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Register a new userX-ray Emission of the $gamma$-ray Loud Young Radio Galaxy NGC 3894
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Added by
Karthik Balasubramaniam Mr
Publication date
2021
fields
Physics
and research's language is
English
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The radio source 1146+596 is hosted by an elliptical/S0 galaxy NGC,3894, with a low-luminosity active nucleus. The radio structure is compact, suggesting a very young age of the jets in the system. Recently, the source has been confirmed as a high-energy (HE, $>0.1$,GeV) $gamma$-ray emitter, in the most recent accumulation of the {it Fermi} Large Area Telescope (LAT) data. Here we report on the analysis of the archival {it Chandra} X-ray Observatory data for the central part of the galaxy, consisting of a single 40,ksec-long exposure. We have found that the core spectrum is best fitted by a combination of an ionized thermal plasma with the temperature of $simeq 0.8$,keV, and a moderately absorbed power-law component (photon index $Gamma = 1.4pm 0.4$, hydrogen column density $N_{rm H}/10^{22}$,cm$^{-2}$,$= 2.4pm 0.7$). We have also detected the iron K$alpha$ line at $6.5pm 0.1$,keV, with a large equivalent width of EW,$= 1.0_{-0.5}^{+0.9}$,keV. Based on the simulations of the {it Chandra}s Point Spread Function (PSF), we have concluded that, while the soft thermal component is extended on the scale of the galaxy host, the hard X-ray emission within the narrow photon energy range 6.0--7.0,keV originates within the unresolved core (effectively the central kpc radius). The line is therefore indicative of the X-ray reflection from a cold neutral gas in the central regions of NGC,3894. We discuss the implications of our findings in the context of the X-ray Baldwin effect. NGC,3894 is the first young radio galaxy detected in HE $gamma$-rays with the iron K$alpha$ line.
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We analyzed Suzaku/XIS data of 2006--2015 observations of a gamma-ray emitting radio galaxy NGC 1275, and brightening of the nucleus in the X-ray band was found in 2013--2015, correlating with GeV Gamma-ray brightening. This is the first evidence of variability with correlation between GeV gamma-ray and X-ray for NGC 1275. We also analyzed Swift/XRT data of NGC 1275, and found that X-ray was flaring by a factor of $sim$5 in several days in 2006, 2010, and 2013. The X-ray spectrum during the flare was featureless and somewhat steeper with a photon index of $sim$2 against $sim$1.7 in the normal state, indicating that a synchrotron component became brighter. A large Xray to GeV gamma-ray flux ratio in the flare could be explained by the shock-in-jet scenario. On the other hand, a long-term gradual brightening of radio, X-ray, and GeV gamma-ray with a larger gamma-ray amplitude could be origin of other than internal shocks, and then we discuss some possibilities.
NGC 1275 is a gamma-ray-emitting radio galaxy at the center of the Perseus cluster. Its multi-wavelength spectrum is similar to that of blazers, and thus a jet-origin of gamma-ray emissions is believed. In the optical and X-ray region, NGC 1275 also shows a bright core, but their origin has not been understood, since a disk emission is not ruled out. In fact, NGC 1275 exhibits optical broad emission lines and a X-ray Fe-K line, which are typical for Seyfert galaxies. In our precious studies of NGC 1275 with Suzaku/XIS, no X-ray time variability was found from 2006 to 2011, regardless of moderate gamma-ray variability observed by {it Fermi}-LAT~cite{Yamazaki}. We have continued monitoring observations of NGC 1275 with Suzaku/XIS. In 2013-2014, MeV/GeV gams-ray flux of NGC 1275 gradually increased and reached the maximum at the beginning of 2014. Correlated with this recent gamma-ray activity, we found that X-ray flux also increased, and this is the first evidence of X-ray variability of NGC 1275. Following these results, we discuss the emission component during the time variability, but we cannot decide the origin of X-ray variability correlating with gamma-ray. Therefore, for future observation, it is important to observe NGC 1275 by using Fermi gamma-ray, XMM-Newton, NuStar, ASTRO-H X-ray, CTA TeV gamma-ray and Kanata optical telescope.
