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Register a new userSuzaku Observations of Luminous Quasars: Revealing the Nature of High-Energy Blazar Emission in Quiescent States
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We present the results from the Suzaku X-ray observations of five flat-spectrum radio quasars (FSRQs), namely PKS0208-512, Q0827+243, PKS1127-145, PKS1510-089 and 3C 454.3. All these sources were additionally monitored simultaneously or quasi-simultaneously by the Fermi satellite in gamma-rays and the Swift UVOT in the UV and optical bands, respectively. We constructed their broad-band spectra covering the frequency range from 10^14 Hz up to 10^25 Hz, and those reveal the nature of high-energy emission of luminous blazars in their low-activity states. The analyzed X-ray spectra are well fitted by a power-law model with photoelectric absorption. In the case of PKS0208-512, PKS1127-145, and 3C 454.3, the X-ray continuum showed indication of hard-ening at low-energies. Moreover, when compared with the previous X-ray observations, we see a significantly increasing contribution of low-energy photons to the total X-ray fluxes when the sources are getting fainter. The same behavior can be noted in the Suzaku data alone. A likely explanation involves a variable, flat-spectrum component produced via inverse-Compton (IC) emission, plus an additional, possibly steady soft X-ray component prominent when the source gets fainter. This soft X-ray excess is represented either by a steep powerlaw (photon indices Gamma ~ 3 - 5) or a blackbody-type emission with temperatures kT ~ 0.1-0.2 keV. We model the broad-band spectra spectra of the five observed FSRQs using synchrotron self-Compton (SSC) and/or external-Compton radiation (ECR) models. Our modeling suggests that the difference between the low and high-activity states in luminous blazars is due to the different total kinetic power of the jet, most likely related to varying bulk Lorentz factor of the outflow within the blazar emission zone.
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The latest flare of the regular $sim$ 12 years quasi-periodic optical outbursts in the binary SMBH candidate system OJ 287 occurred in December 2015. Following this, the source has exhibited enhanced multi-wavelength (MW) variability in spectral, temporal and polarization domains with new features never seen before. Our MW investigation show that the overall MW variability can be divided into two-phase, (i) November 2015 -- May 2016 with variability from near-infrared (NIR) to Fermi-LAT $rm gamma$-ray energies (0.1 -- 300 GeV), and (ii) September 2016 -- July 2017 with intense NIR to X-ray variability but without any activity in the Fermi-LAT band, and the very first detection at very high energies (VHEs, E $>$ 100 GeV) by VERITAS. The broadband SEDs during the first phase show a thermal bump in the NIR-optical region and a hardening in the $rm gamma$-ray spectra with a shift in its peak. The thermal bump like feature is consistent with the description of the standard accretion-disk associated with the primary SMBH of mass $sim 1.8times10^{10} M_odot$ while the $rm gamma$-ray emission can be naturally reproduced by inverse Compton scattering of photons from the broad line region which has been seen during the close encounter duration of the binary SMBHs, thereby suggesting a sub-parsec scale origin. The SEDs during the second phase (VHE detection) is a mixture of typical OJ 287 SED with hardened $rm gamma$-ray spectra and an HBL SED and can be explained in a two-zone model, one located at sub-parsec scales and other at parsec scales. During both the phases, the MW variability is simultaneous and almost always accompanied by changes in the polarization properties, exhibiting random and systematic variations, suggesting a strong role of magnetic field and turbulence.
We study the linear polarization of the radio cores of eight blazars simultaneously at 22, 43, and 86 GHz with observations obtained by the Korean VLBI Network (KVN) in three epochs between late 2016 and early 2017 in the frame of the Plasma-physics of Active Galactic Nuclei (PAGaN) project. We investigate the Faraday rotation measure (RM) of the cores; the RM is expected to increase with observing frequency if core positions depend on frequency due to synchrotron self-absorption. We find a systematic increase of RMs at higher observing frequencies in our targets. The RM--$ u$ relations follow power-laws with indices distributed around 2, indicating conically expanding outflows serving as Faraday rotating media. Comparing our KVN data with contemporaneous optical polarization data from the Steward Observatory for a few sources, we find indication that the increase of RM with frequency saturates at frequencies of a few hundreds GHz. This suggests that blazar cores are physical structures rather than simple $tau=1$ surfaces. A single region, e.g. a recollimation shock, might dominate the jet emission downstream of the jet launching region. We detect a sign change in the observed RMs of CTA 102 on a time scale of $approx$1 month, which might be related to new superluminal components emerging from its core undergoing acceleration/deceleration and/or bending. We see indication for quasars having higher core RMs than BL Lac objects, which could be due to denser inflows/outflows in quasars.
