It's deja vu, all over again

Our recent discovery of 552 Hz burst oscillations in the well-known binary EXO 0748-676 was fairly unremarkable, except perhaps for the extreme rarity of the oscillations (detected in only 2 of ~160 bursts observed by RXTE). However, oscillations had already been detected in this source back in 2004, at 45 Hz. The 552 Hz signal is much stronger, recommending it as the neutron star spin; unfortunately, the Doppler broadening that would be expected by such a spin means that the narrow spectral features previously identified as arising from the neutron star surface, could not have. Our paper has been submitted to ApJL.

Read the paper (arXiv.org/0910.5546)

The apparent neutron-star radius during bursts

Measuring the radius of neutron stars is hard — ask anyone. (They're really small, and really far away; imagine trying to measure something the width of a human hair... on Pluto). The X-ray emission during the bursts seems to come from the entire surface, and is consistent with a blackbody, so it should be possible to infer the radius indirectly, but the apparent radius behaves in very unexpected ways — usually increasing or decreasing during the burst. Sudip Bhattacharyya & Cole Miller found a correlation between the slope of the radius evolution and the duration of the burst (a proxy for the burst fuel composition), based on the data from the RXTE burst catalog. Our paper was just accepted by MNRAS.

Read the paper (arXiv.org:0908:4245)
Astronomers get neutron star's measure @ ABC Science

Burst catalog paper accepted

The burst catalog paper has just been accepted by ApJS! The referee report, by a team of referees, stretched to 11 pages; it took quite a while to address all the (generally constructive) criticisms. In addition, the authors, all at MIT when the project started, are now scattered to the four corners of the US (as well as Australia). The accepted version is (I think) a big improvement on the original, the extra time spent has really helped our understanding of the global burst properties. Check it out for yourself at astro-ph/0608259v2

Click here to read the abstract and download the full version, as well as data tables

Biases for neutron-star mass, radius and distance measurements

Our paper on the unusually low "touchdown" fluxes for radius-expansion bursts from high-inclination sources was just accepted by MNRAS. Usually the touchdown flux is thought to equal the Eddington flux, but we found that in sources that show X-ray dips — likely arising from structure at the edge of the accretion disk passing across the line of sight, implying that we see these systems almost edge-on — the touchdown flux could be less than half the maximum flux seen earlier in the same burst. The low touchdown fluxes also likely arise from interactions with the disk material, which have some implications for neutron-star distance (but not mass and radius) determination following the method of Özel (2006)

Read the paper (arxiv.org:0712.0412)

The Clocked Burster is running fast

Summer 2007 was a big season for new results on GS 1826-24, the "Clocked Burster". UCSD student Tommy Thompson and I just submitted a new analysis of the X-ray flux–recurrence time relationship in this system. We found a few instances where the thermonuclear bursts — albeit still very regular — occurred more frequently that would be predicted by the relationship derived from a previous study. XMM-Newton observations during one of these episodes revealed the likely presence of an additional soft component, which may account for the "missing" flux. This source may also be useful in future for precision studies of the X-ray flux–accretion rate relationship, which can be measured precisely thanks to the regular bursts.

Read the (accepted) paper (arXiv:0712.3874)

Model lightcurves for bursts from GS 1826-24

Alex's paper comparing the observed burst lightcurves from GS 1826-24 with time-dependent models of nuclear burning is finally out. The correspondence is really remarkable, and confirms the solar composition of the accreted fuel, as we suspected from the 2004 paper. However, in that paper we found that the drop in recurrence time with increased flux was more than predicted by Andrew's ignition models; this discrepancy is resolved with the new modelling, which indicates that thermal/compositional inertia play a role in setting the ignition column.

Read the paper (arXiv:0711.1195)

The Variable Warm Absorber in Circinus X-1

Norbert's paper on the faint-state observations of Cir X-1 has just been accepted to ApJ. While the P-Cygni lines (present in earlier observations when the source was brighter) were absent, we found evidence for complex absorption effects, including warm (moderately ionized) absorbers with varying column density. This Chandra observation has proved popular lately, with another paper analysing the same data posted just weeks earlier.

Read the paper arXiv:0709.3336

An X-ray jet in Circinus X-1

Sebastian did well to spot the X-ray jet in one of Norbert's 2005 zero-phase observations of the enigmatic X-ray binary Cir X-1. This source is thus the first neutron star for which an extended X-ray jet has been detected. Previously ultrarelativistic arcsecond-scale radio jets were detected by Rob Fender's group; see their 2004 Nature paper. The kinetic jet power that we infer is significantly larger than the minimum power required for the jet to inflate the large-scale radio nebula.

