Search

The Fermi -LAT has detected more than 7000 |$\gamma$| -ray sources which show emission above 50 MeV of which more than half are associated with blazars. However, the Fourth Fermi -LAT catalogue (4FGL-DR4) lists 1625 of these as blazar candidates of uncertain type. Increasing the number of classified Fermi -LAT sources is important for improving our understanding of extragalactic |$\gamma$| -ray sources and can be used to search for new classes of very high energy sources. We report on the optical spectroscopy of twelve blazar candidates with hard photon indices included in the Third Catalogue of Hard Fermi -LAT Sources during 2016 and 2017 using the SAAO 1.9-m telescope. We classify all the sources observed as BL Lac objects, and determine the potential spectroscopic redshift for seven of them. [ABSTRACT FROM AUTHOR]

The realtime program for high-energy neutrino track events detected by the IceCube South Pole Neutrino Observatory releases alerts to the astronomical community with the goal of identifying electromagnetic counterparts to astrophysical neutrinos. Gamma-ray observations from the Fermi-Large Area Telescope (LAT) enabled the identification of the flaring gamma-ray blazar TXS 0506+056 as a likely counterpart to the neutrino event IC-170922A. By continuously monitoring the gamma-ray sky, Fermi-LAT plays a key role in the identification of candidate counterparts to realtime neutrino alerts. In this paper, we present the Fermi-LAT strategy for following up high-energy neutrino alerts applied to seven years of IceCube data. Right after receiving an alert, a search is performed in order to identify gamma-ray activity from known and newly detected sources that are positionally consistent with the neutrino localization. In this work, we study the population of blazars found in coincidence with high-energy neutrinos and compare them to the full population of gamma-ray blazars detected by Fermi-LAT. We also evaluate the relationship between the neutrino and gamma-ray luminosities, finding different trends between the two blazar classes BL Lacs and flat-spectrum radio quasars. [ABSTRACT FROM AUTHOR]

The radio galaxy M 87 is a variable very-high energy (VHE) gamma-ray source, exhibiting three major flares, which were reported in 2005, 2008, and 2010. Despite extensive studies, the origin of the VHE gamma-ray emission is not yet fully understood. In this study, we investigate the VHE gamma-ray spectrum of M 87 during states of high gamma-ray activity, utilizing 20.2 h of H.E.S.S. observations. Our findings indicate a preference for a curved spectrum, characterized by a log-parabola model with extra-galactic background light (EBL) model above 0.3 TeV at the 4σ level, compared to a power-law spectrum with EBL. We investigate the degeneracy between the absorption feature and the EBL normalization and derive upper limits on EBL models that are mainly sensitive in the wavelength range of 12.4 μm–40 μm. [ABSTRACT FROM AUTHOR]

Context. Very high-energy (VHE, E > 100 GeV) observations of the blazar Mrk 501 with MAGIC in 2014 provided evidence for an unusual narrow spectral feature at about 3 TeV during an extreme X-ray flaring activity. The one-zone synchrotron-self Compton scenario, widely used in blazar broadband spectral modeling, fails to explain the narrow TeV component. Aims. Motivated by this rare observation, we propose an alternative model for the production of narrow features in the VHE spectra of flaring blazars. These spectral features may result from the decay of neutral pions (π0 bumps) that are in turn produced via interactions of protons (of tens of TeV energy) with energetic photons, whose density increases during hard X-ray flares. Methods. We explored the conditions needed for the emergence of narrow π0 bumps in VHE blazar spectra during X-ray flares reaching synchrotron energies ∼100 keV using time-dependent radiative transfer calculations. We focused on high-synchrotron peaked (HSP) blazars, which comprise the majority of VHE-detected extragalactic sources. Results. We find that synchrotron-dominated flares with peak energies ≳100 keV can be ideal periods for the search of π0 bumps in the VHE spectra of HSP blazars. The flaring region is optically thin to photopion production, its energy content is dominated by the relativistic proton population, and the inferred jet power is highly super-Eddington. Application of the model to the spectral energy distribution of Mrk 501 on MJD 56857.98 shows that the VHE spectrum of the flare is described well by the sum of a synchrotron self-Compton (SSC) component and a distinct π0 bump centered at 3 TeV. Spectral fitting of simulated SSC+π0 spectra for the Cherenkov Telescope Array (CTA) show that a π0 bump could be detected at a 5σ significance level with a 30-min exposure. Conclusions. A harder VHE γ-ray spectrum than the usual SSC prediction or, more occasionally, a distinct narrow bump at VHE energies during hard X-ray flares, can be suggestive of a relativistic hadronic component in blazar jets that otherwise would remain hidden. The production of narrow features or spectral hardenings due to π0 decay in the VHE spectra of blazars is testable with the advent of CTA. [ABSTRACT FROM AUTHOR]

