- ID:
- ivo://CDS.VizieR/J/A+A/647/A195
- Title:
- Unknown active galactic nuclei study
- Short Name:
- J/A+A/647/A195
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We collect data at all frequencies for the new sources classified as unknown active galactic nuclei (AGNs) in the latest Burst Alert Telescope all-sky hard X-ray catalog. Focusing on the 36 sources with measured redshift, we compute their spectral energy distribution (SED) from radio to {gamma}-rays with the aim to classify these objects. We apply emission models that attempt to reproduce the obtained SEDs, including: (i) a standard thin accretion disk together with an obscuring torus and a X-ray corona; (ii) a two temperature thick advection-dominated flow; (iii) an obscured AGN model, accounting for absorption along the line of sight at kiloelectronvolt energies and in the optical band; and (iv) a phenomenological model to describe the jet emission in blazar-like objects. We integrate the models with the SWIRE template libraries to account for the emission of the host galaxy. For every source we found a good agreement between data and our model. Considering that the sources were selected in the hard X-ray band, which is rather unaffected by absorption, we expected and found a large fraction of absorbed radio-quiet AGNs (31 out of 36) and some additional rare radio-loud sources (5 out of 36), since the jet emission in hard X-rays is important for aligned jets owing to the boost produced by the beaming effect. With our work we can confirm the hypothesis that a number of galaxies, whose optical spectra lack AGN emission features, host an obscured active nucleus. The approach we used proved to be efficient in rapidly identifying objects, which commonly used methods were not able to classify.
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Search Results
- ID:
- ivo://CDS.VizieR/J/MNRAS/460/19
- Title:
- Update of INTEGRAL/IBIS AGN catalogue
- Short Name:
- J/MNRAS/460/19
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In the most recent IBIS survey based on observations performed during the first 1000 orbits of INTEGRAL, are listed 363 high-energy emitters firmly associated with AGN, 107 of which are reported here for the first time. We have used X-ray data to image the IBIS 90 per cent error circle of all the AGN in the sample of 107, in order to obtain the correct X-ray counterparts, locate them with arcsec accuracy and therefore pinpoint the correct optical counterparts. This procedure has led to the optical and spectral characterization of the entire sample. This new set consists of 34 broad line or type 1 AGN, 47 narrow line or type 2 AGN, 18 blazars and 8 sources of unknown class. These eight sources have been associated with AGN from their positional coincidence with 2MASX/Radio/X-ray sources. Seven high-energy emitters have been included since they are considered to be good AGN candidates. Spectral analysis has been already performed on 55 objects and the results from the most recent and/or best statistical measurements have been collected. For the remaining 52 sources, we report the spectral analysis for the first time in this work. We have been able to obtain the full X-ray coverage of the sample making use of data from Swift/XRT, XMM-Newtonand NuSTAR. In addition to the spectral characterization of the entire sample, this analysis has enabled us to identify peculiar sources and by comparing different data sets, highlight flux variability in the 2-10keV and 20-40keV bands.
- ID:
- ivo://CDS.VizieR/J/A+A/653/A83
- Title:
- UVB/NIR spectroscopy GRB hosts
- Short Name:
- J/A+A/653/A83
- Date:
- 22 Feb 2022
- Publisher:
- CDS
- Description:
- We present the updated census and statistics of Lyman-alpha emitting long gamma-ray bursts host galaxies (LAE-LGRBs). We investigate the properties of a sub-sample of four LAE-LGRBs (GRBs: 011211, 021004, 060926, 070110). The combined observations of their afterglow and host galaxy allow us to test the shell model commonly used to fit Lyman-alpha emission line spectra.
- ID:
- ivo://CDS.VizieR/J/A+A/581/A125
- Title:
- UV/Optical/NIR spectroscopy GRB hosts
- Short Name:
- J/A+A/581/A125
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present data and initial results from VLT/X-shooter emission-line spectroscopy of 96 galaxies selected by long gamma-ray bursts (GRBs) at 0.1<z<3.6, the largest sample of GRB host spectra available to date. Most of our GRBs were detected by Swift and 76% are at 0.5<z<2.5 with a median z_med_~1.6.
