Low-luminosity X-ray sources and the Galactic ridge X-ray emission

Using the XMM-Newton Slew Survey, we construct a hard-band selected sample of low-luminosity Galactic X-ray sources. Two source populations are represented, namely coronally-active stars and binaries (ASBs) and cataclysmic variables (CVs), with X-ray luminosities collectively spanning the range 10^(28-34) erg/s (2-10 keV). We derive the 2-10 keV X-ray luminosity function (XLF) and volume emissivity of each population. Scaled to the local stellar mass density, the latter is found to be 1.08 +/- 0.16 x 10^28 erg/s/M and 2.5 +/- 0.6 x 10^27 erg/s/M, for the ASBs and CVs respectively, which in total is a factor 2 higher than previous estimates. We employ the new XLFs to predict the X-ray source counts on the Galactic plane at l = 28.5 deg and show that the result is consistent with current observational constraints. The X-ray emission of faint, unresolved ASBs and CVs can account for a substantial fraction of the Galactic ridge X-ray emission (GRXE). We discuss a model in which roughly 80 per cent of the 6-10 keV GRXE intensity is produced in this way, with the remainder attributable to X-ray scattering in the interstellar medium and/or young Galactic source populations. Much of the hard X-ray emission attributed to the ASBs is likely to be produced during flaring episodes

XMM-Newton observations of the Galactic Centre Region - II: The soft thermal emission

We have extended our earlier study (Heard & Warwick 2013, Paper I) of the X-ray emission emanating from the central 100 pc x 100 pc region of our Galaxy to an investigation of several features prominent in the soft X-ray (2-4.5 keV) band. We focus on three specific structures: a putative bipolar outflow from the vicinity of Sgr A*; a high surface brightness region located roughly 12 arcmin to the north-east of Sgr A*; and a lower surface-brightness extended loop feature seen to the south of Sgr A*. We show that all three structures are thermal in nature and have similar temperatures (kT ~ 1 keV). The inferred X-ray luminosities lie in the range (2 - 10) x 10^34 erg s^-1. In the case of the bipolar feature we suggest that the hot plasma is produced by the shock-heating of the winds from massive stars within the Central Cluster, possibly collimated by the Circumnuclear Disc. Alternatively the outflow may be driven by outbursts on Sgr A*, which follow tidal disruption events occurring at a rate of roughly 1 every 4000 yr. The north-east enhancement is centred on a candidate PWN which has a relatively hard non-thermal X-ray spectrum. We suggest that the coincident soft-thermal emission traces the core of a new thermal-composite supernova remnant, designated as SNR G0.13-0.12. There is no clear evidence for an associated radio shell but such a feature may be masked by the bright emission of the nearby Radio Arc and other filamentary structures. SNR G0.13-0.12 is very likely interacting with the nearby molecular cloud, G0.11-0.11, and linked to the Fermi source, 2FGL J1746.4-2851c. Finally we explore a previous suggestion that the elliptically-shaped X-ray loop to the south of Sgr A*, of maximum extent ~45 pc, represents the shell of a superbubble located in the GC region. Although plausible, the interpretation of this feature in terms a coherent physical structure awaits confirmation.Comment: 17 pages, 6 figures, accepted for publication in MNRA

On characterising the variability properties of X-ray light curves from active galaxies

We review some practical aspects of measuring the amplitude of variability in `red noise' light curves typical of those from Active Galactic Nuclei (AGN). The quantities commonly used to estimate the variability amplitude in AGN light curves, such as the fractional rms variability amplitude, F_var, and excess variance, sigma_XS^2, are examined. Their statistical properties, relationship to the power spectrum, and uses for investigating the nature of the variability processes are discussed. We demonstrate that sigma_XS^2 (or similarly F_var) shows large changes from one part of the light curve to the next, even when the variability is produced by a stationary process. This limits the usefulness of these estimators for quantifying differences in variability amplitude between different sources or from epoch to epoch in one source. Some examples of the expected scatter in the variance are tabulated for various typical power spectral shapes, based on Monte Carlo simulations. The excess variance can be useful for comparing the variability amplitudes of light curves in different energy bands from the same observation. Monte Carlo simulations are used to derive a description of the uncertainty in the amplitude expected between different energy bands (due to measurement errors). Finally, these estimators are used to demonstrate some variability properties of the bright Seyfert 1 galaxy Markarian 766. The source is found to show a strong, linear correlation between rms amplitude and flux, and to show significant spectral variability.Comment: 14 pages. 12 figures. Accepted for publication in MNRA

X-ray spectral complexity in narrow-line Seyfert 1 galaxies

We present a systematic analysis of the X-ray spectral properties of a sample of 22 ``narrow-line'' Seyfert 1 galaxies for which data are available from the ASCA public archive. Many of these sources, which were selected on the basis of their relatively narrow H-beta line width (FWHM <= 2000 km/s), show significant spectral complexity in the X-ray band. Their measured hard power-law continua have photon indices spanning the range 1.6 - 2.5 with a mean of 2.1, which is only slightly steeper than the norm for ``broad-line'' Seyfert 1s. All but four of the sources exhibit a soft excess, which can be modelled as blackbody emission (T_{bb} ~ 100 - 300 eV) superposed on the underlying power-law. This soft component is often so strong that, even in the relatively hard bandpass of ASCA, it contains a significant fraction, if not the bulk, of the X-ray luminosity, apparently ruling out models in which the soft excess is produced entirely through reprocessing of the hard continuum. Most notably, 6 of the 22 objects show evidence for a broad absorption feature centred in the energy range 1.1 - 1.4 keV, which could be the signature of resonance absorption in highly ionized material. A further 3 sources exhibit ``warm absorption'' edges in the 0.7 - 0.9 keV bandpass. Remarkably, all 9 ``absorbed'' sources have H-beta line widths below 1000 km/s, which is less than the median value for the sample taken as a whole. This tendency for very narrow line widths to correlate with the presence of ionized absorption features in the soft X-ray spectra of NLS1s, if confirmed in larger samples, may provide a further clue in the puzzle of active galactic nuclei.Comment: 13 pages, 5 figures, accepted for publication in MNRA

