Γ-ray Band Research Articles

Revealing the Variation Mechanism of ON 231 via the Two-component Shock-in-jet Model

The variation mechanism of blazars is a long-standing unresolved problem. In this work, we present a scenario to explain diverse variation phenomena for ON 231, where the jet emissions are composed of the flaring and the less variable components (most probably from the post-flaring blobs), and the variation is dominated by shock-in-jet instead of the Doppler effect. We perform correlation analysis for the multiwavelength light curves and find no significant correlations. For the optical band, ON 231 exhibits a harder when brighter (HWB) trend, and the trend seems to shift at different periods. Correspondingly, the correlation between the degree of polarization and flux exhibits a V-shaped behavior, and a similar translation relation during different periods is also found. These phenomena could be understood via the superposition of the flaring component and slowly varying background component. We also find that the slopes of the HWB trend become smaller at higher flux levels, which indicates the energy-dependent acceleration processes of the radiative particles. For the X-ray band, we discover a trend transition from HWB to softer when brighter (SWB) to HWB. We consider that the X-ray emission is composed of both the synchrotron tail and the synchrotron self-Compton components, which could be described by two log-parabolic functions. By varying the peak frequency, we reproduce the observed trend transition in a quantitative manner. For the γ-ray band, we find the SWB trend, which could be explained naturally if a very-high-energy γ-ray background component exists. Our study elucidates the variation mechanism of intermediate synchrotron-peaked BL Lac objects.

Constraining the PG 1553+113 Binary Hypothesis: Interpreting Hints of a New, 22 yr Period

PG 1553+113 is a well-known blazar exhibiting evidence of a ∼2.2 yr quasiperiodic oscillation (QPO) in the radio, optical, X-ray, and γ-ray bands. Since QPO mechanisms often predict multiple QPOs, we search for a second QPO in its historical optical light curve covering a century of observations. Despite challenging data quality issues, we find hints of a 21.8 ± 4.7 yr oscillation. On its own, this ∼22 yr period has a modest statistical significance of 1.6σ when accounting for the look-elsewhere effect. However, the joint significance of both the 2.2 and 22 yr periods arising from colored noise alone is ∼3.6σ. The next peak of the 22 yr oscillation is predicted to occur around July 2025. We find that such a ∼10:1 relation between two periods can arise in the gas dynamics of a plausible supermassive black hole binary model of PG 1553+113. While the 22 yr QPO is preliminary, an interpretation of PG 1553+113's two QPOs in this binary model suggests that the binary engine has a mass ratio ≳0.2, an eccentricity ≲0.1, and accretes from a disk with characteristic aspect ratio ∼0.03. The putative binary radiates nHz gravitational waves, but the amplitude is ∼10–100 times too low for detection by foreseeable pulsar timing arrays.

Morphology Study for GeV Emission of Nearby Supernova Remnant G332.5-5.6

A spatial template is important to study nearby supernova remnants (SNRs). For SNR G332.5-5.6, we report a Gaussian disk with a radius of about 1.°06 to be a potential good spatial model in the γ-ray band. Employing this new Gaussian disk, its GeV lightcurve shows a significant variability of about seven sigma. The γ-ray observations of this SNR could be explained well either by a leptonic model or a hadronic model, in which a flat spectrum for the ejected electrons/protons is required.

Central Engine and Spectral Energy Distribution Properties of High Redshift Gamma Ray Blazars

We report on the properties of central engines in the γ-ray blazars located at high redshifts beyond z > 0.4, where the extra-galactic background light (EBL) starts affecting their γ-ray spectra. The physical engine that provides power to the blazars of very high bolometric luminosity is assumed to be a highly collimated jet of matter moving relativistically away from the supermassive black hole (SMBH), located in the central region of the host galaxy, in a direction aligned toward the Earth. Due to their peculiar geometry and special physical conditions, blazars at redshifts beyond z > 0.4 are bright enough to be detected in the γ-ray energy band. In this work, we investigate the physical properties of high-z γ-ray blazars detected by the Large Area Telescope (LAT) on board the Fermi satellite. We also study the properties of their emission regions and the central engines and discuss cosmological and astrophysical implications.

