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Register a new userCanonical X-Ray Fluorescence Line Intensities as Column Density Indicators
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X-ray line fluorescence is ubiquitous around powerful accretion sources, namely active galactic nuclei and X-ray binaries. The brightest and best-studied line is the Fe K$alpha$ line at $lambda = 1.937$AA (6.4,keV). This paper presents a survey of all well-measured Chandra/HETG grating spectra featuring several K$alpha$ fluorescence lines from elements between Mg and Ni. Despite the variety of sources and physical conditions, we identify a common trend that dictates the K$alpha$ line intensity ratios between elements. For the most part, the line intensities are well described by a simple, plane-parallel approximation of a near-neutral, solar-abundance, high column density ($N_{textrm{H}} > 10^{24}$ cm$^{-2}$) medium. This approximation gives canonical photon-intensity line ratios for the K$alpha$ fluorescence of all elements, e.g., 0.104:,0.069:,1.0:,0.043 for Si:,S:,Fe:,Ni, respectively. Deviations from these ratios are shown to be primarily due to excess column density along the line of sight beyond the Galactic column. Therefore, measured fluorescence line ratios provide an independent estimate of $N_{textrm{H}}$ and insight into the environment of accretion sources. Residual discrepancies with the canonical ratios could be due to a variety of effects such as a fluorescing medium with $N_{textrm{H}} < 10^{24}$,cm$^{-2}$, a non-neutral medium, variations in the illuminating spectrum, non-solar abundances, or an irregular source geometry. However, evidently and perhaps surprisingly, these are uncommon, and their effect remains minor.
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Be X-ray binaries (BeXRBs) consist of rapidly rotating Be stars with neutron star companions accreting from the circumstellar emission disk. We compare the observed population of BeXRBs in the Small Magellanic Cloud with simulated populations of BeXRB-like systems produced with the COMPAS population synthesis code. We focus on the apparently higher minimal mass of Be stars in BeXRBs than in the Be population at large. Assuming that BeXRBs experienced only dynamically stable mass transfer, their mass distribution suggests that at least 30% of the mass donated by the progenitor of the neutron star is typically accreted by the B-star companion. We expect these results to affect predictions for the population of double compact object mergers. A convolution of the simulated BeXRB population with the star formation history of the Small Magellanic Cloud shows that the excess of BeXRBs is most likely explained by this galaxys burst of star formation around 20--40 Myr ago.
We remark on the utility of an observational relation between the absorption column density in excess of the Galactic absorption column density, $Delta N_{rm H} = N_{rm H, fit} - N_{rm H, gal}$, and redshift, z, determined from all 55 Swift-observed long bursts with spectroscopic redshifts as of 2006 December. The absorption column densities, $N_{rm H, fit}$, are determined from powerlaw fits to the X-ray spectra with the absorption column density left as a free parameter. We find that higher excess absorption column densities with $Delta N_{rm H} > 2times 10^{21}$ cm$^{-2}$ are only present in bursts with redshifts z$<$2. Low absorption column densities with $Delta N_{rm H} < 1times 10^{21}$ cm$^{-2}$ appear preferentially in high-redshift bursts. Our interpretation is that this relation between redshift and excess column density is an observational effect resulting from the shift of the source rest-frame energy range below 1 keV out of the XRT observable energy range for high redshift bursts. We found a clear anti-correlation between $Delta N_{rm H}$ and z that can be used to estimate the range of the maximum redshift of an afterglow. A critical application of our finding is that rapid X-ray observations can be used to optimize the instrumentation used for ground-based optical/NIR follow-up observations. Ground-based spectroscopic redshift measurements of as many bursts as possible are crucial for GRB science.
In high mass X-ray binaries (HMXBs), an accreting compact object orbits a high mass star which loses mass through a dense and inhomogeneous wind. Using the compact object as an X-ray backlight, the time variability of the absorbing column density in the wind can be exploited in order to shed light on the micro-structure of the wind and obtain unbiased stellar mass loss rates for high mass stars. We explore the impact of clumpiness on the variability of the column density with a simplified wind model. In particular, we focus on the standard deviation of the column density and the characteristic duration of enhanced absorption episodes, and compare them with analytical predictions based on the porosity length. We identified the favorable systems and orbital phases to determine the wind micro-structure. The coherence time scale of the column density is shown to be the self-crossing time of a clump in front of the compact object. We provide a recipe to get accurate measurements of the size and of the mass of the clumps, purely based on the observable time variability of the column density. The coherence time scale grants direct access to the size of the clumps while their mass can be deduced separately from the amplitude of the variability. If it is due to unaccreted passing-by clumps, the high column density variations in some HMXBs requires high mass clumps to reproduce the observed peak-to-peak amplitude and coherence time scales. These clump properties are hardly compatible with the ones derived from first principles. Alternatively, other components could contribute to the variability of the column density: larger orbital scale structures produced by a mechanism still to be identified, or a dense environment in the immediate vicinity of the accretor such as an accretion disk, an outflow or a spherical shell around the magnetosphere of the accreting neutron star.
We update a flux-limited complete sample of Swift-based SGRBs (SBAT4, DAvanzo et al. 2014), bringing it to 25 events and doubling its previous redshift range. We then evaluate the column densities of the events in the updated sample, in order to compare them with the NH distribution of LGRBs, using the sample BAT6ext (Arcodia et al. 2016). We rely on Monte Carlo simulations of the two populations and compare the computed NH distributions with a two sample Kolmogorov Smirnov (K-S) test. We then study how the K-S probability varies with respect to the redshift range we consider. We find that the K-S probability keeps decreasing as redshift increases until at z$sim$1.8 the probability that short and long GRBs come from the same parent distribution drops below 1$%$. This testifies for an observational difference among the two populations. This difference may be due to the presence of highly absorbed LGRBs above z$sim$1.3, which have not been observed in the SGRB sample yet, although this may be due to our inability to detect them, or to the relatively small sample size.
We present $NuSTAR$ X-ray observations of the active galactic nucleus (AGN) in NGC 7674. The source shows a flat X-ray spectrum, suggesting that it is obscured by Compton-thick gas columns. Based upon long-term flux dimming, previous work suggested the alternate possibility that the source is a recently switched-off AGN with the observed X-rays being the lagged echo from the torus. Our high-quality data show the source to be reflection-dominated in hard X-rays, but with a relatively weak neutral Fe K$alpha$ emission line (equivalent width [EW] of $approx$ 0.4 keV) and a strong Fe XXVI ionised line (EW $approx$ 0.2 keV). We construct an updated long-term X-ray light curve of NGC 7674 and find that the observed 2-10 keV flux has remained constant for the past $approx$ 20 years, following a high flux state probed by $Ginga$. Light travel time arguments constrain the minimum radius of the reflector to be $sim$ 3.2 pc under the switched-off AGN scenario, $approx$ 30 times larger than the expected dust sublimation radius, rendering this possibility unlikely. A patchy Compton-thick AGN (CTAGN) solution is plausible, requiring a minimum line-of-sight column density ($N_{rm H}$) of 3 $times$ 10$^{24}$ cm$^{-2}$ at present, and yields an intrinsic 2-10 keV luminosity of (3-5) $times$ 10$^{43}$ erg s$^{-1}$. Realistic uncertainties span the range of $approx$ (1-13) $times$ 10$^{43}$ erg s$^{-1}$. The source has one of the weakest fluorescence lines amongst {em bona fide} CTAGN, and is potentially a local analogue of bolometrically luminous systems showing complex neutral and ionised Fe emission. It exemplifies the difficulty of identification and proper characterisation of distant CTAGN based on the strength of the neutral Fe K$alpha$ line.
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