Read the paper (ApJ 663, L93)

Fe Kα Lines in MCVs

Aimee's paper on the Fe Kα line profiles from the accretion columns of magnetic CVs was accepted by MNRAS last month. The paper extends Jason Cullen's nonlinear Monte-Carlo code to predict the degree of broadening expected from lines arising from close to the white dwarf. We used a Chandra observation of GK Per in outburst for comparison, and while the relative line strengths in the model is not set by the physical conditions, the results were promising.

Read the paper (astro-ph/0704.1516)

Intermittent pulsations in HETE J1900.1-2455

The Energizer Bunny of the accretion-powered millisecond pulsars is unusual not just because it has been active for more than a year since its discovery in June 2005 ("he's still goin'!"). Pulsations were only detected in the first few months of the outburst; since then, the source has been indistinguishable from an ordinary low-accretion rate LMXB. In all the other millisecond X-ray pulsars (and, for that matter, all other accretion-powered pulsars) pulsations are always present when the source is active. Our paper on the properties of the pulsations is out now at astro-ph/0609693.

Thermonuclear bursts observed by RXTE

The long-awaited catalog of bursts observed by RXTE has finally been accepted by ApJS, and is also out on astro-ph/0608259. The preprint version omits the extended figures; click below for a full abstract, more complete versions, and data tables.

>> Read more Abstract

We have assembled a sample of 1187 thermonuclear (type-I) X-ray bursts from observations of 48 accreting neutron stars by the Rossi X-ray Timing Explorer, spanning more than ten years. The sample contains examples of two of the three theoretical ignition regimes (confirmed via comparisons with numerical models) and likely examples of the third. We present a detailed analysis of the variation of the burst profiles, energetics, recurrence times, presence of photospheric radius expansion, and presence of burst oscillations, as a function of accretion rate.
We estimated the distance for 35 sources exhibiting radius-expansion bursts, and found that in general the peak flux of such bursts varies typically by 13%, We classified sources into two main groups based on the burst properties: both long and short bursts (indicating mixed H/He accretion), and consistently short bursts (primarily He accretion), and calculated the mean burst rate as a function of accretion rate for the two groups. The decrease in burst rate observed at 0.06 Mdot_Edd (>~2x10³⁷ erg/s) is associated with a transition in the persistent spectral state and (as has been suggested previously) may be related to the increasing role of steady He-burning. We found many examples of bursts with recurrence times <30 min, including burst triplets and even quadruplets.
We describe the oscillation amplitudes for 13 of the 16 burst oscillation sources, as well as the stages and properties of the bursts in which the oscillations are detected. The burst properties are correlated with the burst oscillation frequency; sources spinning at <400 Hz generally have consistently short bursts, while the more rapidly-spinning systems have both long and short bursts. This correlation suggests either that shear-mediated mixing dominates the burst properties, or alternatively that the nature of the mass donor (and hence the evolutionary history) has an influence on the long-term spin evolution.

Preprint with complete figures (10 Mb PDF)
ASCII version of the burst data (table 5 in the accepted version) + IDL template for use with READ_ASCII
ASCII version of table 9 (burst oscillation properties)

Helium-rich bursts and the distance to SAX J1808.4-3658

Our paper on the outburst and thermonuclear burst properties of SAX J1808.4-3658 was just accepted by ApJ. We used RXTE observations to constrain the distance, deriving a likely range of 3.4-3.6 kpc. We also compared the burst properties to Andrew C's ignition model to deduce the H-fraction at ignition; these bursts are the first confirmed He-rich bursts which have been studied in detail.
Read the paper

>> Full abstract Abstract:
We analysed Rossi X-ray Timing Explorer observations of the accretion-powered 401 Hz pulsar SAX J1808.4-3658, in order to precisely determine the source distance. While the fluences for the five transient outbursts observed from 1996 were constant to within the uncertainties, the outburst interval varied signficantly, so that the time-averaged flux (and accretion rate) decreased by around 40%. By equating the time-averaged X-ray flux with the expected mass transfer rate from gravitational radiation, we derived a lower limit on the distance of 3.4 kpc. Combined with an upper limit from assuming that the four radius-expansion thermonuclear bursts observed during the 2002 October outburst reached at most the Eddington limit for a pure He atmosphere, we found that the probable distance range for the source is 3.4-3.6 kpc. The implied inclination, based on the optical/IR properties of the counterpart, is i<~30 degrees.

We compared the properties of the bursts with an ignition model. The time between bursts was long enough for hot CNO burning to significantly deplete the accreted hydrogen, so that ignition occurred in a pure helium layer underlying a stable hydrogen burning shell. This is the first time that this burning regime has been securely observationally identified. The observed energetics of the bursts give a mean hydrogen fraction at ignition of approx. 0.1, and require that the accreted hydrogen fraction X_0 and the CNO metallicity Z_CNO are related by Z_CNO approx. 0.03(X_0/0.7)^2. We show that in this burning regime, a measurement of the burst recurrence time and energetics allows the local accretion rate onto the star to be determined independently of the accreted composition, giving a new method for estimating the source distance which is in good agreement with our other estimates.