Galactic Diffuse Emission from Radio to Ultra-high-energy γ-Rays in Light of Up-to-date Cosmic-Ray Measurements

Cosmic rays (CRs) travel throughout the Galaxy, leaving traces from radio to ultra-high-energy γ-rays due to interactions with the interstellar gas, radiation field, and magnetic field. Therefore, it is necessary to utilize multiwavelength investigations on the Galactic diffuse emission to shed light on the physics of CR production and propagation. In this work, we present a spatially dependent propagation scenario, taking account of a local source contribution, while making allowances for an additional CR component freshly accelerated near their sources. In this picture, after reproducing the particle measurements at the solar system, we calculated the intensity and compared the spectral energy distribution to observations from Fermi-LAT and LHAASO-KM2A in the γ-ray band, and from WMAP and Planck among other radio surveys at lower energies. Multiband data considered in conjunction, the former comparison exhibits sufficiently good consistency in favor of our model, while the latter calls for improvement in data subtraction and processing. From this standpoint, there remains potential for advanced observations at energies from milli-eVs to MeVs toward the Galactic plane, in order to evaluate our model further and more comprehensively in the future.

The invisible black widow PSR J1720-0534: implications for the electron density towards the North Polar Spur

Abstract Radio emission from pulsars can be used to map out their distances through dispersion measure (DM), which quantifies the amount of radio pulse dispersion. However, this method relies on accurately modeling the free electron density in the line of sight. Here, we present a detailed study of the multi-wavelength emission from PSR J1720-0534, a black widow compact binary millisecond pulsar discovered in 2021, which the latest electron density model of the Galaxy (YMW16; Yao et al. 2017) places at only 191 pc. We obtained and analyzed deep multi-wavelength observations in the γ-ray (Fermi-LAT, 2008-2022), optical (LCO, 2.7-h), near-infrared (NOT, 3.5-h) and X-ray (Swift-XRT, 10 ksec) bands. We found no significant detection of γ-ray, optical, near-infrared, or X-ray counterparts around the radio-timing position of PSR J1720-0534, which we thus nickname ‘the invisible black widow’. Employing the most constraining near-infrared limit (J > 23.4 mag), we established a lower limit on the source distance, d > 1.1 kpc, assuming conservative properties for the black widow companion star. This distance lower limit differs drastically (by a factor of more than five) from the YMW16 DM distance estimate. We attribute this difference to the inclusion in the YMW16 model of a large and dense component towards the North Polar Spur. Considering our results and recent parallax distances to other pulsars in this direction, we argue that such a local and large component in the electron density model of the Galaxy is unnecessary.

Broad-band study of gamma-ray blazars at redshifts z = 2.0–2.5

ABSTRACT High redshift blazars are among the most powerful non-explosive sources in the Universe and play a crucial role in understanding the evolution of relativistic jets. To understand these bright objects, we performed a detailed investigation of the multiwavelength properties of 79 γ-ray blazars with redshifts ranging from z = 2.0 to 2.5, using data from Fermi LAT, Swift XRT/UVOT, and NuSTAR observations. In the γ-ray band, the spectral analysis revealed a wide range of flux and photon indices, from 5.32 × 10−10 to 3.40 × 10−7 photon cm−2 s−1 and from 1.66 to 3.15, respectively, highlighting the diverse nature of these sources. The detailed temporal analysis showed that flaring activities were observed in 31 sources. Sources such as 4C+71.07, PKS 1329-049, and 4C + 01.02, demonstrated significant increase in the γ-ray luminosity and flux variations, reaching peak luminosity exceeding 1050 erg s−1. The temporal analysis extended to X-ray and optical/ultraviolet (UV) bands, showed clear flux changes in some sources in different observations. The time-averaged properties of high redshift blazars were derived through modeling the spectral energy distributions with a one-zone leptonic scenario, assuming the emission region is within the broad-line region (BLR) and the X-ray and γ-ray emissions are due to inverse Compton scattering of synchrotron and BLR-reflected photons. This modeling allowed us to constrain the emitting particle distribution, estimate the magnetic field inside the jet, and evaluate the jet luminosity, which is discussed in comparison with the disc luminosity derived from fitting the excess in the UV band.

