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Register a new userChandra observations of five X-ray transient galactic nuclei
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We report on exploratory Chandra observations of five galactic nuclei that were found to be X-ray bright during the ROSAT all-sky survey (with L_X > 10^43 erg s^-1) but subsequently exhibited a dramatic decline in X-ray luminosity. Very little is known about the post-outburst X-ray properties of these enigmatic sources. In all five cases Chandra detects an X-ray source positionally coincident with the nucleus of the host galaxy. The spectrum of the brightest source (IC 3599) appears consistent with a steep power-law (Gamma~3.6). The other sources have too few counts to extract individual, well-determined spectra, but their X-ray spectra appear flatter (Gamma~2) on average. The Chandra fluxes are ~10^2-10^3 fainter than was observed during the outburst (up to 12 years previously). That all post-outburst X-ray observations showed similarly low X-ray luminosities is consistent with these sources having `switched to a persistent low-luminosity state. Unfortunately the relative dearth of long-term monitoring and other data mean that the physical mechanism responsible for this spectacular behaviour is still highly unconstrained.
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We report the results of a programme of dual-epoch Chandra ACIS-S observations of five ultraluminous X-ray sources (ULXs) in nearby spiral galaxies. All five ULXs are detected as unresolved, point-like X-ray sources by Chandra, though two have faded below the 10^39 erg/s luminosity threshold used to first designate these sources as ULXs. Using this same criterion, we detect three further ULXs within the imaged regions of the galaxies. The ULXs appear to be related to the star forming regions of the galaxies, indicating that even in ``normal spiral galaxies the ULX population is predominantly associated with young stellar populations. A detailed study of the Chandra ACIS-S spectra of six of the ULXs shows that five are better described by a powerlaw continuum than a multi-colour disc blackbody model, though there is evidence for additional very soft components to two of the powerlaw continua. The measured photon indices in four out of five cases are consistent with the low/hard state in black hole binaries, contrary to the suggestion that powerlaw-dominated spectra of ULXs originate in the very high state. A simple interpretation of this is that we are observing accretion onto intermediate-mass black holes, though we might also be observing a spectral state unique to very high mass accretion rates in stellar-mass black hole systems. Short-term flux variability is only detected in one of two epochs for two of the ULXs, with the lack of this characteristic arguing that the X-ray emission of this sample of ULXs is not dominated by relativistically-beamed jets. The observational characteristics of this small sample suggest that ULXs are a distinctly heterogeneous source class.
We present the results of Chandra X-ray Observatory observations of the transient anomalous X-ray pulsar (AXP) candidate AX J1845.0-0258 in apparent quiescence. Within the sources error circle, we find a point source and possible counterpart, which we designate CXOU J184454.6-025653. No coherent pulsations are detected, and no extended emission is seen. The sources spectrum is equally well described by a blackbody model of temperature kT~2.0 keV or a power law model with photon index Gamma~1.0. This is considerably harder than was seen for AX J1845.0-0258 during its period of brightening in 1993 (kT~0.6 keV) despite being at least ~13 times fainter. This behavior is opposite to that observed in the case of the established transient AXP, XTE J1810-197. We therefore explore the possibility that CXOU J184454.6-025653 is an unrelated source, and that AX J1845.0-0258 remains undetected since 1993, with flux 260-430 times fainter than at that epoch. If so, this would represent an unprecedented range of variability in AXPs.
We briefly review the synergy between X-ray and infrared observations for Active Galactic Nuclei (AGNs) detected in cosmic X-ray surveys, primarily with XMM-Newton, Chandra, and NuSTAR. We focus on two complementary aspects of this X-ray-infrared synergy (1) the identification of the most heavily obscured AGNs and (2) the connection between star formation and AGN activity. We also briefly discuss future prospects for X-ray-infrared studies over the next decade.
X-ray variation is a ubiquitous feature of active galactic nuclei (AGNs), however, its origin is not well understood. In this paper, we show that the X-ray flux variations in some AGNs, and correspondingly the power spectral densities (PSDs) of the variations, may be interpreted as being caused by absorptions of eclipsing clouds or clumps in the broad line region (BLR) and the dusty torus. By performing Monte-Carlo simulations for a number of plausible cloud models, we systematically investigate the statistics of the X-ray variations resulting from the cloud eclipsing and the PSDs of the variations. For these models, we show that the number of eclipsing events can be significant and the absorption column densities due to those eclipsing clouds can be in the range from 10^{21} to 10^{24} cm^{-2}, leading to significant X-ray variations. We find that the PSDs obtained from the mock observations for the X-ray flux and the absorption column density resulting from these models can be described by a broken double power law, similar to those directly measured from observations of some AGNs. The shape of the PSDs depend strongly on the kinematic structures and the intrinsic properties of the clouds in AGNs. We demonstrate that the X-ray eclipsing model can naturally lead to a strong correlation between the break frequencies (and correspondingly the break timescales) of the PSDs and the masses of the massive black holes (MBHs) in the model AGNs, which can be well consistent with the one obtained from observations. Future studies of the PSDs of the AGN X-ray (and possibly also the optical-UV) flux and column density variations may provide a powerful tool to constrain the structure of the BLR and the torus and to estimate the MBH masses in AGNs.
We present X-ray and optical analysis of 188 AGN identified from 497 hard X-ray (f (2.0-8.0 keV) > 2.7x10^-15 erg cm^-2 s^-1) sources in 20 Chandra fields (1.5 deg^2) forming part of the Chandra Multi-wavelength Project. These medium depth X-ray observations enable us to detect a representative subset of those sources responsible for the bulk of the 2-8 keV Cosmic X-ray Background. Brighter than our optical spectroscopic limit, we achieve a reasonable degree of completeness (77% of X-ray sources with counter-parts r< 22.5 have been classified): broad emission line AGN (62%), narrow emission line galaxies (24%), absorption line galaxies (7%), stars (5%) or clusters (2%). We find that most X-ray unabsorbed AGN (NH<10^22 cm^-2) have optical properties characterized by broad emission lines and blue colors, similiar to optically-selected quasars from the Sloan Digital Sky Survey but with a slighly broader color distribution. However, we also find a significant population of redder (g-i>1.0) AGN with broad optical emission lines. Most of the X-ray absorbed AGN (10^22<NH<10^24 cm^-2) are associated with narrow emission line galaxies, with red optical colors characteristically dominated by luminous, early type galaxy hosts rather than from dust reddening of an AGN. We also find a number of atypical AGN; for instance, several luminous AGN show both strong X-ray absorption (NH>10^22 cm^-2) and broad emission lines. Overall, we find that 81% of X-ray selected AGN can be easily interpreted in the context of current AGN unification models. Most of the deviations seem to be due to an optical contribution from the host galaxies of the low luminosity AGN.
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