-ray energies. This result closes a gap in the multi-wavelength coverage of the nebula, revealing the emission region of the highest energy gamma rays. These gamma rays are a new probe of a previously inaccessible electron and positron energy range. We find that simulations of the electromagnetic emission reproduce our new measurement, providing a non-trivial test of our understanding of particle acceleration in the Crab nebula.

Revealing a new region of gamma-ray emission in the vicinity of HESS J1825–137

HESS J1825-137 is a bright very-high-energy (VHE) gamma-ray source that has been firmly established as a pulsar wind nebula (PWN), and one of the most extended gamma-ray objects within this category. The progenitor supernova remnant (SNR) for this PWN has not been firmly established. We carried out an analysis of gamma-ray observations in the region of HESS J1825-137 with the Fermi-LAT which reveal emission in the direction away from the Galactic plane. The region lies beyond the PWN and reaches a distance from the pulsar compatible with the supposed location of the SNR shock front. The spectrum of the gamma-rays is hard with a photon index of $\sim 1.9$ in the 10-250 GeV range. Several scenarios for the origin of the emission are discussed, including the SNR as a source of high-energy particles and the "leakage"' of leptons from the PWN.

A Method for Locating a High-energy Dissipation Region in a Blazar

Abstract The production site of gamma-rays in a blazar jet is an unresolved problem. We present a method to locate a gamma-ray emission region in the framework of a one-zone emission model. From measurements of the core-shift effect, the relation between the magnetic field strengths (B′) in the radio cores of the jet and the distances (R) of these radio cores from the central supermassive black hole (SMBH) can be inferred. Therefore, once the magnetic field strength in the gamma-ray emission region ( ) is obtained, one can use the relation of B′–R to derive the distance (R diss) of the gamma-ray emission region from the SMBH. Here, we evaluate the lower limit of by using the criteria that the optical variability timescale t var should be longer than or equal to the synchrotron radiation cooling timescale of the electrons that emit optical photons. We test the method with the observations of PSK 1510-089 and BL Lacertae, and derive pc for PSK 1510-089 with t var ∼ a few hours and pc for BL Lacertae with t var ∼ a few minutes. Here, δ D is the Doppler factor and A is the Compton dominance (i.e., the ratio of the Compton to the synchrotron peak luminosities).

Characterization of the energy spectrum of a 137Cs irradiator through measurements using a pulse-mode detector

137Cs irradiators are used for many different applications including food processing, radiobiological experiments, and calibrations of radiation detectors at standards labs. Air-kerma modulation techniques including varying source size, adding attenuation, or varying source-to-detector distance, combined with the geometry of the collimation in the irradiator, may change the energy spectrum of the irradiator. This is unaccounted for during beam characterization, which is completed using a spherical ionization chamber with a minimal energy dependence in the 662 keV 137Cs gamma-ray emission region. This work incorporated a high-purity germanium (HPGe) detector collimated using a lead aperture to measure the energy spectrum of a 5 Ci source in a Hopewell Designs G-10 dual source irradiator with 4.08 cm and 5.54 cm of attenuating material. The measured pulse-height distributions (PHDs) were compared with simulations in MCNP6 to provide validation of the geometry of the irradiator model. PHD measurements were corrected for detector response using MCNP and the true spectrum was obtained using Gold's deconvolution method. Due to the fluence limitations and collimation of the HPGe detector, measurements represented the primary photon energy spectrum. The impact of scattered photons on the energy spectrum was investigated using MCNP6 simulations. In addition, simulations were run to determine the effects of attenuation and source-to-detector distance (SDD) on the energy spectrum. Comparisons of spectra were made in terms of mean energy. Simulated and measured PHDs showed good agreement when comparing spectral features including FWHM of the 662 keV photopeak, mean energy, and the total area under the curve. After deconvolution of the PHDs, the primary energy spectrum mimicked an encapsulated 137Cs source with emission of only the 662 keV photopeak. For 5.54 cm of attenuation, simulations showed that if scattered photons were taken into account, the average energy decreased from 662 keV to 656 keV, which was a 2% difference. Simulations showed that as attenuation increased from 0 cm to 5.54 cm, the mean energy of the spectrum increased 9% from 605 keV to 656 keV. The SDD did not affect the mean energy. This work used spectrum measurements to validate a 137Cs irradiator model in MCNP6, which represents a method that is more sensitive to geometry errors than conventional PDD and profile comparisons. Using the validated model, simulations were run to improve the understanding of the output of the Hopewell G-10 137Cs irradiator.