Context. According to radiative models, radio galaxies may produce gamma-ray emission from the first stages of their evolution. However, very few such galaxies have been detected by the Fermi Large Area Telescope (LAT) so far. Aims. NGC 3894 is a nearby (z = 0.0108) object that belongs to the class of compact symmetric objects (CSOs, i.e., the most compact and youngest radio galaxies), which is associated with a gamma-ray counterpart in the Fourth Fermi-LAT source catalog. Here we present a study of the source in the gamma-ray and radio bands aimed at investigating its high-energy emission and assess its young nature. Methods. We analyzed 10.8 years of Fermi-LAT data between 100 MeV and 300 GeV and determined the spectral and variability characteristics of the source. Multi-epoch very long baseline array (VLBA) observations between 5 and 15 GHz over a period of 35 years were used to study the radio morphology of NGC 3894 and its evolution. Results. NGC 3894 is detected in gamma-rays with a significance >9 sigma over the full period, and no significant variability has been observed in the gamma-ray flux on a yearly time-scale. The spectrum is modeled with a flat power law ($Gamma$ = 2.0$pm$0.1) and a flux on the order of 2.2 $times$ 10$^{-9}$ ph cm$^{-2}$ s$^{-1}$. For the first time, the VLBA data allow us to constrain with high precision the apparent velocity of the jet and counter-jet side to be $beta_{mathrm{app,NW}}$ = 0.132$pm$0.004 and $beta_{mathrm{app,SE}}$ = 0.065$pm$0.003, respectively. Conclusions. Fermi-LAT and VLBA results favor the youth scenario for the inner structure of this object, with an estimated dynamical age of 59$pm$5 years. The estimated range of viewing angle (10{deg} < $theta$ < 21{deg}) does not exclude a possible jet-like origin of the gamma-ray emission.
According to radiative models, radio galaxies are predicted to produce gamma rays from the earliest stages of their evolution onwards.The study of the high-energy emission from young radio sources is crucial for providing information on the most energetic processes associated with these sources, the actual region responsible for this emission, as well as the structure of the newly born radio jets. Despite systematic searches for young radio sources at gamma-ray energies, only a handful of detections have been reported so far. Taking advantage of more than 11 years of textit{Fermi}-LAT data, we investigate the gamma-ray emission of 162 young radio sources (103 galaxies and 59 quasars), the largest sample of young radio sources used so far for a gamma-ray study. We analyse the textit{Fermi}-LAT data of each individual source separately to search for a significant detection. In addition, we perform the first stacking analysis of this class of sources in order to investigate the gamma-ray emission of the young radio sources that are undetected at high energies. We report the detection of significant gamma-ray emission from 11 young radio sources, including the discovery of significant gamma-ray emission from the compact radio galaxy PKS 1007+142. Although the stacking analysis of below-threshold young radio sources does not result in a significant detection, it provides stringent upper limits to constrain the gamma-ray emission from these objects. In this talk we present the results of our study and we discuss their implications for the predictions of gamma-ray emission from this class of sources.
The relativistic jets created by some active galactic nuclei are important agents of AGN feedback. In spite of this, our understanding of what produces these jets is still incomplete. X-ray observations, which can probe the processes operating in the central regions in immediate vicinity of the supermassive black hole, the presumed jet launching point, are potentially particularly valuable in illuminating the jet formation process. Here, we present the hard X-ray NuSTAR observations of the radio-loud quasar 4C 74.26 in a joint analysis with quasi-simultaneous, soft X-ray Swift observations. Our spectral analysis reveals a high-energy cut-off of 183$_{-35}^{+51}$ keV and confirms the presence of ionized reflection in the source. From the average spectrum we detect that the accretion disk is mildly recessed with an inner radius of $R_mathrm{in}=4-180,R_mathrm{g}$. However, no significant evolution of the inner radius is seen during the three months covered by our NuSTAR campaign. This lack of variation could mean that the jet formation in this radio-loud quasar differs from what is observed in broad-line radio galaxies.
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