Context. Spiral galaxies M31 and M33 are among the γ-ray sources detected by the Fermi Large Area Telescope (LAT). Aims. We aim to model the broadband non-thermal emission of the central region of M31 (a LAT point source) and of the disk of M33 (a LAT extended source), as part of our continued survey of non-thermal properties of local galaxies that includes the Magellanic Clouds. Methods. We analysed the observed emission from the central region of M31 (R < 5.5 kpc) and the disk-sized emission from M33 (R ∼ 9 kpc). For each galaxy, we self-consistently modelled the broadband spectral energy distribution of the diffuse non-thermal emission based on published radio and γ-ray data. All relevant radiative processes involving relativistic and thermal electrons (synchrotron, Compton scattering, bremsstrahlung, and free–free emission and absorption), along with relativistic protons (π0 decay following interaction with thermal protons), were considered, using exact emissivity formulae. We also used the Fermi-LAT-validated γ-ray emissivities for pulsars. Results. Joint spectral analyses of the emission from the central region of M31 and the extended disk of M33 indicate that the radio emission is composed of both primary and secondary electron synchrotron and thermal bremsstrahlung, whereas the γ-ray emission may be explained as a combination of diffuse pionic, pulsar, and nuclear-BH-related emissions in M31 and plain diffuse pionic emission (with an average proton energy density of 0.5 eV cm−3) in M33. Conclusions. The observed γ-ray emission from M33 appears to be mainly hadronic. This situation is similar to other local galaxies, namely, the Magellanic Clouds. In contrast, we have found suggestions of a more complex situation in the central region of M31, whose emission could be an admixture of pulsar emission and hadronic emission, with the latter possibly originating from both the disk and the vicinity of the nuclear black hole. The alternative modelling of the spectra of M31 and M33 is motivated by the different hydrogen distribution in the two galaxies: The hydrogen deficiency in the central region of M31 partially unveils emissions from the nuclear BH and the pulsar population in the bulge and inner disk. If this were to be the case in M33 as well, these emissions would be outshined by diffuse pionic emission originating within the flat central-peak gas distribution in M33. [ABSTRACT FROM AUTHOR]

PKS 1830 -211 is a γ-ray emitting, high-redshift (z  =2.507 ± 0.002), lensed flat-spectrum radio quasar. During the period 2019 mid-February to mid-April, this source underwent a series of strong γ-ray flares that were detected by both AGILE-GRID (Gamma-Ray Imaging Detector) and Fermi Large Area Telescope (Fermi -LAT), reaching a maximum γ-ray flux of |$F_{\rm E\gt 100\, MeV}\approx 2.3\times 10^{-5}$|  photons cm−2 s−1. Here, we report on a coordinated campaign from both on-ground [Medicina, Owens Valley Radio Observatory (OVRO), Rapid Eye Mount (REM), and Sardinia Radio Telescope (SRT)] and orbiting facilities (AGILE, Fermi, INTEGRAL, NuSTAR, Swift , and Chandra), with the aim of investigating the multiwavelength properties of PKS 1830-211 through nearly simultaneous observations presented here for the first time. We find a possible break in the radio spectra in different epochs above 15 GHz, and a clear maximum of the 15 GHz data approximately 110 d after the γ-ray main activity periods. The spectral energy distribution shows a very pronounced Compton dominance (> 200) which challenges the canonical one-component emission model. Therefore, we propose that the cooled electrons of the first component are re-accelerated to a second component by, for example, kink or tearing instability during the γ-ray flaring periods. We also note that PKS 1830-211 could be a promising candidate for future observations with both Compton satellites [e.g. enhanced ASTROGAM (e-ASTROGAM)] and Cherenkov arrays [Cherenkov Telescope Array Observatory (CTAO)] which will help, thanks to their improved sensitivity, in extending the data availability in energy bands currently uncovered. [ABSTRACT FROM AUTHOR]

