Abstract
We report the results of joint Chandra/ACIS—NuSTAR deep observations of NGC 1167, the host galaxy of the young radio jet B2 0258+35. In the ACIS data, we detect X-ray emission, extended both ...along and orthogonal to the jet. At the end of the southeast radio jet, we find lower-energy X-ray emission that coincides with a region of CO turbulence and fast outflow motions. This suggests that the hot interstellar medium (ISM) may be compressed by the jet and molecular outflow, resulting in more efficient cooling. Hydrodynamic simulations of jet–ISM interaction tailored to NGC 1167 are in agreement with this conclusion and with the overall morphology and spectra of the X-ray emission. The faint hard nuclear source detected with Chandra and the stringent NuSTAR upper limits on the harder X-ray emission show that the active galactic nucleus (AGN) in NGC 1167 is in a very low-accretion state. However, the characteristics of the extended X-ray emission are more consonant to those of luminous Compton-thick (CT) AGNs, suggesting that we may be observing the remnants of a past high accretion rate episode, with sustained strong activity lasting ∼2 × 10
3
yr. We conclude that NGC1167 is presently a Low-Ionization Nuclear Emission-line Region (LINER) , but was an AGN in the past, given the properties of the extended X-ray emission and their similarity with those of CT AGN extended emission.
Abstract
We report the detection of extended X-ray emission from two high-redshift radio quasars. These quasars, J1405+0415 at
z
= 3.208 and J1610+1811 at
z
= 3.118, were observed in a Chandra ...snapshot survey selected from a complete sample of the radio-brightest quasars in the overlap area of the VLA-FIRST radio survey and the Sloan Digital Sky Survey. The extended X-ray emission is located along the line connecting the core to a radio knot or hotspot, favoring the interpretation of X-ray jets. The inferred rest-frame jet X-ray luminosities from 2 to 30 keV would be of order 10
45
erg s
−1
if emitted isotropically and without relativistic beaming. In the scenario of inverse Compton scattering of the cosmic microwave background (CMB), X-ray jets without a coincident radio counterpart may be common, and should be readily detectable to redshifts even beyond 3.2 due to the (1+
z
)
4
increase of the CMB energy density compensating for the (1+
z
)
−4
cosmological diminution of surface brightness. If these can be X-ray confirmed, they would be the second and third examples of quasar X-ray jets without detection of underlying continuous radio jets.
We present a systematic analysis of the mid-infrared (MIR) properties of the youngest radio galaxies, based on low-resolution data provided by the Wide-field Infrared Survey Explorer and IRAS ...satellites. We restrict our analysis to sources with available X-ray data that constitute the earliest phase of radio galaxy evolution, i.e., those classified as gigahertz-peaked spectrum and/or compact symmetric objects. In our sample of 29 objects, we find that the host galaxies are predominantly red/yellow ellipticals, with some of them displaying distorted morphology. We find a variety of MIR colors and observe that the sources in which the MIR emission is dominated by the ISM component uniformly populate the region occupied by galaxies with a wide range of pronounced (≥0.5M yr−1) star formation activity. We compare the MIR color distribution in our sample to that in the general population of local active galactic nuclei (AGNs), in the population of evolved FR II radio galaxies, and also in the population of radio galaxies with recurrent jet activity. We conclude that the triggering of radio jets in AGNs does not differentiate between elliptical hosts with substantially different fractions of young stars; instead, there is a relationship between the jet duty cycle and the ongoing star formation. The distribution of the subsample of our sources with z < 0.4 on the low-resolution MIR versus absorption-corrected X-ray luminosity plane is consistent with the distribution of a sample of local AGNs. Finally, we comment on the star formation rates of the two γ-ray-detected sources in our sample, 1146+596 and 1718-649.