MIRAX workshop proceedings

Here's the paper "Accretion-powered Millisecond Pulsar Outbursts" I submitted to the proceedings of the MIRAX workshop last December.

>> Click for the abstract Abstract: The population of accretion-powered millisecond pulsars has grown rapidly over the last four years, with the discovery of six new examples to bring the total sample to seven. While the first six discovered are transients active for a few weeks every two or more years, the most recently-discovered source HETE J1900.1-2455, has been active for more than 8 months. We summarise the transient behaviour of the population to estimate long-term time-averaged fluxes, and equate these fluxes to the expected mass transfer rate driven by gravitational radiation in order to constrain the distances. We also estimate an upper limit of 6 kpc to the distance of IGR J00291+5934 based on the non-detection of bursts from this source.

Korea | Sydney | 4U 1636-536

A busy few weeks, with a trip to Seoul to attend the 7th Pacific Rim Conference on Stellar Astrophysics. I gave a talk (PDF file) on accretion-powered millisecond pulsars, highlighting the recent observations of the latest discovery, HETE J1900.1-2455. Returned via Sydney where I visited a collaborator at U. Sydney and caught up with friends and family.
In the meantime, the long awaited paper on the radius-expansion bursts from 4U 1636-536 was finally accepted. In a study of all the bursts observed by RXTE, we found a bimodal distribution of peak radius-expansion burst fluxes, separated by a factor of 1.7. This is exactly what you would expect if the fainter bursts reach the Eddington limit in an atmosphere containing hydrogen at approximately solar abundances, while the brighter bursts instead occur in a pure helium environment. It remains a mystery exactly how the accreted hydrogen is eliminated from the atmosphere in the brighter radius-expansion bursts.
Read the paper

Discovery of pulsations in the X-ray transient 4U 1901+03

Back in 2003 we made RXTE observations of a new outburst of an old transient last seen in the early '70s, 4U 1901+03. We found pulsations at 2.763 s and an almost-circular orbit with period 22.58 d. Like KS 1947+30, this likely Be-X-ray binary has a much smaller than expected eccentricity of 0.036, suggesting that the natal kick from the supernova explosion which formed the neutron star was unusually small.
Read the paper (accepted by ApJ)
See also ATel #121

New millisecond X-ray pulsar IGR J00291+5934

Initially identified as a new X-ray transient by ESA's INTEGRAL satellite, IGR J00291+5934 was found in RXTE observations to be a 599 Hz millisecond pulsar. Subsequent observations revealed a 2.46 hr orbit, making the source very similar to the original millisecond X-ray pulsar, SAX J1808.4-3658.

Read the paper (ApJ 622, 45L 2005)
INTEGRAL team discovery paper (Shaw et al. 2005, A&A, 432, L13)
U. Southampton press release Feb 16^th 2005
New Scientist breaking news Feb 22^nd 2005

An X-ray Jet in CH Cygni

Analysis of archival Chandra data of the symbiotic system CH Cygni reveals faint extended emission to the south, aligned with the optical and radio jets seen in earlier HST and VLA observations. CH Cygni is only the second known white dwarf with an X-ray jet, after R Aquarii.
Read the paper (ApJL 613, L61)

The optical counterpart of XTE J1709-267

Jonker et al. report on Chandra and CTIO observations, as well as RXTE observations revealing bursts with an unusual precursor. A similar event occurred in the well-known burster 4U 1636-536
Accepted by MNRAS 22 July 2004

A frequency glitch in an accreting pulsar

RXTE observations reveal the 18.7 s pulsar KS 1947+30 to be in an almost circular 40.415 d orbit. We found evidence for a "glitch" - an abrupt increase in the neutron star spin frequency - previously observed only in radio pulsars and AXPs.
Read the paper (accepted by ApJ 6/11/2004)

New results from the "Clocked Burster" GS 1826-24

Analysis of the regular thermonuclear bursts from GS 1826-24 provide the best verification yet of theoretical ignition models. Solar metallicity models naturally reproduce the observed burst energies, but not the recurrence time variation.
Read the paper (ApJ 601, 466)

Gravitational waves may set speed limit for pulsar spin

Gravitational radiation - ripples in the fabric of space predicted by Albert Einstein - may serve as a cosmic traffic enforcer, protecting reckless pulsars from spinning too fast and blowing apart.

Read the paper (Nature 424:6944, 42) or the press release

Going, going, almost gone

MIT scientists have found a pulsar in a binary system that has all but completely whittled away its companion star, leaving this companion only about 10 times more massive than Jupiter.

Read the paper (s) (ApJ 576, L137; ApJ, 587, 754) or the press release