Systematic variation in the apparent burning area of thermonuclear bursts and its implication for neutron star radius measurement

Precision measurements of neutron star radii can provide a powerful probe of the properties of cold matter beyond nuclear density. Beginning in the late 1970s it was proposed that the radius could be obtained from the apparent or inferred emitting area during the decay portions of thermonuclear (type I) X-ray bursts. However, this apparent area is generally not constant, preventing reliable measurement of the source radius. Here we report for the first time a correlation between the variation of the inferred area and the burst properties, measured in a sample of almost 900 bursts from 43 sources. We found that the rate of change of the inferred area during decay is anticorrelated with the burst decay duration. A Spearman rank correlation test shows that this relation is significant at the <10^{-45} level for our entire sample, and at the 7x10^{-37} level for the 625 bursts without photospheric radius expansion. This anticorrelation is also highly significant for individual sources exhibiting a wide range of burst durations, such as 4U 1636-536 and Aql X-1. We suggest that variations in the colour factor, which relates the colour temperature resulted from the scattering in the neutron star atmosphere to the effective temperature of the burning layer, may explain the correlation. This in turn implies significant variations in the composition of the atmosphere between bursts with long and short durations.Comment: 8 pages, 4 figures, accepted for publication in MNRA

Infrared Candidates for the Intense Galactic X-ray Source GX 17+2

We present new astrometric solutions and infrared Hubble Space Telescope observations of GX 17+2 (X1813-140), one of the brightest X-ray sources on the celestial sphere. Despite 30 years of intensive study, and the existence of a strong radio counterpart with a sub-arcsecond position, the object remains optically unidentified. The observed X-ray characteristics strongly suggest that it is a so-called "Z-source," the rare but important category that includes Sco X-1 and Cyg X-2. Use of the USNO-A2.0 catalog enables us to measure the position of optical and infrared objects near the radio source to sub-arcsecond precision within the International Celestial Reference Frame, for direct comparison with the radio position, which we also recompute using modern calibrators. With high confidence we eliminate the V~17.5 star NP Ser, often listed as the probable optical counterpart of the X-ray source, as a candidate. Our HST NICMOS observations show two faint objects within our 0.5" radius 90% confidence error circle. Even the brighter of the two, Star A, is far fainter than expected (H~19.8), given multiple estimates of the extinction in this field and our previous understanding of Z sources, but it becomes the best candidate for the counterpart of GX 17+2. The probability of a chance coincidence of an unrelated faint object on the radio position is high. However, if the true counterpart is not Star A, it is fainter still, and our conclusion that the optical counterpart is surprisingly underluminous is but strengthened.Comment: 15 pages including 3 figures and 3 tables. Accepted for publication in The Astrophysical Journa

A variable near-infrared counterpart to the neutron-star low-mass X-ray binary 4U 1705-440

We report the discovery of a near-infrared (nIR) counterpart to the persistent neutron-star low-mass X-ray binary 4U 1705-440, at a location consistent with its recently determined Chandra X-ray position. The nIR source is highly variable, with K_s-band magnitudes varying between 15.2 and 17.3 and additional J- and H-band observations revealing color variations. A comparison with contemporaneous X-ray monitoring observations shows that the nIR brightness correlates well with X-ray flux and X-ray spectral state. We also find possible indications for a change in the slope of the nIR/X-ray flux relation between different X-ray states. We discuss and test various proposed mechanisms for the nIR emission from neutron-star low-mass X-ray binaries and conclude that the nIR emission in 4U 1705-440 is most likely dominated by X-ray heating of the outer accretion disk and the secondary star.Comment: Accepted for publication in Ap

Accretion rate and the occurrence of multi-peaked X-ray bursts

Most Type I X-ray bursts from accreting neutron stars have a lightcurve with a single peak, but there is a rare population of faint bursts that are double or even triple peaked. Suggested mechanisms include polar ignition with equatorial stalling, or multi-step energy release; the latter being caused by hydrodynamic instabilities or waiting points in the nuclear reaction sequence. We present an analysis of the accretion rate dependence of the multi-peak bursts, and discuss the consequences for the various models. The observations pose particular challenges for the polar ignition mechanism given current models of ignition latitude dependence.Comment: 5 pages, 4 figures, accepted for publication in A&A Letter

