Carbon Flashes in the Heavy Element Ocean on Accreting Neutron Stars

We show that burning of a small mass fraction of carbon in a neutron star ocean is thermally unstable at low accumulated masses when the ocean contains heavy ashes from the hydrogen burning rapid proton (rp) process. The key to early unstable ignition is the low thermal conductivity of a heavy element ocean. The instability requires accretion rates in excess of one-tenth the Eddington limit when the carbon mass fraction is 0.1 or less. The unstable flashes release 10^{42} to 10^{43} ergs over hours to days, and are likely the cause of the recently discovered large Type I X-ray bursts (so-called ``superbursts'') from six Galactic low mass X-ray binaries. In addition to explaining the energetics, recurrence times, and durations of the superbursts, these mixed carbon/heavy element flashes have an accretion rate dependence of unstable burning similar to that observed. Though the instability is present at accretion rates near Eddington, there is less contrast with the accretion luminosity there, explaining why most detections are made at accretion rates between 0.1 and 0.3 Eddington. Future comparisons of time dependent calculations with observations will provide new insights into the rp process.Comment: Submitted to Astrophysical Journal Letters (6 pages, 3 figures

Unstable Nonradial Oscillations on Helium Burning Neutron Stars

Material accreted onto a neutron star can stably burn in steady state only when the accretion rate is high (typically super-Eddington) or if a large flux from the neutron star crust permeates the outer atmosphere. For such situations we have analyzed the stability of nonradial oscillations, finding one unstable mode for pure helium accretion. This is a shallow surface wave which resides in the helium atmosphere above the heavier ashes of the ocean. It is excited by the increase in the nuclear reaction rate during the oscillations, and it grows on the timescale of a second. For a slowly rotating star, this mode has a frequency of approximately 20-30 Hz (for l=1), and we calculate the full spectrum that a rapidly rotating (>>30 Hz) neutron star would support. The short period X-ray binary 4U 1820--30 is accreting helium rich material and is the system most likely to show this unstable mode,especially when it is not exhibiting X-ray bursts. Our discovery of an unstable mode in a thermally stable atmosphere shows that nonradial perturbations have a different stability criterion than the spherically symmetric thermal perturbations that generate type I X-ray bursts.Comment: Accepted for publication in Astrophysical Journal, 22 pages, 14 figure

INTEGRAL, Swift, and RXTE observations of the 518 Hz accreting transient pulsar Swift J1749.4-2807

The burst-only Swift J1749.4-2807 source was discovered in a high X-ray-active state, while during an {INTEGRAL observations of the Galactic bulge on 2010 April 10. Pulsations at 518 Hz were discovered in the RXTE data, confirming previous suggestions of possible associations between burst-only sources and accreting millisecond X-ray pulsars. The subsequent discovery of X-ray eclipses made Swift J1749.4$-$2807 the first eclipsing accreting millisecond X-ray pulsar. We obtain additional information on Swift J1749.4-2807 and other burst-only sources. We report on the results of a monitoring campaign on the source, carried out for about two weeks with the Swift, INTEGRAL, and RXTE satellites. The observations showed that the X-ray spectrum (energy range 0.5-40 keV) of Swift J1749.4-2807 during the entire event was accurately modeled by an absorbed power-law model (N_H~3e2 cm^-2, Gamma~1.7). X-ray eclipses were also detected in the Swift data and provides a clear evidence of a dust-scattering halo located along the line of sight to the source. Only one type-I X-ray burst was observed throughout the two-weeks long monitoring. The X-ray flux of Swift J1749.4-2807 decayed below the detection threshold of Swift/XRT about 11 days after the discovery, in a exponential fashion (e-folding time of tau=12^+7_-3 days). We compare the properties of the outburst observed from Swift J1749.4-2807 with those of the previously known millisecond X-ray pulsars and other transient low mass X-ray binaries.Comment: Accepted for publication on A&

Theoretical Models of Superbursts on Accreting Neutron Stars

We carry out a general-relativistic global linear stability analysis of the amassed carbon fuel on the surface of an accreting neutron star to determine the conditions under which superbursts occur. We reproduce the general observational characteristics of superbursts, including burst fluences, recurrence times, and the absence of superbursts on stars with accretion rates below 10% of the Eddington limit. By comparing our results with observations, we are able to set constraints on neutron star parameters such as the stellar radius and neutrino cooling mechanism in the core. Specifically, we find that accreting neutron stars with ordered crusts and highly efficient neutrino emission in their cores (due to direct URCA or pionic reactions, for example) produce extremely energetic (> 10^44 ergs) superbursts which are inconsistent with observations, in agreement with previous investigations. Also, because of pycnonuclear burning of carbon, they do not have superbursts in the range of accretion rates at which superbursts are actually observed unless the crust is very impure. Stars with less efficient neutrino emission (due to modified URCA reactions, for example) produce bursts that agree better with observations. Stars with highly inefficient neutrino emission in their cores produce bursts that agree best with observations. All systems that accrete primarily hydrogen and in which superbursts are observed show evidence of H- and He-burning delayed mixed bursts. We speculate that delayed mixed bursts provide sufficient amounts of carbon fuel for superbursts and are thus a prerequisite for having superbursts. We compare our global stability analysis to approximate one-zone criteria used by other authors and identify a particular set of approximations that give accurate results for most choices of parameters. (abstract truncated)Comment: 43 pages, 18 figures, accepted by Ap

On the mass of the neutron star in Cyg X-2

We present new high resolution spectroscopy of the low mass X-ray binary Cyg X-2 which enables us to refine the orbital solution and rotational broadening of the donor star. In contrast with Elebert et al (2009) we find a good agreement with results reported in Casares et al. (1998). We measure $P=9.84450\pm0.00019$ day, $K_2=86.5\pm1.2$ km s$^{-1}$ and $V \sin i=33.7\pm0.9$ km s$^{-1}$. These values imply $q=M_{2}/M_{1}=0.34 \pm 0.02$ and $M_{1}=1.71\pm 0.21$ M$_{\odot}$ (for $i=62.5 \pm 4^{\circ}$). Therefore, the neutron star in Cyg X-2 can be more massive than canonical. We also find no evidence for irradiation effects in our radial velocity curve which could explain the discrepancy between Elebert et al's and our $K_2$ values.Comment: Accepted for publication in MNRA

A new bursting X-ray transient: SAX J1750.8-2900

We have analysed in detail the discovery measurements of the X-ray burster SAX J1750.8-2900 by the Wide Field Cameras on board BeppoSAX in spring 1997, at a position ~1.2 degrees off the Galactic Centre. The source was in outburst on March 13th when the first observation started and showed X-ray emission for ~ 2 weeks. A total of 9 bursts were detected, with peak intensities varying from ~ 0.4 to 1.0 Crab in the 2-10 keV range. Most bursts showed a fast rise time (~ 1s), an exponential decay profile with e-folding time of ~ 5s, spectral softening during decay, and a spectrum which is consistent with few keV blackbody radiation. These features identify them as type-I X-ray bursts of thermonuclear origin. The presence of type-I bursts and the source position close to the Galactic Centre favours the classification of this object as a neutron star low mass X-ray binary. X-ray emission from SAX J1750.8-2900 was not detected in the previous and subsequent Galactic bulge monitoring, and the source was never seen bursting again.Comment: 13 pages, 3 Postscript figures, aaspp4 styl