We use joint observations by the Neil Gehrels Swift X-ray Telescope (XRT) and the Fermi Large Area Telescope (LAT) of gamma-ray burst (GRB) afterglows to investigate the nature of the long-lived high-energy emission observed by Fermi LAT. Joint broadband spectral modeling of XRT and LAT data reveal that LAT nondetections of bright X-ray afterglows are consistent with a cooling break in the inferred electron synchrotron spectrum below the LAT and/or XRT energy ranges. Such a break is sufficient to suppress the high-energy emission so as to be below the LAT detection threshold. By contrast, LAT-detected bursts are best fit by a synchrotron spectrum with a cooling break that lies either between or above the XRT and LAT energy ranges. We speculate that the primary difference between GRBs with LAT afterglow detections and the non-detected population may be in the type of circumstellar environment in which these bursts occur, with late-time LAT detections preferentially selecting GRBs that occur in low wind-like circumburst density profiles. Furthermore, we find no evidence of high-energy emission in the LAT-detected population significantly in excess of the flux expected from the electron synchrotron spectrum fit to the observed X-ray emission. The lack of excess emission at high energies could be due to a shocked external medium in which the energy density in the magnetic field is stronger than or comparable to that of the relativistic electrons behind the shock, precluding the production of a dominant synchrotron self-Compton (SSC) component in the LAT energy range. Alternatively, the peak of the SSC emission could be beyond the 0.1-100 GeV energy range considered for this analysis.
PKS 2155-304 is a blazar located in the Southern Hemisphere, monitored with the High Energy Stereoscopic System (H.E.S.S.) at very high energy (VHE, E>100 GeV) $gamma$ rays every year since 2002. Thanks to the large data set collected in the VHE range and simultaneous coverage in optical, ultraviolet (UV), X-ray and high energy $gamma$-ray ranges, this object is an excellent laboratory to study spectral and temporal variability in blazars. However, despite many years of dense monitoring, the nature of the variability observed in PKS 2155-304 remains puzzling. In this paper, we discuss the complex spectral and temporal variability observed in PKS 2155-304. The data discussed include VHE $gamma$-ray data collected with H.E.S.S. between 2013 and 2016, complemented with multiwavelength (MWL) observations from Fermi-LAT, Swift-XRT, Swift-UVOT, SMARTS, and the ATOM telescope. During the period of monitoring, PKS 2155-304 was transitioning from its lower state to the flaring states, and exhibiting different flavors of outbursts. For the first time, orphan optical flare lasting a few months was observed. Correlation studies show an indication of correlation between the X-ray and VHE $gamma$-ray fluxes. Interestingly, a comparison of optical and X-ray or VHE $gamma$-ray fluxes does not show global correlation. However, two distinct tracks in the diagram were found, which correspond to the different flaring activity states of PKS 2155-304.
We discuss a probe of the contribution of wind-related shocks to the radio emission in otherwise radio-quiet quasars. Given 1) the non-linear correlation between UV and X-ray luminosity in quasars, 2) that such correlation leads to higher likelihood of radiation-line-driven winds in more luminous quasars, and 3) that luminous quasars are more abundant at high redshift, deep radio observations of high-redshift quasars are needed to probe potential contributions from accretion disk winds. We target a sample of 50 $zsimeq 1.65$ color-selected quasars that span the range of expected accretion disk wind properties as traced by broad CIV emission. 3-GHz observations with the Very Large Array to an rms of $approx10mu$Jy beam$^{-1}$ probe to star formation rates of $approx400,M_{rm Sun},{rm yr}^{-1}$, leading to 22 detections. Supplementing these pointed observations are survey data of 388 sources from the LOFAR Two-metre Sky Survey Data Release 1 that reach comparable depth (for a typical radio spectral index), where 123 sources are detected. These combined observations reveal a radio detection fraction that is a non-linear function of civ emission-line properties and suggest that the data may require multiple origins of radio emission in radio-quiet quasars. We find evidence for radio emission from weak jets or coronae in radio-quiet quasars with low Eddingtion ratios, with either (or both) star formation and accretion disk winds playing an important role in optically luminous quasars and correlated with increasing Eddington ratio. Additional pointed radio observations are needed to fully establish the nature of radio emission in radio-quiet quasars.
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