Stimulated by recent observations of a giant radio-to-X-ray synchrotron flare from Hubble Space Telescope (HST)-1, the innermost knot of the M 87 jet, as well as by the detection of a very high ...energy γ-ray emission from M 87, we investigated the dynamics and multiwavelength emission of the HST-1 region. We study thermal pressure of the hot interstellar medium in M 87 and argue for the presence of a gaseous condensation in its central parts. We postulate that this additional feature is linked to the observed central stellar cusp of the elliptical host. Interaction of the jet with such a feature is likely to result in the formation of a stationary converging/diverging reconfinement/reflected shock structure in the innermost parts of the M 87 jet. We show that for a realistic set of the outflow parameters, a stationary and a flaring part of the HST-1 knot located ∼100 pc away from the active centre can be associated with the decelerated portion of the jet matter placed immediately downstream of the point where the reconfinement shock reaches the jet axis. We discuss a possible scenario explaining a broad-band brightening of the HST-1 region related to the variable activity of the central core. In particular, we show that assuming a previous epoch of the high central black hole activity resulting in ejection of excess particles and photons down along the jet, one may first expect a high-energy flare of HST-1 due to inverse-Comptonization of the nuclear radiation, followed after a few years by an increase in the synchrotron continuum of this region. The synchrotron flare itself could be accompanied by a subsequent inverse-Compton brightening due to upscattering of the ambient (mostly starlight) photons. If this is the case, then the recently observed order-of-magnitude increase in the knot luminosity in all spectral bands could be regarded as an unusual echo of the order-of-magnitude outburst that had happened previously (and could be eventually observed some ∼40 yr ago) in the highly relativistic active core of the M 87 radio galaxy. We show that very high energy γ-ray fluxes expected in a framework of the proposed scenario are consistent with the observed ones.
We present an analysis of the new, deep (94 ksec) Chandra ACIS-S observation of radio-loud active galaxy CGCG 292−057, characterized by a LINER-type nucleus and a complex radio structure that ...indicates intermittent jet activity. On the scale of the host galaxy bulge, we detected excess X-ray emission with a spectrum best fit by a thermal plasma model with a temperature of ∼0.8 keV. We argue that this excess emission results from compression and heating of the hot diffuse fraction of the interstellar medium displaced by the expanding inner, ∼20 kpc-scale lobes observed in this restarted radio galaxy. The nuclear X-ray spectrum of the target clearly displays an ionized iron line at ∼6.7 keV, and is best fitted with a phenomenological model consisting of a power-law (photon index 1.8) continuum absorbed by a relatively large amount of cold matter (hydrogen column density 0.7 × 1023 cm−2), and partly scattered (fraction ∼3%) by ionized gas, giving rise to a soft excess component and K line from iron ions. We demonstrate that the observed X-ray spectrum, particularly the equivalent width of Fe XXV K (of order 0.3 keV) can in principle, be explained in a scenario involving a Compton-thin gas located at the scale of the broad-lined region in this source and photoionized by nuclear illumination. We compare the general spectral properties of the CGCG 292−057 nucleus, with those of other nearby LINERs studied in X-rays.
We report on the first X-ray Chandra observations of a sample of seven low-luminosity compact sources, which belong to a class of young compact steep spectrum (CSS) radio sources. Four of these have ...been detected, while the other three have upper limit estimations for X-ray flux; one CSS galaxy is associated with an X-ray cluster. We have used the new observations, together with the observational data for known strong CSS and gigahertz-peaked spectrum (GPS) objects and large-scale Fanaroff-Riley types I and II objects (FR I and II), to study the relation between morphology, X-ray properties and excitation modes in radio-loud active galactic nuclei (AGNs). We have found the following. (i) The low-power objects fit well with the already established X-ray-radio luminosity correlation for AGNs and occupy the space among FR I objects, which are weaker in X-rays. (ii) The high-excitation and low-excitation galaxies occupy a distinct locus in the radio/X-ray luminosity plane, notwithstanding their evolutionary stage. This is in agreement with the postulated different origins of the X-ray emission in these two groups of objects. (iii) We have tested the AGN evolution models by comparing the radio/X-ray luminosity ratio with the size of the sources and, indirectly, with their age. We conclude that the division for two different X-ray emission modes, which originate in the base of the relativistic jet (FR Is) or in the accretion disc (FR IIs) is already present among the younger compact AGNs. (iv) Finally, we have found that the CSS sources are less obscured than the more compact GPS objects in X-rays. However, the anticorrelation between X-ray column density and radio size does not hold for the whole sample of GPS and CSS objects.