The XMM-Newton slew survey in the 2-10 keV band

The XMM-Newton Slew Survey (XSS) covers a significant fraction of the sky in a broad X-ray bandpass. Although shallow by contemporary standards, in the `classical' 2-10 keV band of X-ray astronomy, the XSS provides significantly better sensitivity than any currently available all-sky survey. We investigate the source content of the XSS, focussing on detections in the 2-10 keV band down to a very low threshold (> 4 counts net of background). At the faint end, the survey reaches a flux sensitivity of roughly 3e-12 erg/cm2/s (2-10 keV). Our starting point was a sample of 487 sources detected in the XMMSL1d2 XSS at high galactic latitude in the hard band. Through cross-correlation with published source catalogues from surveys spanning the electromagnetic spectrum from radio to gamma-rays, we find that 45% of the sources have likely identifications with normal/active galaxies, 18% are associated with other classes of X-ray object (nearby coronally active stars, accreting binaries, clusters of galaxies), leaving 37% of the XSS sources with no current identification. We go on to define an XSS extragalactic hard band sample comprised of 219 galaxies and active galaxies. We investigate the properties of this extragalactic sample including its X-ray logN-logS distribution. We find that in the low-count limit, the XSS is strongly affected by Eddington bias. There is also a very strong bias in the XSS against the detection of extended sources, most notably clusters of galaxies. A significant fraction of the detections at and around the low-count limit may be spurious. Nevertheless, it is possible to use the XSS to extract a reasonably robust sample of extragalactic sources, excluding galaxy clusters. The differential logN-logS relation of these extragalactic sources matches very well to the HEAO-1 A2 all-sky survey measurements at bright fluxes and to the 2XMM source counts at the faint end.Comment: 16 pages, 13 figures, FITS table of XSS extragalactic sample available from http://www.star.le.ac.uk/~amr30/Slew

Fe Ka line emission from the Arches cluster region - evidence for ongoing particle bombardment?

We present the results of eight years of XMM-Newton observations of the region surrounding the Arches cluster in the Galactic Center. We study the spatial distribution and temporal behaviour of the Fe-Ka line emission with the objective of identifying the likely source of the excitation. We investigate the variability of the 6.4-keV line emission of four clouds through spectral fitting of the EPIC MOS data with the use of a modelled background, which avoids many of the systematics inherent in local background subtraction. We also employ spectral stacking of both EPIC PN and MOS data to search for evidence of an Fe-K edge feature imprinted on the underlying X-ray continuum. The lightcurves of the Fe-Ka line from three bright molecular knots close to the Arches cluster are found to be constant over the 8-year observation window. West of the cluster, however, we found a bright cloud exhibiting the fastest Fe-Ka variability yet seen in a molecular cloud in the Galactic Center region. The time-averaged spectra of the molecular clouds reveal no convincing evidence of the 7.1-keV edge feature. The EW of the 6.4-keV line emitted by the clouds near the cluster is found to be ~1.0 keV. The observed Fe-Ka line flux and the high EW suggest the fluorescence has a photoionization origin, although excitation by cosmic-ray particles is not specifically excluded. For the three clouds nearest to the cluster, an identification of the source of photo-ionizing photons with an earlier outburst of Sgr A* is however at best tentative. The hardness of the nonthermal component associated with the 6.4-keV line emission might be best explained in terms of bombardment by cosmic-ray particles from the Arches cluster itself. The relatively short-timescale variability seen in the 6.4-keV line emission from the cloud to the West of the cluster is most likely the result of illumination by a nearby transient X-ray source.Comment: 13 pages, 6 figures, accepted for publication in Astronomy and Astrophysic

The Galactic plane at faint X-ray fluxes - II. Stacked X-ray spectra of a sample of serendipitous XMM-Newton sources

We have investigated the X-ray spectral properties of a sample of 138 X-ray sources detected serendipitously in $XMM-Newton$ observations of the Galactic plane, at an intermediate to faint flux level. We divide our sample into 5 subgroups according to the spectral hardness of the sources, and stack (i.e. co-add) the individual source spectra within each subgroup. As expected these stacked spectra show a softening trend from the hardest to the softest subgroups, which is reflected in the inferred line-of-sight column density. The spectra of the three hardest subgroups are characterized by a hard continuum plus superimpose Fe-line emission in the 6--7 keV bandpass. The average equivalent width (EW) of the 6.7-keV He-like Fe-K$\alpha$ line is 170$^{+35}_{-32}$ eV, whereas the 6.4-keV Fe-K fluorescence line from neutral iron and the 6.9-keV H-like Fe-Ly$\alpha$ line have EWs of 89$^{+26}_{-25}$ eV and 81$^{+30}_{-29}$ eV respectively, i.e. roughly half that of the 6.7-keV line. The remaining subgroups exhibit soft thermal spectra. Virtually all of the spectrally-soft X-ray sources can be associated with relatively nearby coronally-active late-type stars, which are evident as bright near-infrared (NIR) objects within the X-ray error circles. On a similar basis only a minority of the spectrally-hard X-ray sources have likely NIR identifications. The average continuum and Fe-line properties of the spectrally-hard sources are consistent with those of magnetic cataclysmic variables but the direct identification of large numbers of such systems in Galactic X-ray surveys, probing intermediate to faint flux levels, remains challenging.Comment: 14 pages, 9 figures, 4 tables, accepted for publication in MNRA