Revisiting the quasi-periodic oscillations in blazar PG 1553＋113 with multi-wavebands data

The blazar PG 1553＋113 is hypothesized to harbor a supermassive black hole binary system, a scenario that aligns with its observed physical characteristics. In this study, we re-examine the authenticity of the periodicity of PG 1553＋113 by conducting a comprehensive analysis of multi-wavebands periodic light variations, using the updated light curve data of more than 15 years. We used two methods to search for the light curves data of this blazar in γ-ray, X-ray, optical and radio bands. The multi-wavebands analysis approach enables a thorough verification of the observed periodic patterns. The result of γ-ray detection showed a quasi-periodic oscillation (QPO) of 2.16 years, which verified the results given by Ackermann et al. (2015). And the optical band shows a QPO of 2.24 years. We analyzed the correlation among γ-ray, optical and radio bands, and we found that there is a strong correlation among them, and the emission of different bands coming from the same region (the same electron group). Finally, we estimated the black hole mass of PG 1553＋113 to be M≃4.3×109M⨀ based on the binary black hole model.

A high-energy study of the supernova remnant G296.5+10.0

ABSTRACT We report a detailed multi-wavelength study of the supernova remnant G296.5+10.0 using archival data from XMM–Newton and Fermi-LAT complemented with ATCA observations. In the X-ray band, we performed an adaptive smoothing on the double background subtracted images to construct an X-ray mosaic map with six individual observations. Below 2.0 keV, G296.5+10.0 is asymmetrical, with the south-east side of the radio shell brighter than the south-west one. The spatially resolved X-ray spectral study confirms the thermal origin of the plasma, with enhanced metal abundances, probably arising from ejecta material according to the H i and infrared ($140~{\mu \rm m}$) distributions. In the γ-ray band, we analysed 14 years of accumulated Fermi observations below 500 GeV via different fitting processes. To discuss the origin of the γ-ray emission, we compare the GeV results with H i structures probably associated with the SNR and with the radio spectral indices found at various positions towards the radio shell. Moreover, we identified diverse sources candidates to contribute γ-ray emissions observed. Also, we calculated the lepto-hadronic spectral energy distribution of the remnant for synchrotron, inverse Compton, Bremsstrahlung, and proton–proton processes. The emission at low energies can be explained by electron-synchrotron radiation, with a weak magnetic field of $B=25\, {\rm \mu G}$, while the γ-ray data can be explained by hadronic interactions. Employing the reddening-distance method, we computed a distance of 1.4 kpc for the SNR, implying an age of 14 000 yr.

Can FSRQ 3C 345 be a very high energy blazar candidate?

ABSTRACT The recent detection of very high energy (VHE) emissions from flat spectrum radio quasars (FSRQs) at high redshifts has revealed that the universe is more transparent to VHE γ-rays than it was expected. It has also questioned the plausible VHE emission mechanism responsible for these objects. Particularly for FSRQs, the γ-ray emission is attributed to the external Compton (EC) process. We perform a detailed spectral study of Fermi-detected FSRQ 3C 345 using synchrotron, synchrotron self-Compton, and EC emission mechanisms. The simultaneous data available in optical, ultraviolet, X-ray, and γ-ray energy bands is statistically fitted under these emission mechanisms using the χ2-minimization technique. Three high flux states and one low flux state are chosen for spectral fitting. The broad-band spectral energy distribution during these flux states is fitted under different target photon temperatures, and the model VHE flux is compared with the 50 h Cherenkov Telescope Array sensitivity. Our results indicate a significant VHE emission could be attained during the high flux state from MJD 59635−59715 when the target photon temperature is within 900–1200 K. Furthermore, our study shows a clear trend of variation in the bulk Lorentz factor of the emission region as the source transits through different flux states. We also note that during high γ-ray flux states, an increase in external photon temperature demands high bulk Lorentz factors, while this behaviour reverses in case of low γ-ray flux state.