Long-term Study of the Light Curve of PKS 1510-089 in GeV Energies

Abstract We have analyzed data from the flat-spectrum radio quasar PKS 1510-089 collected over a period of eight years from 2008 August to 2016 December with the Fermi-LAT. We have identified several flares of this highly variable source, studied their temporal and spectral properties in detail, and compared with previous works on flares of PKS 1510-089. Five major flares and a few subflares or substructures have been identified in our study. The fastest variability time is found to be 1.30 ± 0.18 hr between MJD 55852.063 and 55852.188, where we estimate the minimum size of the emission region to be 4.85 × 1015 cm. In most of the flares, the spectral energy distributions are better fitted with a log-parabolic distribution compared to a simple power law or a power law with exponential cutoffs. This has strong physics implications regarding the nature of the high-energy gamma-ray emission region.

Constraints on the Location of the Gamma-ray Emission Region for the Gamma-ray-loud Radio Source GB 1310+487

Abstract We employ a single-zone leptonic jet model, with synchrotron, synchrotron self-Compton, and external Compton (EC) processes, to reproduce the quasi-simultaneous multi-wavelength spectral energy distributions in active and quiescent states of the narrow-line gamma-ray-loud radio source GB 1310+487. In the case of the EC process, the external seed photons from both the broad line region (BLR) and the dust torus are considered by assuming that the gamma-ray emission region is located at the outside boundary of the BLR and inside the dust torus. Comparing the energy density of external photon fields obtained by model fitting with that constrained from the BLR observations. We find that the location of the gamma-ray-emitting region of GB 1310+487 can be tightly constrained at the outer edge of the BLR (the dissipation distance of the γ-ray emission region from central black hole ). The ratio of magnetic energy and emitting-electron energy in the radiation blob ( ) is gradually increased from Flare 1, to Flare 2, to Post-flare, where the magnetic energy increases and the matter energy decreases. These results suggest that the conversion of the magnetic field and the matter (radiation electrons) energy and the location of the γ-ray emission region (or ambient photon field) may play an important role in different radiation states of GB 1310+487.

Cosmic Ray Astrophysics using The High Altitude Water Cherenkov (HAWC) Observatory in México

The High-Altitude Water Cherenkov (HAWC) TeV gamma–ray Observatory in Mexico is ready to search and study gamma-ray emission regions, extremely high-energy cosmic-ray sources, and to identify transient phenomena. With a better Gamma/Hadron rejection method than other similar experiments, it will play a key role in triggering multi–wavelength and multi–messenger studies of active galaxies (AGN), gamma-ray bursts (GRB), supernova remnants (SNR), pulsar wind nebulae (PWN), Galactic Plane Sources, and Cosmic Ray Anisotropies. It has an instantaneous field-of-view of ∼2 str, equivalent to 15% of the whole sky and continuous operation (24 hours per day). The results obtained by HAWC–111 (111 detectors in operation) were presented on the proceedings of the International Cosmic Ray Conference 2015 and in [1]. The results obtained by HAWC–300 (full operation) are now under analysis and will be published in forthcoming papers starting in 2017 (see preliminary results on http://www.hawc-observatory.org/news/). Here we present the HAWC contributions on cosmic ray astrophysics via anisotropies studies, summarizing the HAWC detector and its upgrading by the installation of “outriggers”.

