Models of neutron star atmospheres enriched with nuclear burning ashes

Low-mass X-ray binaries hosting neutron stars (NS) exhibit thermonuclear (type-I) X-ray bursts, which are powered by unstable nuclear burning of helium and/or hydrogen into heavier elements deep in the NS "ocean". In some cases the burning ashes may rise from the burning depths up to the NS photosphere by convection, leading to the appearance of the metal absorption edges in the spectra, which then force the emergent X-ray burst spectra to shift toward lower energies. These effects may have a substantial impact on the color correction factor $f_c$ and the dilution factor $w$, the parameters of the diluted blackbody model $F_E \approx w B_E(f_c T_{eff})$ that is commonly used to describe the emergent spectra from NSs. The aim of this paper is to quantify how much the metal enrichment can change these factors. We have developed a new NS atmosphere modeling code, which has a few important improvements compared to our previous code required by inclusion of the metals. The opacities and the internal partition functions (used in the ionization fraction calculations) are now taken into account for all atomic species. In addition, the code is now parallelized to counter the increased computational load. We compute a detailed grid of atmosphere models with different exotic chemical compositions that mimic the presence of the burning ashes. From the emerging model spectra we compute the color correction factors $f_c$ and the dilution factors $w$ that can then be compared to the observations. We find that the metals may change $f_c$ by up to about 40%, which is enough to explain the scatter seen in the blackbody radius measurements. The presented models open up the possibility for determining NS mass and radii more accurately, and may also act as a tool to probe the nuclear burning mechanisms of X-ray bursts.Comment: 14 pages, 7 figures, to be published in A&

The Magellanic system X-ray sources

Using archival X-ray data from the second XMM-Newton serendipitous source catalogue, we present comparative analysis of the overall population of X-ray sources in the Large and Small Magellanic Clouds. We see a difference between the characteristics of the brighter sources in the two populations in the X-ray band. Utilising flux measurements in different energy bands we are able to sort the X-ray sources based on similarities to other previously identified and classified objects. In this manner we are able to identify the probable nature of some of the unknown objects, identifying a number of possible X-ray binaries and Super Soft Sources.Comment: 4 pages, 2 figures. Poster to appear in proceedings of IAU Symposium 256, The Magellanic System: Stars, Gas, and Galaxies. Keele Univeristy, U

The 2009 outburst of accreting millisecond pulsar IGR J17511-3057 as observed by SWIFT and RXTE

The twelfth accretion-powered millisecond pulsar, IGR J17511-3057, was discovered in September 2009. In this work we study its spectral and timing properties during the 2009 outburst based on Swift and RXTE data. Our spectral analysis of the source indicates only slight spectral shape evolution during the entire outburst. The equivalent width of the iron line and the apparent area of the blackbody emission associated with the hotspot at the stellar surface both decrease significantly during the outburst. This is consistent with a gradual receding of the accretion disc as the accretion rate drops. The pulse profile analysis shows absence of dramatic shape evolution with a moderate decrease in pulse amplitude. This behaviour might result from a movement of the accretion column footprint towards the magnetic pole as the disc retreats. The time lag between the soft and the hard energy pulses increase by a factor of two during the outburst. A physical displacement of the centroid of the accretion shock relative to the blackbody spot or changes in the emissivity pattern of the Comptonization component related to the variations of the accretion column structure could cause this evolution. We have found that IGR J17511-3057 demonstrates outburst stages similar to those seen in SAX J1808.4-3658. A transition from the "slow decay" into the "rapid drop" stage, associated with the dramatic flux decrease, is also accompanied by a pulse phase shift which could result from an appearance of the secondary spot due to the increasing inner disc radius.Comment: 12 pages, 10 figures, MNRAS, in press. Title correcte