Hunting Gamma-Ray-emitting FR0 Radio Galaxies in Wide-field Sky Surveys

Abstract The latest entry in the jetted active galactic nuclei (AGN) family is the Fanaroff–Riley type 0 (FR0) radio galaxies. They share several observational characteristics, e.g., nuclear emission and host galaxy morphology, with FR I sources; however, they lack extended, kiloparsec-scale radio structures, which are the defining features of canonical FR I and II sources. Here we report the identification of seven γ-ray-emitting AGN as FR0 radio sources by utilizing the high-quality observations delivered by ongoing multiwavelength wide-field sky surveys, e.g., Very Large Array Sky Survey. The broadband observational properties of these objects are found to be similar to their γ-ray undetected counterparts. In the γ-ray band, FR0 radio galaxies exhibit spectral features similar to more common FR I and II radio galaxies, indicating a common γ-ray production mechanism and the presence of misaligned jets. Although the parsec-scale radio structure of FR0s generally exhibits a wide range, with about half having emission on opposite sides of the core, the γ-ray-detected FR0s tend to have dominant cores with core-jet structures. We conclude that dedicated, high-resolution observations are needed to unravel the origin of relativistic jets in this enigmatic class of faint yet numerous population of compact radio sources.

Extreme photometric and polarimetric variability of blazar S4 0954+65 at its maximum optical and γ-ray brightness levels

ABSTRACT In 2022 the BL Lac object S4 0954+65 underwent a major variability phase, reaching its historical maximum brightness in the optical and γ-ray bands. We present optical photometric and polarimetric data acquired by the Whole Earth Blazar Telescope (WEBT) Collaboration from 2022 April 6 to July 6. Many episodes of unprecedented fast variability were detected, implying an upper limit to the size of the emitting region as low as $10^{-4}$ parsec. The WEBT data show rapid variability in both the degree and angle of polarization. We analyse different models to explain the polarization behaviour in the framework of a twisting jet model, which assumes that the long-term trend of the flux is produced by variations in the emitting region viewing angle. All the models can reproduce the average trend of the polarization degree, and can account for its general anticorrelation with the flux, but the dispersion of the data requires the presence of intrinsic mechanisms, such as turbulence, shocks, or magnetic reconnection. The WEBT optical data are compared to γ-ray data from the Fermi satellite. These are analysed with both fixed and adaptive binning procedures. We show that the strong correlation between optical and γ-ray data without measurable delay assumes different slopes in faint and high brightness states, and this is compatible with a scenario where in faint states we mainly see the imprint of the geometrical effects, while in bright states the synchrotron self-Compton process dominates.

Electromagnetic Cascade Emission from Neutrino-coincident Tidal Disruption Events

The potential association between Tidal Disruption Events and high-energy astrophysical neutrinos implies the acceleration of cosmic rays. These accelerated particles will initiate electromagnetic (EM) cascades spanning from keV to GeV energies via the processes related to neutrino production. We model the EM cascade and neutrino emissions by numerically solving the time-dependent transport equations, and discuss the implications for AT2019dsg and AT2019fdr in the X-ray and γ-ray bands. We show that the γ-ray constraints from Fermi can constrain the size of the radiation zone and the maximum energy of injected protons, and that the corresponding expected neutrino event numbers in follow-up searches are limited to be less than about 0.1. Depending on the efficiency of p γ interactions and the time at which the target photons peak, the X-ray and γ-ray signals can be expected closer to the peak of the optical-ultraviolet luminosity or to the time of the neutrino production.

