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)

US/Netherlands tour

I just got back from a very busy trip around the US, with a stop in Amsterdam on the way home. I presented a couple of posters at the AAS/High Energy Astrophysics Division Meeting in Los Angeles, and went on to visit Ed Brown at JINA, where I gave a lunch talk on the following Monday.

Then it was on to Boston for a brief visit to MIT to catch up with collaborators and friends, as well as a pilgrimage to Toscaninis (sadly the Harvard Square location has closed). After that I had a couple of nights in St. Louis for the APS meeting, and gave an invited talk on burst observations in a special session on the physics of X-ray bursts (L3).

Finally I headed home via Amsterdam for another invited talk at the Decade of Accreting Millisecond X-ray Pulsars workshop at UvA.

HEAD posters 1 2 (0.24/1.22 MB PDF)
APS talk (10.2 MB PPT)
AMSPs workshop talk (1.5 MB PPT) and the proceedings paper

Two accretion-powered millisecond pulsars active in June

I'm not sorry to be wrong about HETE J1900.1-2455 returning to quiescence; Swift and RXTE observations following the nondetection last month indicate that the source is as bright as before. This is bad news for anticipated measurements of the quiescent flux with Chandra, but good news for our ongoing RXTE monitoring and pulsation searches.

Also last month the 8th accretion-powered millisecond pulsar was discovered, Swift J1756.9-2508. At 182 Hz the source is the slowest of the class (by 3 Hz); the 54-minute orbital period indicates an ultracompact (and thus likely H-poor) mass donor. The pulsar was only active for two weeks, and was undetected with Swift by June 21. A paper is in preparation by the Swift team.

HETE J1900.1-2455 is returning to quiescence

The most recently-discovered millisecond pulsar, HETE J1900.1-2455, has finally begun to fade into quiescence. The last couple of observations in May saw the flux below 10^-10 erg cm^-2 s^-1, and fading, as reported in ATel #1086. Collaborators in the Netherlands, led by Rudy Wijnands, triggered a series of Swift observations, which confirmed the decline. Swift observations are ongoing, and a Chandra pointing is planned for the end of June. HETE J1900.1-2455 was active for almost 2 years — longer by far than any of the other accretion-powered millisecond pulsars. We're sorry to see it go!

Flux history (.PS) from RXTE

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.

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.

IGR J00291+5934 in the news

Maurizio Falanga (CEA), Chris Wanjek (NASA) and I prepared a press release (courtesy SpaceflightNow) following the acceptance of Maurizio's A&A paper. The release first appeared at ESA on 6^th September 2005. See also Une étoile « cannibale » at CEA (French) and the more detailed Festin stellaire: un pulsar milliseconde s'échauffe et accélère at SAp (French)
Earlier post: New millisecond X-ray pulsar IGR J00291+5934

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

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)

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