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Register a new userAwakening of The High Redshift Blazar CGRaBS J0809+5341
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Vaidehi Sharan Paliya Mr.
Publication date
2015
fields
Physics
and research's language is
English
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CGRaBS J0809+5341, a high redshift blazar at z = 2.144, underwent a giant optical outburst on 2014 April 19 when it brightened by $sim$5 mag and reached an unfiltered apparent magnitude of 15.7 mag. This implies an absolute magnitude of -30.5 mag, making it one of the brightest quasars in the Universe. This optical flaring triggered us to carry out observations during the decaying part of the flare covering a wide energy range using the {it Nuclear Spectroscopic Telescope Array}, {it Swift}, and ground based optical facilities. For the first time, the source is detected in $gamma$-rays by the Large Area Telescope onboard the {it Fermi Gamma-Ray Space Telescope}. A high optical polarization of $sim$10% is also observed. Using the Sloan Digital Sky Survey spectrum, accretion disk luminosity and black hole mass are estimated as $1.5 times 10^{45}$ erg s$^{-1}$ and $10^{8.4}~M_{odot}$ respectively. Using a single zone leptonic emission model, we reproduce the spectral energy distribution of the source during the flaring activity. This analysis suggests that the emission region is probably located outside the broad line region, and the jet becomes radiatively efficient. We also show that the overall properties of CGRaBS J0809+5341 seems not to be in agreement with the general properties observed in high redshift blazars up to now.
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We analyse Very Long Baseline Interferometry (VLBI) observations of the blazar CGRaBS J0809+5341 using Bayesian inference methods. The observation was carried out at 5 GHz using 8 telescopes that form part of the European VLBI Network. Imaging and deconvolution using traditional methods imply that the blazar is unresolved. To search for source structure beyond the diffraction limit, we perform Bayesian model selection between three source models (point, elliptical Gaussian, and circular Gaussian). Our modelling jointly accounts for antenna-dependent gains and system equivalent flux densities. We obtain posterior distributions for the various source and instrumental parameters along with the corresponding uncertainties and correlations between them. We find that there is very strong evidence (>1e9 :1) in favour of elliptical Gaussian structure and using this model derive the apparent brightness temperature distribution of the blazar, accounting for uncertainties in the shape estimates. To test the integrity of our method, we also perform model selection on synthetic observations and use this to develop a Bayesian criterion for the minimum resolvable source size and consequently the maximum measurable brightness temperature for a given interferometer, dependent on the signal-to-noise ratio (SNR) of the data incorporating the aforementioned systematics. We find that calibration errors play an increasingly important role in determining the over-resolution limit for SNR>>100. We show that it is possible to exploit the resolving power of future VLBI arrays down to about 5 per cent of the size of the naturally-weighted restoring beam, if the gain calibration is precise to 1 per cent or less.
High-redshift ($z>2$) blazars are the most powerful members of the blazar family. Yet, only a handful of them have both X-ray and $gamma$-ray detection, thereby making it difficult to characterize the energetics of the most luminous jets. Here, we report, for the first time, the Fermi-Large Area Telescope detection of the significant $gamma$-ray emission from the high-redshift blazar DA 193 ($z=2.363$). Its time-averaged $gamma$-ray spectrum is soft ($gamma$-ray photon index = $2.9pm0.1$) and together with a relatively flat hard X-ray spectrum (14$-$195 keV photon index = $1.5pm0.4$), DA 193 presents a case to study a typical high-redshift blazar with inverse Compton peak being located at MeV energies. An intense GeV flare was observed from this object in the first week of 2018 January, a phenomenon rarely observed from high-redshift sources. What makes this event a rare one is the observation of an extremely hard $gamma$-ray spectrum (photon index = $1.7pm0.2$), which is somewhat unexpected since high-redshift blazars typically exhibit a steep falling spectrum at GeV energies. The results of our multi-frequency campaign, including both space- (Fermi, NuSTAR, and Swift) and ground-based (Steward and Nordic Optical Telescope) observatories, are presented and this peculiar $gamma$-ray flare is studied within the framework of a single-zone leptonic emission scenario.