The spectra of IceCube Neutrino (SIN) candidate sources – IV. Spectral energy distributions and multiwavelength variability

ABSTRACT We present hybrid spectral energy distributions, combining photon, and neutrino fluxes, for a sample of blazars, which are candidate IceCube neutrino sources. We furthermore check for differences in our sources’ variability in the near-infrared, optical, X-ray, and γ-ray bands compared to a sample of non-neutrino source candidate blazars, and investigate the state of each blazar at the arrival time of high-energy neutrinos. We find no significant differences when comparing our sample with control sources, also in terms of their spectral energy distributions, and no correlation between flaring states and neutrino arrival times. Looking for signatures of hadronic production, we check for similar strengths of the γ-ray and neutrino fluxes and find a $2.2\, \sigma$ signal for our source candidates. The hybrid spectral energy distributions assembled here will form the basis of the next step of our project, namely lepto-hadronic modelling of these blazars to assess the physical likelihood of a neutrino connection.

Analysis of γ-ray emission above 30 GeV from the LMC pulsar wind nebula N 157B with Fermi-LAT

ABSTRACT N 157B located in the Large Magellanic Cloud is the first pulsar wind nebula detected outside of the Galaxy in γ-rays. In this paper, we analyse the emission above 30 GeV from N 157B using ∼14.3 yr of Fermi-LAT data. The γ-ray spectrum between 30 and 500 GeV is well described by a single power-law function with a photon index of 1.83 ± 0.26, and its integral photon flux is (4.10 ± 0.83) × 10−11 photons cm−2 s−1. We adopt a one-zone leptonic model to investigate whether the multiband non-thermal emission of the target source can be generated by synchrotron radiation and inverse-Compton scattering of the electrons/positrons. Assuming the electrons/positrons in the nebula have a broken power-law spectrum with two breaks, the model can reproduce the observed fluxes in the radio, X-ray, and γ-ray bands. This result indicates that the γ-ray emission from N 157B can be explained by the leptonic process of electrons/positrons via inverse-Compton scattering.

Multiwavelength Observations of the Blazar PKS 0735+178 in Spatial and Temporal Coincidence with an Astrophysical Neutrino Candidate IceCube-211208A

Abstract We report on multiwavelength target-of-opportunity observations of the blazar PKS 0735+178, located 2.°2 away from the best-fit position of the IceCube neutrino event IceCube-211208A detected on 2021 December 8. The source was in a high-flux state in the optical, ultraviolet, X-ray, and GeV γ-ray bands around the time of the neutrino event, exhibiting daily variability in the soft X-ray flux. The X-ray data from Swift-XRT and NuSTAR characterize the transition between the low-energy and high-energy components of the broadband spectral energy distribution (SED), and the γ-ray data from Fermi-LAT, VERITAS, and H.E.S.S. require a spectral cutoff near 100 GeV. Both the X-ray and γ-ray measurements provide strong constraints on the leptonic and hadronic models. We analytically explore a synchrotron self-Compton model, an external Compton model, and a lepto-hadronic model. Models that are entirely based on internal photon fields face serious difficulties in matching the observed SED. The existence of an external photon field in the source would instead explain the observed γ-ray spectral cutoff in both the leptonic and lepto-hadronic models and allow a proton jet power that marginally agrees with the Eddington limit in the lepto-hadronic model. We show a numerical lepto-hadronic model with external target photons that reproduces the observed SED and is reasonably consistent with the neutrino event despite requiring a high jet power.

The Vanishing of the Primary Emission Region in PKS 1510–089

In 2021 July, PKS 1510−089 exhibited a significant flux drop in the high-energy γ-ray (by a factor 10) and optical (by a factor 5) bands and remained in this low state throughout 2022. Similarly, the optical polarization in the source vanished, resulting in the optical spectrum being fully explained through the steady flux of the accretion disk and the broad-line region. Unlike the aforementioned bands, the very-high-energy γ-ray and X-ray fluxes did not exhibit a significant flux drop from year to year. This suggests that the steady-state very-high-energy γ-ray and X-ray fluxes originate from a different emission region than the vanished parts of the high-energy γ-ray and optical jet fluxes. The latter component has disappeared through either a swing of the jet away from the line of sight or a significant drop in the photon production efficiency of the jet close to the black hole. Either change could become visible in high-resolution radio images.