Context. The origin of the diffuse astrophysical neutrino flux observed by the IceCube experiment is still under debate. Multiple associations have been reported between high-energy neutrino events and individual bla/ars, such as the source TXS 0506+056, which are active galaxies with relativistic jets pointing toward Earth. From a theoretical perspective, the properties of these sources as neutrino emitters are not yet well understood. Aims. By systematically modeling the effect of cosmic-ray protons on the multiwavelength data from the largest sample of bright gamma-ray bla/ars to date, we expect to learn about the multi-messenger nature of the active galaxy population as a whole, as well as the relationship between neutrino production and the multiwavelength spectrum of these sources. Methods. We predict the emitted multiwavelength and neutrino spectrum using a self-consistent numerical radiation model applied individually to each source in the sample. We then study the properties of the full population and identify empirical relations. We focus on public multiwavelength data from the radio to the gamma-ray bands from a sample of 324 bla/ars detected by the Fermi Large Area Telescope (LAT), most of which are flat-spectrum radio quasars (FSRQs). This amounts to 34% of all FSRQs in the latest Fermi catalog. Results. We demonstrate that the optical and gigaelectronvolt gamma-ray broadband features are generally well described by electron emission, which helps for the location of the emission region relative to the central black hole to be constrained. For 33% of the bla/ars in our sample, a description of the observed X-ray spectrum benefits from an additional component from proton interactions, in agreement with recent studies of individual IceCube candidate bla/ars. We show that, on average, bla/ars that are brighter in gigaelectronvolt gamma rays have a higher neutrino production efficiency but a lower best-fit baryonic loading. The predicted neutrino luminosity shows a positive correlation both with the observed flux of gigaelectronvolt gamma rays and with the predicted flux of megaelectronvolt gamma rays. We also estimate the diffuse neutrino flux from gamma-ray bla/ars by extrapolating the result to the Fermi population, and we show that it may be at the level of ~20% of the diffuse neutrino flux observed by IceCube, in agreement with current limits from stacking analyses. We discuss the implications of our results for future neutrino searches and suggest promising sources for potential detections. [ABSTRACT FROM AUTHOR]

We systematically analyzed Swift/XRT X-ray data of gamma-ray-emitting radio galaxies (RGs) to study the origin of X-ray radiation. We studied the short-duration variability of spectra by using Swift/XRT data for 20 RGs. We found that gamma-ray-emitting RGs with a higher gamma-ray-to-X-ray flux ratio show a strong harder-when-brighter behavior. In addition, gamma-ray-emitting RGs with soft gamma-ray spectral index tend to have a hard X-ray spectral index and vice versa. Based on these properties, we imply a trend that gamma-ray-emitting RGs could be roughly classified into three types of X-ray emission: jet-dominated type, disk/corona-dominated type, and intermediate type. This classification is quite consistent with the optical and radio classification of RGs. In addition, we also discuss this result in terms of the Eddington luminosity ratio. These interpretations should be confirmed using larger samples of RGs in the future. [ABSTRACT FROM AUTHOR]

Many experimental quantities show a power-law distribution p (x) ∝  x −α. In astrophysics, examples are: size distribution of dust grains or luminosity function of galaxies. Such distributions are characterized by the exponent α and by the extremes x min x max where the distribution extends. There are no mathematical tools that derive the three unknowns at the same time. In general, one estimates a set of α corresponding to different guesses of x min x max. Then, the best set of values describing the observed data is selected a posteriori. In this paper, we present a tool that finds contextually the three parameters based on simple assumptions on how the observed values xi populate the unknown range between x min and x max for a given α. Our tool, freely downloadable, finds the best values through a non-linear least-squares fit. We compare our technique with the maximum likelihood estimators for power-law distributions, both truncated and not. Through simulated data, we show for each method the reliability of the computed parameters as a function of the number N of data in the sample. We then apply our method to observed data to derive: (i) the slope of the core mass function in the Perseus star-forming region, finding two power-law distributions: α = 2.576 between |$1.06\, \mathrm{M}_{\odot }$| and |$3.35\, \mathrm{M}_{\odot }$|⁠ , α = 3.39 between |$3.48\, \mathrm{M}_{\odot }$| and |$33.4\, \mathrm{M}_{\odot }$|⁠ ; (ii) the slope of the γ-ray spectrum of the blazar J0011.4+0057, extracted from the Fermi-LAT archive. For the latter case, we derive α = 2.89 between 1484 MeV and 28.7 GeV; then we derive the time-resolved slopes using subsets of 200 photons each. [ABSTRACT FROM AUTHOR]