Rapid late-time X-ray brightening of the tidal disruption event OGLE16aaa

Stars that pass too close to a super-massive black hole may be disrupted by strong tidal forces. OGLE16aaa is one such tidal disruption event (TDE) which rapidly brightened and peaked in the optical/UV bands in early 2016 and subsequently decayed over the rest of the year. OGLE16aaa was detected in an XMM-Newton X-ray observation on June 9, 2016 with a flux slightly below the Swift/XRT upper limits obtained during the optical light curve peak. Between June 16-21, 2016, Swift/XRT also detected OGLE16aaa and based on the stacked spectrum, we could infer that the X-ray luminosity had jumped up by more than a factor of ten in just one week. No brightening signal was seen in the simultaneous optical/UV data to cause the X-ray luminosity to exceed the optical/UV one. A further XMM-Newton observation on November 30, 2016 showed that almost a year after the optical/UV peak, the X-ray emission was still at an elevated level, while the optical/UV flux decay had already leveled off to values comparable to those of the host galaxy. In all X-ray observations, the spectra were nicely modeled with a 50-70 eV thermal component with no intrinsic absorption, with a weak X-ray tail seen only in the November 30 XMM-Newton observation. The late-time X-ray behavior of OGLE16aaa strongly resembles the tidal disruption events ASASSN-15oi and AT2019azh. We were able to pinpoint the time delay between the initial optical TDE onset and the X-ray brightening to $182 \pm 5$ days, which may possibly represent the timescale between the initial circularization of the disrupted star around the super-massive black hole and the subsequent delayed accretion. Alternatively, the delayed X-ray brightening could be related to a rapid clearing of a thick envelope that covers the central X-ray engine during the first six months.Comment: 8 pages, 2 figures, Accepted for publication in A&

The influence of accretion geometry on the spectral evolution during thermonuclear (type-I) X-ray bursts

Neutron star (NS) masses and radii can be estimated from observations of photospheric radius-expansion X-ray bursts, provided the chemical composition of the photosphere, the spectral colour-correction factors in the observed luminosity range, and the emission area during the bursts are known. By analysing 246 X-ray bursts observed by the Rossi X-ray Timing Explorer from 11 low-mass X-ray binaries, we find a dependence between the persistent spectral properties and the time evolution of the black body normalisation during the bursts. All NS atmosphere models predict that the colour-correction factor decreases in the early cooling phase when the luminosity first drops below the limiting Eddington value, leading to a characteristic pattern of variability in the measured blackbody normalisation. However, the model predictions agree with the observations for most bursts occurring in hard, low-luminosity, &#39;island&#39; spectral states, but rarely during soft, high-luminosity, &#39;banana&#39; states. The observed behaviour may be attributed to the accretion flow, which influences cooling of the NS preferentially during the soft state bursts. This result implies that only the bursts occurring in the hard, low-luminosity spectral states can be reliably used for NS mass and radius determination.</p

Spectral variability of ultraluminous X-ray sources

We study spectral variability of 11 ultraluminous X-ray sources (ULX) using archived XMM-Newton and Chandra observations. We use three models to describe the observed spectra; a power-law, a multi-colour disk (MCD) and a combination of these two models. We find that out of the 11 ULXs in our sample, 7 ULXs show a correlation between the luminosity and the photon index Gamma (hereafter L-Gamma correlation). Furthermore, out of the 7 ULXs that have the L-Gamma correlation, 4 ULXs also show spectral pivoting in the observed energy band. We also find that two ULXs show an L-Gamma anti-correlation. The spectra of 4 ULXs in the sample can be adequately fitted with a MCD model. We compare these sources to known black hole binaries (BHB) and find that they follow similar paths in their luminosity-temperature (hereafter L-T) diagrams. Finally we show that the 'soft excess' reported for many of these ULXs at 0.2 keV seem to follow a trend L \propto T^{-4} when modeled with a power-law plus a 'cool' MCD model. This is contrary to the expected L \propto T^4 relation that is expected from theory and what is seen for many accreting BHBs.Comment: 9 pages, 3 figures; to appear in the proceedings of "Cool discs, hot flows: The Varying Faces of Accreting Compact Objects", Funasdalen, Sweden, 2008, ed. M. Axelsson (New York: AIP