Exceptional outburst of the blazar CTA 102 in 2012: the GASP–WEBT campaign and its extension

After several years of quiescence, the blazar CTA 102 underwent an exceptional outburst in 2012 September-October. The flare was tracked from gamma-ray to near-infrared frequencies, including Fermi and Swift data as well as photometric and polarimetric data from several observatories. An intensive GASP-WEBT collaboration campaign in optical and NIR bands, with an addition of previously unpublished archival data and extension through fall 2015, allows comparison of this outburst with the previous activity period of this blazar in 2004-2005. We find remarkable similarity between the optical and gamma-ray behaviour of CTA 102 during the outburst, with a time lag between the two light curves of ~1 hour, indicative of co-spatiality of the optical and gamma-ray emission regions. The relation between the gamma-ray and optical fluxes is consistent with the SSC mechanism, with a quadratic dependence of the SSC gamma-ray flux on the synchrotron optical flux evident in the post-outburst stage. However, the gamma-ray/optical relationship is linear during the outburst; we attribute this to changes in the Doppler factor. A strong harder-when-brighter spectral dependence is seen both the in gamma-ray and optical non-thermal emission. This hardening can be explained by convexity of the UV-NIR spectrum that moves to higher frequencies owing to an increased Doppler shift as the viewing angle decreases during the outburst stage. The overall pattern of Stokes parameter variations agrees with a model of a radiating blob or shock wave that moves along a helical path down the jet.

Exploring the nature of the broadband variability in the flat spectrum radio quasar 3C 273

Detailed investigation of broadband flux variability in the blazar 3C 273 allows us to probe the location and size of emission regions and their physical conditions. We report the results on correlation studies of the flaring activity observed between 2008 and 2012. The observed broadband variations were investigated using the structure function and the discrete correlation function, and power spectral density analysis (PSD) methods. The PSD analysis showed that the optical/IR light curve slopes are consistent with the slope of white noise processes, while, the PSD slopes at radio, X-ray and gamma-ray energies are consistent with red-noise processes. The flux variations at gamma-ray and mm-radio bands are found to be significantly correlated. Using the estimated time lag of (110\pm27) days between gamma-ray and radio light curves, we constrained the location of the gamma-ray emission region at a de-projected distance of 1.2\pm0.9 pc from the jet apex. Flux variations at X-ray bands were found to have a significant correlation with variations at both radio and \gamma-rays energies. The correlation between X-rays and gamma-rays light curves suggests presence of two components responsible for the X-ray emission. A negative time lag of -(50\pm20) days, where the X-rays are leading the emission, suggests X-rays are emitted closer to the jet apex from a compact region at a distance of ~(0.5\pm0.4) pc from the jet apex. A positive time lag of (110\pm20) days suggests jet-base origin of the other X-ray component at ~(4--5)~pc from the jet apex. The flux variations at radio frequencies were found to be well correlated with each other such that the variations at higher frequencies are leading the lower frequencies, which could be expected in the standard shock-in-jet model.

What can the 2008/10 broadband flare of PKS 1502+106 tell us?

Context. The origin of blazar variability, seen from radio up to gamma rays, is still a heavily debated matter and broadband flares offer a unique testbed towards a better understanding of these extreme objects. Such an energetic outburst was detected by Fermi/LAT in 2008 from the blazar PKS 1502+106. The outburst was observed from gamma rays down to radio frequencies. Aims. Through the delay between flare maxima at different radio frequencies, we study the frequency-dependent position of the unit-opacity surface and infer its absolute position with respect to the jet base. This nuclear opacity profile enables the magnetic field tomography of the jet. We also localize the gamma-ray emission region and explore the mechanism producing the flare. Methods. The radio flare of PKS 1502+106 is studied through single-dish flux density measurements at 12 frequencies in the range 2.64 to 226.5 GHz. To quantify it, we employ both a Gaussian process regression and a discrete cross-correlation function analysis. Results. We find that the light curve parameters (flare amplitude and cross-band delays) show a power-law dependence on frequency. Delays decrease with frequency, and the flare amplitudes increase up to about 43 GHz and then decay. This behavior is consistent with a shock propagating downstream the jet. The self-absorbed radio cores are located between about 10 and 4 pc from the jet base and their magnetic field strengths range between 14 and 176 mG, at the frequencies 2.64 to 86.24 GHz. Finally, the gamma-ray active region is located at (1.9 +/- 1.1) pc away from the jet base.

