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Register a new userExtended X-ray emission in PKS 1718$-$649
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PKS 1718$-$649 is one of the closest and most comprehensively studied candidates of a young active galactic nucleus (AGN) that is still embedded in its optical host galaxy. The compact radio structure, with a maximal extent of a few parsecs, makes it a member of the group of compact symmetric objects (CSO). Its environment imposes a turnover of the radio synchrotron spectrum towards lower frequencies, also classifying PKS 1718$-$649 as gigahertz-peaked radio spectrum (GPS) source. Its close proximity has allowed the first detection of extended X-ray emission in a GPS/CSO source with Chandra that is for the most part unrelated to nuclear feedback. However, not much is known about the nature of this emission. By co-adding all archival Chandra data and complementing these datasets with the large effective area of XMM-Newton, we are able to study the detailed physics of the environment of PKS 1718$-$649. Not only can we confirm that the bulk of the $lesssim$kiloparsec-scale environment emits in the soft X-rays, but we also identify the emitting gas to form a hot, collisionally ionized medium. While the feedback of the central AGN still seems to be constrained to the inner few parsecs, we argue that supernovae are capable of producing the observed large-scale X-ray emission at a rate inferred from its estimated star formation rate.
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We report the $gamma$-ray detection of a young radio galaxy, PKS 1718$-$649, belonging to the class of Compact Symmetric Objects (CSOs), with the Large Area Telescope (LAT) on board the {it Fermi} satellite. The third {it Fermi} Gamma-ray LAT catalog (3FGL) includes an unassociated $gamma$-ray source, 3FGL J1728.0$-$6446, located close to PKS 1718$-$649. Using the latest Pass 8 calibration, we confirm that the best fit $1 sigma$ position of the $gamma$-ray source is compatible with the radio location of PKS 1718$-$649. Cross-matching of the $gamma$-ray source position with the positions of blazar sources from several catalogs yields negative results. Thus, we conclude that PKS 1718$-$649 is the most likely counterpart to the unassociated LAT source. We obtain a detection test statistics TS$sim 36$ ($>$5$sigma$) with a best fit photon spectral index $Gamma=$2.9$pm$0.3 and a 0.1-100 GeV photon flux density $F_{rm 0.1-100GeV}=$(11.5$pm$0.3)$times{rm 10^{-9}}$ ph cm$^{-2}$ s$^{-1}$. We argue that the linear size ($sim$2 pc), the kinematic age ($sim$100 years), and the source distance ($z=0.014$) make PKS 1718$-$649 an ideal candidate for $gamma$-ray detection in the framework of the model proposing that the most compact and the youngest CSOs can efficiently produce GeV radiation via inverse-Compton scattering of the ambient photon fields by the radio lobe non-thermal electrons. Thus, our detection of the source in $gamma$-rays establishes young radio galaxies as a distinct class of extragalactic high-energy emitters, and yields an unique insight on the physical conditions in compact radio lobes interacting with the interstellar medium of the host galaxy.
Using the new wideband capabilities of the Australia Telescope Compact Array (ATCA), we obtain spectra for PKS 1718-649, a well-known gigahertz-peaked spectrum radio source. The observations, between approximately 1 and 10 GHz over three epochs spanning approximately 21 months, reveal variability both above the spectral peak at ~3 GHz and below the peak. The combination of the low and high frequency variability cannot be easily explained using a single absorption mechanism, such as free-free absorption or synchrotron self-absorption. We find that the PKS 1718-649 spectrum and its variability are best explained by variations in the free-free optical depth on our line-of-sight to the radio source at low frequencies (below the spectral peak) and the adiabatic expansion of the radio source itself at high frequencies (above the spectral peak). The optical depth variations are found to be plausible when X-ray continuum absorption variability seen in samples of Active Galactic Nuclei is considered. We find that the cause of the peaked spectrum in PKS 1718-649 is most likely due to free-free absorption. In agreement with previous studies, we find that the spectrum at each epoch of observation is best fit by a free-free absorption model characterised by a power-law distribution of free-free absorbing clouds. This agreement is extended to frequencies below the 1 GHz lower limit of the ATCA by considering new observations with Parkes at 725 MHz and 199 MHz observations with the newly operational Murchison Widefield Array. These lower frequency observations argue against families of absorption models (both free-free and synchrotron self-absorption) that are based on simple homogenous structures.
We present new Australia Telescope Compact Array (ATCA) observations of the young (< 10^2 years) radio galaxy PKS B1718-649. We study the morphology and the kinematics of the neutral hydrogen (HI) disk (M(HI) = 1.1x 10^10 M(sun), radius ~ 30 kpc). In particular, we focus on the analysis of the cold gas in relation to the triggering of the nuclear activity. The asymmetries at the edges of the disk date the last interaction with a companion to more than 1 Gyr ago. The tilted-ring model of the HI disk shows that this event may have formed the disk as we see it now, but that it may have not been responsible for triggering the AGN. The long timescales of the interaction are incompatible with the short ones of the radio activity. In absorption, we identify two clouds with radial motions which may represent a population that could be involved in the triggering of the radio activity. We argue that PKS B1718-649 may belong to a family of young low-excitation radio AGN where, rather than through a gas rich merger, the active nuclei (AGN) are triggered by local mechanisms such as accretion of small gas clouds.
We report NuSTAR observations of a sample of six X-ray weak broad absorption line (BAL) quasars. These targets, at z=0.148-1.223, are among the optically brightest and most luminous BAL quasars known at z<1.3. However, their rest-frame 2 keV luminosities are 14 to >330 times weaker than expected for typical quasars. Our results from a pilot NuSTAR study of two low-redshift BAL quasars, a Chandra stacking analysis of a sample of high-redshift BAL quasars, and a NuSTAR spectral analysis of the local BAL quasar Mrk 231 have already suggested the existence of intrinsically X-ray weak BAL quasars, i.e., quasars not emitting X-rays at the level expected from their optical/UV emission. The aim of the current program is to extend the search for such extraordinary objects. Three of the six new targets are weakly detected by NuSTAR with <45 counts in the 3-24 keV band, and the other three are not detected. The hard X-ray (8-24 keV) weakness observed by NuSTAR requires Compton-thick absorption if these objects have nominal underlying X-ray emission. However, a soft stacked effective photon index ({Gamma}~1.8) for this sample disfavors Compton-thick absorption in general. The uniform hard X-ray weakness observed by NuSTAR for this and the pilot samples selected with <10 keV weakness also suggests that the X-ray weakness is intrinsic in at least some of the targets. We conclude that the NuSTAR observations have likely discovered a significant population (>33%) of intrinsically X-ray weak objects among the BAL quasars with significantly weak <10 keV emission. We suggest that intrinsically X-ray weak quasars might be preferentially observed as BAL quasars.
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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