skip to main content

Title: IGR J18249−3243: a new GeV-emitting FR II and the emerging population of high-energy radio galaxies

The advent of new all-sky radio surveys such as the VLA Sky Survey and the Rapid ASKAP Continuum Survey, performed with the latest generation radio telescopes, is opening new possibilities on the classification and study of extragalactic γ-ray sources, specially the underrepresented ones like radio galaxies. In particular, the enhanced sensitivity (sub-mJy level) and resolution (a few arcsec) provides a better morphological and spectral classification. In this work, we present the reclassification of a Fermi/Large Area Telescope (LAT) source as a new Fanaroff–Riley II radio galaxy from the International Gamma-Ray Astrophysics Laboratory sample found to emit at GeV energies. Through a broad-band spectral fitting from radio to γ-ray, we find that the commonly invoked jet contribution is not sufficient to account for the observed γ-ray flux. Our modelling suggests that the observed emission could mainly originate in the lobes (rather than in the radio core) by inverse Compton scattering of radio-emitting electrons off the ambient photon fields. In addition, we cross-correlated the latest generation radio surveys with a list of Fermi/LAT candidate misaligned AGN from the literature, finding four new radio galaxies with a double-lobed morphology. Additional four objects could be classified as such thanks to previous studies in the literature, for a total of nine new radio galaxies with GeV emission presented in this work. We foresee that further objects of this class might be found in the near future with the advent of the Square Kilometer Array, populating the GeV sky.

more » « less
Author(s) / Creator(s):
; ; ; ; ; ; ; ; ; ; ; ;
Publisher / Repository:
Oxford University Press
Date Published:
Journal Name:
Monthly Notices of the Royal Astronomical Society
Page Range / eLocation ID:
p. 886-899
Medium: X
Sponsoring Org:
National Science Foundation
More Like this

    The flat-spectrum radio quasar Ton 599 attained its highest ever γ-ray flux state during the first week of 2017 November. Observations of the source by the Swift satellite during this period made it possible to generate a simultaneous high flux state broad-band spectral energy distribution (SED). The high flux state activity of Ton 599 is modelled in this work for the first time. We modelled one high flux state and one quiescent state of the source in order to characterize the evolution of SEDs covering the entire dynamic range of γ-ray flux observed by Fermi-LAT. An attempt was made to model the 2017 November state of the source using an external Compton (EC) model in the leptonic scenario. We reproduce the broad-band flaring state SED using a two-component leptonic emission model. We considered one component as an EC+synchrotron self-Compton (SSC) component and the other as pure SSC, lying further down in the jet. The EC+SSC component was located outside the broad-line region (BLR). It mainly reproduces the GeV emission by an EC process with a dusty torus (DT) photon field providing seed photons. We reproduce the broad-band emission from Ton 599 satisfactorily during its peculiar flaring state with a leptonic two-component model. Besides this, we compare the model parameters of a quiescent-state SED with the available average state model parameters in the literature.

    more » « less
  2. Abstract

    We report the detection of very high energy gamma-ray emission from the blazar S3 1227+25 (VER J1230+253) with the Very Energetic Radiation Imaging Telescope Array System (VERITAS). VERITAS observations of the source were triggered by the detection of a hard-spectrum GeV flare on 2015 May 15 with the Fermi-Large Area Telescope (LAT). A combined 5 hr VERITAS exposure on May 16 and 18 resulted in a strong 13σdetection with a differential photon spectral index, Γ = 3.8 ± 0.4, and a flux level at 9% of the Crab Nebula above 120 GeV. This also triggered target-of-opportunity observations with Swift, optical photometry, polarimetry, and radio measurements, also presented in this work, in addition to the VERITAS and Fermi-LAT data. A temporal analysis of the gamma-ray flux during this period finds evidence of a shortest variability timescale ofτobs= 6.2 ± 0.9 hr, indicating emission from compact regions within the jet, and the combined gamma-ray spectrum shows no strong evidence of a spectral cutoff. An investigation into correlations between the multiwavelength observations found evidence of optical and gamma-ray correlations, suggesting a single-zone model of emission. Finally, the multiwavelength spectral energy distribution is well described by a simple one-zone leptonic synchrotron self-Compton radiation model.