- ID:
- ivo://CDS.VizieR/J/ApJ/748/134
- Title:
- Variability components in BATSE GRB light curves
- Short Name:
- J/ApJ/748/134
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Gamma-ray bursts (GRBs) have variable light curves. Although most models attribute the observed variability to one physical origin (e.g., central engine activity, clumpy circumburst medium, or relativistic turbulence), some models invoke two physically distinct variability components. We develop a method, namely, the stepwise filter correlation method, to decompose the variability components in a GRB light curve. Based on a low-pass filter technique, we progressively filter the high-frequency signals from the light curve, and then perform a correlation analysis between each adjunct pair of filtered light curves. Our simulations suggest that if a mock light curve contains a "slow" variability component superposed on a rapidly varying time sequence, the correlation coefficient as a function of the filter frequency would display a prominent "dip" feature around the frequency of the slow component. Through simulations, we demonstrate that this method can identify significant clustering structures of a light curve in the frequency domain, and we prove that it can catch superposed signals that are otherwise not easy to retrieve based on other methods (e.g., the power density spectrum analysis method). We apply this method to 266 Burst and Transient Source Experiment bright GRBs. We find that the majority of the bursts have clear evidence of such a superposition effect. We perform a statistical analysis of the identified variability components and discuss the implications for GRB physics.
- ID:
- ivo://CDS.VizieR/J/A+A/370/468
- Title:
- Variability of gamma-ray sources
- Short Name:
- J/A+A/370/468
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a study of 40 low-latitude unidentified 3EG gamma-ray sources which were found to be not positionally coincident with any known class of potential gamma-ray emitters in the Galaxy (Romero et al., 1999A&A...348..868R). We have performed a variability analysis which reveals that many of these 40 sources are variable. These sources have, in addition, a steep mean value of the gamma-ray spectral index, <{Gamma}>=2.41+/-0.2, which, combined with the high level of variability, seems to rule out a pulsar origin. The positional coincidences with uncatalogued candidates to supernova remnants were also studied. Only 7 sources in the sample are spatially coincident with these candidates, a result that is shown to be consistent with the expected level of pure chance association. A complementary search for weak radio counterparts was also conducted and the results are presented as an extensive table containing all significant point-like radio sources within the 40 EGRET fields. We argue that in order to produce the high variability, steep gamma-ray spectra, and absence of strong radio counterparts observed in some of the gamma-ray sources of our sample, a new class of objects should be postulated, and we analyze a viable candidate.
- ID:
- ivo://CDS.VizieR/J/A+A/522/A68
- Title:
- Variable sources detected by INTEGRAL
- Short Name:
- J/A+A/522/A68
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In 6 years of operation, INTEGRAL/ISGRI revealed more than 500 sources. Many of these sources are variable. Taking into account that nearly half of INTEGRAL/ISGRI sources are new and many of them are still unidentified, the variability properties of the sources can serve as additional parameters that may help to classify and identify the unknown sources. In order to study the variability properties of the sources detected by INTEGRAL/ISGRI we develop a method to quantify the variability of a source. We describe here our techniques and compile a catalog of the sources that fit our criteria of variability.
- ID:
- ivo://CDS.VizieR/J/A+A/612/A7
- Title:
- Vela Junior (RX J0852.0-4622) HESS image
- Short Name:
- J/A+A/612/A7
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We study {gamma}-ray emission from the shell-type supernova remnant (SNR) RX J0852.0-4622 to better characterize its spectral properties and its distribution over the SNR. The analysis of an extended High Energy Spectroscopic System (H.E.S.S.) data set at very high energies (E>100GeV) permits detailed studies, as well as spatially resolved spectroscopy, of the morphology and spectrum of the whole RX J0852.0-4622 region. The H.E.S.S. data are combined with archival data from other wavebands and interpreted in the framework of leptonic and hadronic models. The joint Fermi-LAT-H.E.S.S. spectrum allows the direct determination of the spectral characteristics of the parent particle population in leptonic and hadronic scenarios using only GeV-TeV data. An updated analysis of the H.E.S.S. data shows that the spectrum of the entire SNR connects smoothly to the high-energy spectrum measured by Fermi-LAT. The increased data set makes it possible to demonstrate that the H.E.S.S. spectrum deviates significantly from a power law and is well described by both a curved power law and a power law with an exponential cutoff at an energy of E_cut_=(6.7+/-1.2_stat_+/-1.2_syst_)TeV. The joint Fermi-LAT-H.E.S.S. spectrum allows the unambiguous identification of the spectral shape as a power law with an exponential cutoff. No significant evidence is found for a variation of the spectral parameters across the SNR, suggesting similar conditions of particle acceleration across the remnant. A simple modeling using one particle population to model the SNR emission demonstrates that both leptonic and hadronic emission scenarios remain plausible. It is also shown that at least a part of the shell emission is likely due to the presence of a pulsar wind nebula around PSR J0855-4644.