Models of Type I X-ray Bursts from GS 1826-24: A Probe of rp-Process Hydrogen Burning

The X-ray burster GS 1826-24 shows extremely regular Type I X-ray bursts whose energetics and recurrence times agree well with thermonuclear ignition models. We present calculations of sequences of burst lightcurves using multizone models which follow the rp-process nucleosynthesis with an extensive nuclear reaction network. The theoretical and observed burst lightcurves show remarkable agreement. The models naturally explain the slow ~5s rise and long ~100s tails of these bursts, as well as their dependence on mass accretion rate. This comparison provides further evidence for solar metallicity in the accreted material in this source, and constrains the distance to the source. The main difference is that the observed lightcurves do not show the distinct two-stage rise of the models. This may reflect the time for burning to spread over the stellar surface, or may indicate that our treatment of heat transport or nuclear physics needs to be revised. The trends in burst properties with accretion rate are well-reproduced by our spherically symmetric models which include chemical and thermal inertia from the ashes of previous bursts. Changes in the covering fraction of the accreted fuel are not required.Comment: 5 pages, 4 figures, to appear in ApJ letter

Systematic Uncertainties in the Spectroscopic Measurements of Neutron-Star Masses and Radii from Thermonuclear X-ray Bursts. I. Apparent Radii

The masses and radii of low-magnetic field neutron stars can be measured by combining different observable quantities obtained from their X-ray spectra during thermonuclear X-ray bursts. One of these quantities is the apparent radius of each neutron star as inferred from the X-ray flux and spectral temperature measured during the cooling tails of bursts, when the thermonuclear flash is believed to have engulfed the entire star. In this paper, we analyze 13,095 X-ray spectra of 446 X-ray bursts observed from 12 sources in order to assess possible systematic effects in the measurements of the apparent radii of neutron stars. We first show that the vast majority of the observed X-ray spectra are consistent with blackbody functions to within a few percent. We find that most X-ray bursts follow a very well determined correlation between X-ray flux and temperature, which is consistent with the whole neutron-star surface emitting uniformly during the cooling tails. We develop a Bayesian Gaussian mixture algorithm to measure the apparent radii of the neutron stars in these sources, while detecting and excluding a small number of X-ray bursts that show irregular cooling behavior. This algorithm also provides us with a quantitative measure of the systematic uncertainties in the measurements. We find that those errors in the spectroscopic determination of neutron-star radii that are introduced by systematic effects in the cooling tails of X-ray bursts are in the range $\simeq 3-8$%. Such small errors are adequate to distinguish between different equations of state provided that uncertainties in the distance to each source and the absolute calibration of X-ray detectors do not dominate the error budget.Comment: accepted for publication in the February 2012 issue of the Astrophysical Journa

A comparison of neutron star blackbody luminosities in LMXB with the theory of accretion flow spreading on the stellar surface

We present a comparison of the results of the ASCA survey of LMXB with the Inogamov and Sunyaev theory of accretion flow spreading on the surface of neutron stars. The ASCA survey of LMXB of Church and Balucinska-Church (2001) revealed a systematic variation of the luminosity of blackbody emission from the neutron star spanning 3 decades in total X-ray luminosity suggesting that the level of blackbody emission is controlled by the physics of the inner disk/stellar interface, which we can hope to understand. Two types of explanation exist: firstly that there is radial flow between the inner disk and star at all vertical positions above the orbital plane so that the height of the disk directly determines the area of star emitting. Secondly, the height of the emitting region on the star is not directly related to the disk properties but depends on the mass accretion rate as suggested by Inogamov and Sunyaev (1999) in their theory of accretion flow spreading on the stellar surface. We find that the survey results for the emitting area agree with this theory at the lowest luminosities. However, for higher luminosities, the blackbody emission is stronger than predicted by spreading theory suggesting that the emitting area is controlled by radial flow between disk and star.Comment: 8 pages, 6 ps figures; Astron. and Astrophysics in pres