    more » « less
  3. Extragalactic background light (EBL) plays an important role in cosmology since it traces the history of galaxy formation and evolution. Such diffuse radiation from near-UV to far-infrared wavelengths can interact with γ -rays from distant sources such as active galactic nuclei (AGNs), and is responsible for the high-energy absorption observed in their spectra. However, probing the EBL from γ -ray spectra of AGNs is not trivial due to internal processes that can mimic its effect. Such processes are usually taken into account in terms of curvature of the intrinsic spectrum. Hence, an improper choice of parametrization for the latter can seriously affect EBL reconstruction. In this paper, we propose a statistical approach that avoids a priori assumptions on the intrinsic spectral curvature and that, for each source, selects the best-fit model on a solid statistical basis. By combining the Fermi -LAT observations of 490 blazars, we determine the γ -ray-inferred level of EBL for various state-of-the-art EBL models. We discuss the EBL level obtained from the spectra of both BL Lacs and flat spectrum radio quasars (FSRQ) in order to investigate the impact of internal absorption in different classes of objects. We further scrutinize constraints on the EBL evolution from γ -ray observations by reconstructing the EBL level in four redshift ranges, up to z  ∼ 2.5. The approach implemented in this paper, carefully addressing the question of the modeling of the intrinsic emission at the source, can serve as a solid stepping stone for studies of hundreds of high-quality spectra acquired by next-generation γ -ray instruments. 
    more » « less
  4. null (Ed.)
    The presence of relativistic electrons within the diffuse gas phase of galaxy clusters is now well established, thanks to deep radio observations obtained over the last decade, but their detailed origin remains unclear. Cosmic ray protons are also expected to accumulate during the formation of clusters. They may explain part of the radio signal and would lead to γ -ray emission through hadronic interactions within the thermal gas. Recently, the detection of γ -ray emission has been reported toward the Coma cluster with Fermi -LAT. Assuming that this γ -ray emission arises essentially from pion decay produced in proton-proton collisions within the intracluster medium (ICM), we aim at exploring the implication of this signal on the cosmic ray content of the Coma cluster and comparing it to observations at other wavelengths. We use the MINOT software to build a physical model of the Coma cluster, which includes the thermal target gas, the magnetic field strength, and the cosmic rays, to compute the corresponding expected γ -ray signal. We apply this model to the Fermi -LAT data using a binned likelihood approach, together with constraints from X-ray and Sunyaev-Zel’dovich observations. We also consider contamination from compact sources and the impact of various systematic effects on the results. We confirm that a significant γ -ray signal is observed within the characteristic radius θ 500 of the Coma cluster, with a test statistic TS ≃ 27 for our baseline model. The presence of a possible point source (4FGL J1256.9+2736) may account for most of the observed signal. However, this source could also correspond to the peak of the diffuse emission of the cluster itself as it is strongly degenerate with the expected ICM emission, and extended models match the data better. Given the Fermi -LAT angular resolution and the faintness of the signal, it is not possible to strongly constrain the shape of the cosmic ray proton spatial distribution when assuming an ICM origin of the signal, but preference is found in a relatively flat distribution elongated toward the southwest, which, based on data at other wavelengths, matches the spatial distribution of the other cluster components well. Assuming that the whole γ -ray signal is associated with hadronic interactions in the ICM, we constrain the cosmic ray to thermal energy ratio within R 500 to X CRp = 1.79 −0.30 +1.11 % and the slope of the energy spectrum of cosmic rays to α = 2.80 −0.13 +0.67 ( X CRp = 1.06 −0.22 +0.96 % and α = 2.58 −0.09 +1.12 when including both the cluster and 4FGL J1256.9+2736 in our model). Finally, we compute the synchrotron emission associated with the secondary electrons produced in hadronic interactions assuming steady state. This emission is about four times lower than the overall observed radio signal (six times lower when including 4FGL J1256.9+2736), so that primary cosmic ray electrons or reacceleration of secondary electrons is necessary to explain the total emission. We constrain the amplitude of the primary to secondary electrons, or the required boost from reacceleration with respect to the steady state hadronic case, depending on the scenario, as a function of radius. Our results confirm that γ -ray emission is detected in the direction of the Coma cluster. Assuming that the emission is due to hadronic interactions in the intracluster gas, they provide the first quantitative measurement of the cosmic ray proton content in a galaxy cluster and its implication for the cosmic ray electron populations. 
    more » « less

    More than 100 millisecond pulsars (MSPs) have been discovered in radio observations of gamma-ray sources detected by the Fermi Large Area Telescope (LAT), but hundreds of pulsar-like sources remain unidentified. Here, we present the first results from the targeted survey of Fermi-LAT sources being performed by the Transients and Pulsars with MeerKAT (TRAPUM) Large Survey Project. We observed 79 sources identified as possible gamma-ray pulsar candidates by a Random Forest classification of unassociated sources from the 4FGL catalogue. Each source was observed for 10 min on two separate epochs using MeerKAT’s L-band receiver (856–1712 MHz), with typical pulsed flux density sensitivities of $\sim 100\, \mu$Jy. Nine new MSPs were discovered, eight of which are in binary systems, including two eclipsing redbacks and one system, PSR J1526−2744, that appears to have a white dwarf companion in an unusually compact 5 h orbit. We obtained phase-connected timing solutions for two of these MSPs, enabling the detection of gamma-ray pulsations in the Fermi-LAT data. A follow-up search for continuous gravitational waves from PSR J1526−2744 in Advanced LIGO data using the resulting Fermi-LAT timing ephemeris yielded no detection, but sets an upper limit on the neutron star ellipticity of 2.45 × 10−8. We also detected X-ray emission from the redback PSR J1803−6707 in data from the first eROSITA all-sky survey, likely due to emission from an intrabinary shock.

    more » « less