Young and middle age pulsar light-curve morphology: Comparison ofFermiobservations withγ-ray and radio emission geometries

Thanks to the huge amount of gamma-ray pulsar photons collected by the Fermi Large Area Telescope since June 2008, it is now possible to constrain gamma-ray geometrical models by comparing simulated and observed light-curve morphological characteristics. We assumed vacuum-retarded dipole pulsar magnetic field and tested simulated and observed morphological light-curve characteristics in the framework of two pole emission geometries, Polar Cap (PC), radio, and Slot Gap (SG), and Outer Gap (OG)/One Pole Caustic (OPC) emission geometries. We compared simulated and observed/estimated light-curve morphological parameters as a function of observable and non-observable pulsar parameters. The PC model gives the poorest description of the LAT pulsar light-curve morphology. The OPC best explains both the observed gamma-ray peak multiplicity and shape classes. The OPC and SG models describe the observed gamma-ray peak-separation distribution for low- and high-peak separations, respectively. This suggests that the OPC geometry best explains the single-peak structure but does not manage to describe the widely separated peaks predicted in the framework of the SG model as the emission from the two magnetic hemispheres. The OPC radio-lag distribution shows higher agreement with observations suggesting that assuming polar radio emission, the gamma-ray emission regions are likely to be located in the outer magnetosphere. The larger agreement between simulated and LAT estimations in the framework of the OPC suggests that the OPC model best predicts the observed variety of profile shapes. The larger agreement between observations and the OPC model jointly with the need to explain the abundant 0.5 separated peaks with two-pole emission geometries, calls for thin OPC gaps to explain the single-peak geometry but highlights the need of two-pole caustic emission geometry to explain widely separated peaks.

Recent Highlights from VERITAS

VERITAS is a ground-based gamma-ray observatory consisting of an array of four atmospheric Cherenkov telescopes located in southern Arizona, USA. VERITAS carries out an extensive observation program of the gamma-ray sky at energies above 0.1 TeV. Observations of Galactic and extragalactic sources in the TeV band are sensitive probes of the highly energetic processes occurring in these objects. Observations by VERITAS of the Galactic center and nearby dwarf spheroidal galaxies provide constraints on particle dark matter with masses above a few hundred GeV. VERITAS observations also provide constraints on fundamental physics and cosmology, such as probing the history of galaxy formation and studying Lorentz invariance violation (LIV). The majority of the sources detected by VERITAS are active galactic nuclei (AGN), with gamma-ray emission originating in their relativistic jets. TeV observations of AGN help us constrain models of particle acceleration and energy dissipation in relativistic jets, and the size and location of the gamma-ray emission region. Galactic sources at TeV energies include supernova remnants, pulsar wind nebulae, and binary systems, and TeV emission is a key diagnostic of the highly energetic particles in these objects. VERITAS observations provide important clues on the origin of cosmic rays and on particle acceleration in supernova blast shocks and relativistic pulsar wind-termination shocks. In this article I will present some highlights of particle-astrophysics measurements made with VERITAS.

Microlensingconstraintson the size of the gamma-ray emission region in blazar B0218+357

Context. Observations of the effect of microlensing in gravitationally lensed quasars could potentially be used to study the structure of the source on distance scales down to the size of the supermassive black hole powering the quasar activity. Aims. We search for the microlensing effect in the gamma-ray band using the signal from a gravitationally lensed blazar B0218+357. Methonds. We develop a method of deconvolution of contributions of two images of the source into the gamma-ray band flaring lightcurve. We use this method to study the evolution of the magnification factor ratio between the two images throughout the flaring episodes. We interpret the time variability of the ratio as a signature of the microlensing effect and derive constraints on the physical parameters of the gamma-ray source by comparing the observed variability properties of the magnification factor ratio with those derived from numerical simulations of the microlensing caustics networks. Results. We find that the magnification factor ratio has experienced a change characteristic for a microlensing caustic crossing event during a 100 d flaring period in 2012. It has further changed between 2012 and a recent flaring episode in 2014. We use the measurement of the maximal magnification and duration of the caustic crossing event to derive an estimate of the projected size of the gamma-ray emission region in B0218+357, $R_\gamma \sim 10^{14}$ cm. This estimate is compatible with a complementary estimate found from the minimal variability time scale. The microlensing / minimal variability time scale measurements of the source size suggest that the gamma-ray emission is produced at the base of the blazar jet, in the direct vicinity of the central supermassive black hole.