Observation of several very high energy (VHE) flaring events of the BL Lac object VER J0521+211 was reported by the VERITAS and MAGIC collaborations between 2009 and 2014. The redshift of this source is uncertain, and several analyses have derived different limits for it. In the framework of the photohadronic model and using three different extragalactic background light (EBL) models, we analyse seven independent VHE spectra of VER J0521+211 and determine the limiting values on its redshift. It is observed that the photohadronic scenario provides excellent fits to the reported observations. It is further observed that the photohadronic scenario, along with the EBL model of Domínguez et al. puts the most restrictive limits on the redshift z of VER J0521+211: 0.29 ≤ z ≤ 0.31 from the confidence level (CL) intervals at 2σ, or a more conservative 0.28 ≤ z ≤ 0.33 at 3σ. [ABSTRACT FROM AUTHOR]

We study the long-term behaviour of the bright gamma-ray blazar PKS 0402-362. We collected approximately 13 yr of Fermi-LAT data between August 2008 to January 2021 and identified three bright γ-ray activity epochs. The second was found to be the brightest epoch ever seen in this source. We observed most of the γ-ray flare peaks to be asymmetric in profile suggesting a slow cooling time of particles or the varying Doppler factor as the main cause of these flares. The γ-ray spectrum is fitted with PL and LP spectral models, and in both cases, the spectral index is very steep. The γ-ray spectrum does not extend beyond 10 GeV energy suggesting the emission is produced within the BLR. The absence of time lags between optical-IR and γ-ray suggest one zone emission model. Using the above information, we have modelled the broad-band SED with a simple one-zone emission model using the publicly available code 'GAMERA'. The particle distribution index is found to be the same as expected in diffusive shock acceleration suggesting it as the main mechanism of particle acceleration to very high energy up to 4–6 GeV. Throughout the various flux phases, we noticed that the optical emission is dominated by the thermal disc, suggesting it to be a good source to examine the disc-jet coupling. We postulate that the observed broad-band flares could be linked with perturbation produced in the disc, which propagates to the jet and interacts with the standing shock. However, a more detailed examination is required. [ABSTRACT FROM AUTHOR]

Context. The detection of quasi-periodic oscillations (QPOs) in the light curves of active galactic nuclei (AGNs) can provide insights into the physics of the super-massive black holes (SMBHs) powering these systems and could represent a signature of the existence of SMBH binaries, setting fundamental constraints on SMBH evolution in the Universe. Aims. The identification of long-term QPOs, characterized by periods on the order of several months to years, is particularly challenging and can only be achieved via all-sky monitoring instruments that provide unbiased, continuous light curves of astrophysical objects. The Fermi-LAT satellite, thanks to its monitoring observing strategy, is an ideal instrument for such a goal. Here, we aim to identify QPOs in the-ray light curves of the brightest AGNs within the Fermi-LAT catalog. Methods. We analyzed the light curves of the 35 brightest Fermi-LAT AGNs, including data from the beginning of the Fermi mission (August 2008) to April 2021, with energies from 100MeV to 300 GeV. Two time binnings were investigated: 7 and 30 days. The search for quasi-periodic features was then performed using the continuous wavelet transform. The significance of the result was tested via Monte Carlo simulations of artificial light curves with the same power spectral density and probability distribution function as the original light curves. The significances were then corrected for the look-elsewhere effect and provided as post-trials. Results. We identified 24 quasars with candidate QPOs. Several of our candidates coincide with previous claims in the literature, namely: PKS 0537441, S5 0716+714, Mrk 421, B2 1520+31, and PKS 2247131. All our candidates are transient. The most significant multi-year QPO, with a period of about 1100 days, was observed in the quasar S5 1044+71. It is reported here for the first time. [ABSTRACT FROM AUTHOR]