- ID:
- ivo://CDS.VizieR/J/ApJ/833/117
- Title:
- VERITAS gamma-ray TeV LCs of 6 blazars
- Short Name:
- J/ApJ/833/117
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a targeted search for blazar flux-correlated high-energy ({epsilon}_{nu}_>~1TeV) neutrinos from six bright northern blazars, using the public database of northern hemisphere neutrinos detected during "IC40" 40-string operations of the IceCube neutrino observatory (2008 April to 2009 May). Our six targeted blazars are subjects of long-term monitoring campaigns by the VERITAS TeV {gamma}-ray observatory. We use the publicly available VERITAS light curves to identify periods of excess and flaring emission. These predefined intervals serve as our "active temporal windows" in a search for an excess of neutrinos, relative to Poisson fluctuations of the near-isotropic atmospheric neutrino background, which dominates at these energies. After defining the parameters of an optimized search, we confirm the expected Poisson behavior with Monte Carlo simulations prior to testing for excess neutrinos in the actual data. We make two searches: one for excess neutrinos associated with the bright flares of Mrk 421 that occurred during the IC40 run, and one for excess neutrinos associated with the brightest emission periods of five other blazars (Mrk 501, 1ES 0806+524, 1ES 1218+304, 3C 66A, and W Comae), all significantly fainter than the Mrk 421 flares. We find no significant excess of neutrinos from the preselected blazar directions during the selected temporal windows. We derive 90% confidence upper limits on the number of expected flux-associated neutrinos from each search. These limits are consistent with previous point-source searches and Fermi GeV flux-correlated searches. Our upper limits are sufficiently close to the physically interesting regime that we anticipate that future analyses using already-collected data will either constrain models or yield discovery of the first blazar-associated high-energy neutrinos.
- ID:
- ivo://CDS.VizieR/J/ApJ/808/162
- Title:
- VLBA 15 and 43GHz obs. of 3C 120
- Short Name:
- J/ApJ/808/162
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the analysis of the radio jet evolution of the radio galaxy 3C 120 during a period of prolonged {gamma}-ray activity detected by the Fermi satellite between 2012 December and 2014 October. We find a clear connection between the {gamma}-ray and radio emission, such that every period of {gamma}-ray activity is accompanied by the flaring of the millimeter very long baseline interferometry (VLBI) core and subsequent ejection of a new superluminal component. However, not all ejections of components are associated with {gamma}-ray events detectable by Fermi. Clear {gamma}-ray detections are obtained only when components are moving in a direction closer to our line of sight. This suggests that the observed {gamma}-ray emission depends not only on the interaction of moving components with the millimeter VLBI core, but also on their orientation with respect to the observer. Timing of the {gamma}-ray detections and ejection of superluminal components locate the {gamma}-ray production to within ~0.13pc from the millimeter VLBI core, which was previously estimated to lie about 0.24pc from the central black hole. This corresponds to about twice the estimated extension of the broad line region, limiting the external photon field and therefore suggesting synchrotron self Compton as the most probable mechanism for the production of the {gamma}-ray emission. Alternatively, the interaction of components with the jet sheath can provide the necessary photon field to produced the observed {gamma}-rays by Compton scattering.