Multiwavelength study of TeV blazar 1ES 1218+304 using gamma-ray, X-ray and optical observations

ABSTRACT We report on a multiwavelength study of the high-synchrotron-peaked BL Lac 1ES 1218+304 using near-simultaneous data obtained during the period from 2018 January 1 to 2021 May 31 (MJD 58119–59365) from various instruments, including Fermi–LAT, Swift–XRT, AstroSat, and optical data from Swift-UVOT and the TUBITAK observatory in Turkey. The source was reported to be flaring in the TeV γ-ray band during 2019, but no significant variation was observed with Fermi–LAT. A sub-hour variability is seen in the SXT light curve, suggesting a compact emission region for the variability. However, hour-scale variability is observed in the γ-ray light curve. A ‘softer-when-brighter’ trend is observed in the γ-ray, and an opposite trend is seen in the X-ray, suggesting that the two emissions are produced through two different processes, as expected from a high-frequency-peaked BL Lac source. We have chosen the two epochs in 2019 January to study and compare their physical parameters. A joint fit of SXT and LAXPC provides a constraint on the synchrotron peak, estimated to be ∼1.6 keV. A clear shift in the synchrotron peak is observed from ∼1 keV to above 10 keV, revealing its extreme nature or behaviour like an extreme blazar-type source. The optical observation provides a colour-index variation as ‘blue-when-brighter’. The broad-band spectral energy distribution is fitted with a single-zone synchrotron-self Compton model, and their parameters are discussed in the context of a TeV blazar and the possible mechanism behind the broad-band emission.

Fermi Blazars in the Zwicky Transient Facility Survey: A Correlation Study

We report on our correlation analysis for the γ-ray blazars detected with the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope (Fermi), for which we use the Fermi-LAT data and the optical zr and zg band data from the Zwicky Transient Facility survey in the time period approximately from 2018 July to 2022 February. We start from the full sample of the Fermi-LAT blazars and find ∼38% of them as significant variables in both γ-ray and optical bands. Among the variables, which are 705 blazars, 95 are selected as our targets based on the requirement for the γ-ray brightnesses. Then 15 out of the 95 blazars are found to show the γ-ray–optical correlations in the ∼3.7 yr time period. Most of the 15 blazars are classified as low synchrotron peaked (LSP) ones with the time lags between the γ-ray and optical bands in a range from −19 to +25 days. The results generally fit the often-considered emission scenario for the LSP blazars. Two cases of showing ∼−100 days time lags are discussed, which require further studies for confirmation. Our analysis results suggest that among the detected significant variables, ∼23% LSPs can show γ-ray and optical correlations, while their average γ-ray fluxes are above ∼4 × 10−8 photon cm−2 s−1.

The Peculiar Variable X-Ray Spectrum of the Active Galactic Nucleus PKS 2005–489

PKS 2005−489 is a well-known, bright southern BL Lac object that has been detected up to TeV energies. In a low-flux state it exhibits the expected multiwavelength double-peaked spectrum in the radio−γ-ray band. The high-flux state shows extreme flux variations in the X-ray band with a hardening as well as a peculiar curved feature in the spectrum. Thus far, PKS 2005−489 is the only source to exhibit such a feature. To study the X-ray variability further, we obtained the first hard X-ray spectrum of the source with the Nuclear Spectroscopic Telescope Array. We compare quasi-simultaneous radio, optical, UV, soft and hard X-ray, and γ-ray data of PKS 2005−489 to archival data in order to study its broadband behavior. We find a very consistent quiet state in the spectral energy distribution, with little variation in spectral shape or flux between the 2012 and 2020 data. A possible explanation for the peculiar X-ray spectrum in the flaring state is an additional component in the jet, possibly accelerated via magnetic reconnection, that is not co-spatial to the low-flux state emission region.