In this work we have updated the list of active galactic nuclei (AGN) detected by INTEGRAL taking into account the new objects listed in the last published INTEGRAL/IBIS survey. We have collected 83 new AGN, increasing the number of INTEGRAL-detected active galaxies (436) by 19%. Half of these new additions are located behind the Galactic plane, and for most of them we have full X-ray coverage, obtained through archival data from Swift/XRT, XMM-Newton, and NuSTAR. The soft X-ray data allowed us to associate each high-energy emitter with a single or multiple X-ray counterpart(s) and characterise the spectral shape of these new AGN by estimating the photon index, the intrinsic absorption, and the 2–10 keV flux. A few cases where two soft X-ray counterparts fall within the INTEGRAL error circle and at least one is classified as an AGN have been found and discussed in detail. Thirty-four sources originally listed as AGN candidates or unidentified objects have been recognised as AGN by employing three diagnostic tests: WISE colours, radio emission, and morphology. For 12 sources among the 34 AGN candidates, we reduced the optical spectra and confirmed their AGN nature, providing also their optical class and redshift. This paper is part of an ongoing effort to keep the INTEGRAL AGN catalogue updated in order to provide the scientific community with a hard X-ray selected sample of well-classified and spectrally characterised active galaxies. [ABSTRACT FROM AUTHOR]

Recent radio surveys have discovered a large number of low luminosity coredominated radio galaxies that are much more abundant than those at higherluminosities. These objects will be too faint in gamma-rays to be detectedindividually by Fermi. Nevertheless, they may contribute significantly to theunresolved extragalactic gamma-ray background. We consider here the possiblecontribution of these core dominated radio galaxies to the diffuseextragalactic gamma-ray background. Using published data available for all 45of the radiogalaxies listed as detected counterparts in the Fermi FL8Y sourcelist update to the 3FGL catalog, we have searched for radio maps which canresolve the core flux from the total source flux. Using high resolution radiomaps we were able to obtain core fluxes for virtually every source. We thenderived a relation between core radio flux and gamma-ray flux that weextrapolated to sources with low radio luminosities that are known to be highlycore dominated. We then employed a very recent determination of the luminosityfunction for core dominated radio galaxies in order to obtain the contributionof all possible gamma-ray emitting radio galaxies to the unresolvedextragalactic gamma-ray background. We find this contribution to be a possiblynon-negligible, 4% - 18% of the background.

The Fermi γ -ray telescope has detected 6658 sources, with 1845 of them remaining unidentified. We show that polarimetry of γ -ray fields is a powerful asset in the hunt of active galactic nuclei (AGN) as potential optical counterparts for γ -ray sources. We have studied an unidentified Fermi field (3FGL J0221.2+2518) and found a previously-unknown highly-polarized extragalactic object as a potential optical counterpart within the 1-sigma error ellipse of the corresponding γ -ray source. Based on a collection of data, we find that it most probably is a composite object: a star-forming galaxy accompanied by AGN. PASIPHAE is a large polarimetric experiment which will measure the polarisation of sources away from the galactic plane. This will provide an excellent opportunity to study hundreds of unidentified γ -ray sources and unveil potential optical counterparts, using polarimetry. [ABSTRACT FROM AUTHOR]

The gamma-ray emission from Starburst and Star-forming Galaxies (SBGs and SFGs) strongly suggests a correlation between star-forming activity and gamma-ray luminosity. However, the very nature of cosmic ray (CR) transport and the degree of their confinement within SBG cores are still open questions. We aim at probing the imprints left by CR transport on gamma-ray and neutrino observations of point-like SFGs and SBGs, looking into quantitative ways to discriminate among different transport models. We analyse the 10-yr Fermi-LAT spectral energy distributions of 13 nearby galaxies with two different CR transport models, taking into account the corresponding IR and UV observations. We also generate mock gamma-ray data to simulate the CTA performance in detecting these sources. In this way, we propose a test to discriminate between the two CR models, quantifying the statistical confidence at which one model can be preferred over the other. We point out that the current data already give a slight preference to CR models that are dominated by advection. Moreover, we show that CTA will allow us to firmly disfavour models dominated by diffusion over self-induced turbulence, compared to advection-dominated models, with Bayes factors, which can be as large as 107 for some of the SBGs. Finally, we estimate the diffuse gamma-ray and neutrino fluxes of SFGs and SBGs, showing that they can explain |$25{{\,\rm per\ cent}}$| of the diffuse HESE data while remaining consistent with gamma-ray limits on non-blazar sources. [ABSTRACT FROM AUTHOR]