Our goal is to study the termination of an AGN jet in the young universe and to deduce physical parameters of the jet and the intergalactic medium. We use LOFAR to image the long-wavelength radio emission of the high-redshift blazar S5 0836+710 on arcsecond scales between 120 MHz and 160 MHz. The LOFAR image shows a compact unresolved core and a resolved emission region about 1.5 arcsec to the southwest of the radio core. This structure is in general agreement with previous higher-frequency radio observations with MERLIN and the VLA. The southern component shows a moderately steep spectrum with a spectral index of about $gtrsim -1$ while the spectral index of the core is flat to slightly inverted. In addition, we detect for the first time a resolved steep-spectrum halo with a spectral index of about $-1$ surrounding the core. The arcsecond-scale radio structure of S5 0836+710 can be understood as an FR II-like radio galaxy observed at a small viewing angle. The southern component can be interpreted as the region of the approaching jets terminal hotspot and the halo-like diffuse component near the core can be interpreted as the counter-hotspot region. From the differential Doppler boosting of both features, we can derive the hotspot advance speed to $(0.01-0.036)$ c. At a constant advance speed, the derived age of the source would exceed the total lifetime of such a powerful FR II-like radio galaxy substantially. Thus, the hotspot advance speed must have been higher in the past in agreement with a scenario in which the originally highly relativistic jet has lost collimation due to the growth of instabilities and has transformed into an only mildly relativistic flow. Our data suggest that the density of the intergalactic medium around this distant ($z=2.22$) AGN could be substantially higher than the values typically found in less distant FR II radio galaxies.
The flat spectrum radio quasar (FSRQ) PKS 2123-463 was associated in the First Fermi-LAT source catalog with the gamma-ray source 1FGL J2126.1-4603, but when considering the full first two years of Fermi observations, no gamma-ray source at a position consistent with this FSRQ was detected, and thus PKS 2123-463 was not reported in the Second Fermi-LAT source catalog. On 2011 December 14 a gamma-ray source positionally consistent with PKS 2123-463 was detected in flaring activity by Fermi-LAT. This activity triggered radio-to-X-ray observations by the Swift, GROND, ATCA, Ceduna, and KAT-7 observatories. Results of the localization of the gamma-ray source over 41 months of Fermi-LAT operation are reported here in conjunction with the results of the analysis of radio, optical, UV and X-ray data collected soon after the gamma-ray flare. The strict spatial association with the lower energy counterpart together with a simultaneous increase of the activity in optical, UV, X-ray and gamma-ray bands led to a firm identification of the gamma-ray source with PKS 2123-463. A new photometric redshift has been estimated as z = 1.46+/-0.05 using GROND and Swift/UVOT observations, in rough agreement with the disputed spectroscopic redshift of z = 1.67. We fit the broadband spectral energy distribution with a synchrotron/external Compton model. We find that a thermal disk component is necessary to explain the optical/UV emission detected by Swift/UVOT. This disk has a luminosity of about 1.8x10^46 erg/s, and a fit to the disk emission assuming a Schwarzschild (i.e., nonrotating) black hole gives a mass of about 2x10^9 solar masses. This is the first black hole mass estimate for this source.
We present the results of a multi-frequency, time-averaged analysis of blazars included in the Candidate Gamma-ray Blazar Survey catalog. Our sample consists of 324 $gamma$-ray detected ($gamma$-ray loud) and 191 non $gamma$-ray detected ($gamma$-ray quiet) blazars, and we consider all the data up to 2016 April 1. We find that both the $gamma$-ray loud and the $gamma$-ray quiet blazar populations occupy similar regions in the WISE color-color diagram, and in the radio and X-ray bands $gamma$-ray loud sources are brighter. A simple one-zone synchrotron inverse-Compton emission model is applied to derive the physical properties of both populations. We find that the central black hole mass and the accretion disk luminosity ($L_{rm disk}$) computed from the modeling of the optical-UV emission with a Shakura-Sunyaev disk reasonably matches with that estimated from the optical spectroscopic emission-line information. A significantly larger Doppler boosting in the $gamma$-ray loud blazars is noted, and their jets are more radiatively efficient. On the other hand, the $gamma$-ray quiet objects are more MeV-peaked, thus could be potential targets for next-generation MeV missions. Our results confirm the earlier findings about the accretion-jet connection in blazars; however, many of the $gamma$-ray quiet blazars tend to deviate from the recent claim that the jet power exceeds $L_{rm disk}$ in blazars. A broadband study, considering a larger set of $gamma$-ray quiet objects and also including BL Lacs, will be needed to confirm/reject this hypothesis and also to verify the evolution of the powerful high-redshift blazars into their low-power nearby counterparts.
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