Abstract
The radio source 1146+596 is hosted by the elliptical/S0 galaxy NGC 3894, with a low-luminosity active nucleus. The radio structure is compact, suggesting a very young age for the jets in ...the system. Recently, the source has been confirmed as a high-energy (HE; >0.1 GeV)
γ
-ray emitter in the most recent accumulation of Fermi Large Area Telescope data. Here we report on the analysis of the archival Chandra X-ray Observatory data for the central part of the galaxy, consisting of a single 40 ks long exposure. We have found that the core spectrum is best fitted by a combination of an ionized thermal plasma with a temperature of ≃0.8 keV, and a moderately absorbed power-law component (photon index Γ = 1.4 ± 0.4, hydrogen column density
N
H
/10
22
cm
−2
= 2.4 ± 0.7). We have also detected the iron K
α
line at 6.5 ± 0.1 keV, with a large equivalent width of
1.0
−
0.5
+
0.9
keV. Based on the simulations of Chandra's point-spread function, 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 kiloparsec 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
γ
-rays with the iron K
α
line.
Abstract
In 2015, a radio transient named Cygnus A-2 was discovered in Cygnus A with the Very Large Array. Because of its radio brightness (νFν ≈ 6 × 1039 erg s−1), this transient likely represents a ...secondary black hole in orbit around the active galactic nucleus. Using Chandra ACIS observations from 2015 to 2017, we have looked for an X-ray counterpart to Cygnus A-2. The separation of 0.42 arcsec means that Cygnus A-2 cannot be spatially resolved, but by comparing the data with simulated marx data, we put an upper limit to the 2–10 keV X-ray luminosity of Cygnus A-2 of 1 × 1043 erg s−1. Using the Fundamental Plane for accreting black holes, we find that our upper limit to the X-ray flux of Cygnus A-2 in 2015–2017 disfavours the interpretation of Cygnus A-2 as a steadily accreting black hole. We suggest instead that Cygnus A-2 is the radio afterglow of a tidal disruption event (TDE) and that a peak in the 2–10 keV luminosity of the nuclear region in 2013, when it was observed by Swift and NuSTAR, is X-ray emission from the TDE. A TDE could naturally explain the X-ray light curve of the nuclear region, as well as the appearance of a short-lived, fast, and ionized outflow previously detected in the 2013 NuSTAR spectrum. Both the radio and X-ray luminosities fall in between typical luminosities for ‘thermal’ and ‘jetted’ TDE types, suggesting that Cygnus A-2 would be unlike previously seen TDEs.
We investigate the contribution of the beamed jet component to the high-energy emission in young and compact extragalactic radio sources, focusing for the first time on the gamma -ray band. We derive ...predictions on the gamma -ray luminosities associated with the relativistic jet assuming a leptonic radiative model. The high-energy emission is produced via Compton scattering by the relativistic electrons in a spherical region at the considered scales (lap 10 kpc). Simulations show a wide range of gamma -ray luminosities, with intensities up to ~10 super(46)-10 super(48) erg s super(-1) depending on the assumed jet parameters. We find a highly linear relation between the simulated X-ray and gamma -ray luminosities that can be used to select candidates for gamma -ray detection. We compare the simulated luminosity distributions in the radio, X-ray, and gamma -ray regimes with observations for the largest sample of X-ray-detected young radio quasars. Our analysis of ~4-yr Fermi Large Area Telescope (LAT) data does not yield any statistically significant detections. However, the majority of the model-predicted gamma -ray fluxes for the sample are near or below the current Fermi-LAT flux threshold and compatible with the derived upper limits. Our study gives constraints on the minimum jet power (L sub(jet,kin)/L sub(disk) > 0.01 ) of a potential jet contribution to the X-ray emission in the most compact sources (<, ~ 1 kpc) and on the particle-to-magnetic field energy density ratio that are in broad agreement with equipartition assumptions.
Abstract
We present Chandra X-ray and Very Large Array radio observations of the radio galaxy 3C 305. The X-ray observations reveal the details of the previously known extended X-ray halo around the ...radio galaxy. We show using X-ray spectroscopy that the X-ray emission is consistent with being shock-heated material and can be modelled with standard collisional-ionization models, rather than being photoionized by the active nucleus. On this basis, we can make a self-consistent model in which the X-ray-emitting plasma is responsible for the depolarization of some regions of the radio emission from the jets and hotspots, and to place lower and upper limits on the magnetic field strength in the depolarizing medium. On the assumption that the X-ray-emitting material, together with the previously known extended emission-line region and the outflow in neutral hydrogen, is all being driven out of the centre of the galaxy by an interaction with the jets, we derive a detailed energy budget for the radio galaxy, showing that the X-ray-emitting gas dominates the other phases in terms of its energy content. The power supplied by the jets must be ∼1043 erg s−1.
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