The recent discovery of a diffuse cosmic neutrino flux extending up to PeV energies raises the question of which astrophysical sources generate this signal. Blazars are one class of extragalactic sources which may produce such high-energy neutrinos. We present a likelihood analysis searching for cumulative neutrino emission from blazars in the 2nd Fermi-LAT AGN catalog (2LAC) using IceCube neutrino data set 2009-12, which was optimized for the detection of individual sources. In contrast to those in previous searches with IceCube, the populations investigated contain up to hundreds of sources, the largest one being the entire blazar sample in the 2LAC catalog. No significant excess is observed, and upper limits for the cumulative flux from these populations are obtained. These constrain the maximum contribution of 2LAC blazars to the observed astrophysical neutrino flux to 27% or less between around 10 TeV and 2 PeV, assuming the equipartition of flavors on Earth and a single power-law spectrum with a spectral index of -2.5. We can still exclude the fact that 2LAC blazars (and their subpopulations) emit more than 50% of the observed neutrinos up to a spectral index as hard as -2.2 in the same energy range. Our result takes into account the fact that the neutrino source count distribution is unknown, and it does not assume strict proportionality of the neutrino flux to the measured 2LAC γ-ray signal for each source. Additionally, we constrain recent models for neutrino emission by blazars.

Radio and γ-ray loud narrow-line Seyfert 1 galaxies (NLS1s) are unique objects to study the formation and evolution of relativistic jets, as they are believed to have high accretion rates and powered by low mass black holes contrary to that known for blazars. However, only about a dozen γ-ray-detected NLS1s (γ-NLS1s) are known to date and all of them are at |$z$| ≤ 1. Here, we report the identification of a new γ-ray-emitting NLS1 TXS 1206 + 549 at |$z$|  = 1.344. A near-infrared spectrum taken with the Subaru Telescope showed H β emission line with FWHM of 1194 ± 77 km s−1 and weak [O  iii ] emission line but no optical Fe  ii lines, due to the limited wavelength coverage and poor signal-to-noise ratio. However, UV Fe  ii lines are present in the SDSS optical spectrum. The source is very radio-loud, unresolved, and has a flat radio spectrum. The broad-band spectral energy distribution of the source has the typical two hump structure shown by blazars and other γ-NLS1s. The source exhibits strong variability at all wavelengths such as the optical, infrared, and γ-ray bands. All these observed characteristics show that TXS 1206 + 549 is the most distant γ-NLS1 known to date. [ABSTRACT FROM AUTHOR]

We present deep VERITAS observations of the blazar PKS 1424+240, along with contemporaneous Fermi Large Area Telescope, Swift X-ray Telescope, and Swift UV Optical Telescope data between 2009 February 19 and 2013 June 8. This blazar resides at a redshift of z ≥ 0.6035, displaying a significantly attenuated gamma-ray flux above 100 GeV due to photon absorption via pair-production with the extragalactic background light. We present more than 100 hr of VERITAS observations over three years, a multiwavelength light curve, and the contemporaneous spectral energy distributions. The source shows a higher flux of (2.1 ± 0.3) × 10-7 photons m-2 s -1 above 120 GeV in 2009 and 2011 as compared to the flux measured in 2013, corresponding to (1.02 ± 0.08) × 10-7 photons m-2 s-1 above 120 GeV. The measured differential very high energy (VHE; E ≥ 100 GeV) spectral indices are Γ = 3.8 ± 0.3, 4.3 ± 0.6 and 4.5 ± 0.2 in 2009, 2011, and 2013, respectively. No significant spectral change across the observation epochs is detected. We find no evidence for variability at gamma-ray opacities of greater than τ = 2, where it is postulated that any variability would be small and occur on timescales longer than a year if hadronic cosmic-ray interactions with extragalactic photon fields provide a secondary VHE photon flux. The data cannot rule out such variability due to low statistics. © 2014. The American Astronomical Society. All rights reserved..