R. Kaur

Multiwavelength observations of 1RXH J173523.7−354013: revealing an unusual bursting neutron star

On 2008 May 14, the Burst Alert Telescope onboard the Swift mission triggered on a typeI X-ray burst from the previously unclassified ROSAT object 1RXH J173523.7−354013, establishing the source as a neutron star X-ray binary. We report on X-ray, optical and nearinfrared observations of this system. The X-ray burst had a duration of ∼2 h and belongs to the class of rare, intermediately long type-I X-ray bursts. From the bolometric peak flux of ∼3.5 × 10 −8 erg cm −2 s −1 , we infer a source distance of D 9.5 kpc. Photometry of the field reveals an optical counterpart that declined from R = 15.9 during the X-ray burst to R = 18.9 thereafter. Analysis of post-burst Swift/X-ray Telescope observations as well as archival XMM– Newton and ROSAT data suggests that the system is persistent at a 0.5–10 keV luminosity of ∼2 × 10 35 (D/9.5 kpc) 2 erg s −1 . Optical and infrared photometry together with the detection of a narrow Hα emission line (full width at half maximum = 292 ± 9k m s −1 , equivalent width =− 9.0 ± 0.4 A) in the optical spectrum confirms that 1RXH J173523.7−354013 is a neutron star low-mass X-ray binary. The Hα emission demonstrates that the donor star is hydrogen rich, which effectively rules out that this system is an ultracompact X-ray binary.

The Return of the Bursts: Thermonuclear Flashes from Circinus X-1

We report the detection of 15 X-ray bursts with RXTE and Swift observations of the peculiar X-ray binary Circinus X-1 (Cir X-1) during its 2010 May X-ray re-brightening. These are the first X-ray bursts observed from the source after the initial discovery by Tennant and collaborators, 25 years ago. By studying their spectral evolution, we firmly identify nine of the bursts as type I (thermonuclear) X-ray bursts. We obtain an arcsecond location of the bursts that confirms once and for all the identification of Cir X-1 as a type I X-ray burst source, and therefore as a low magnetic field accreting neutron star. The first five bursts observed by RXTE are weak and show approximately symmetric light curves, without detectable signs of cooling along the burst decay. We discuss their possible nature. Finally, we explore a scenario to explain why Cir X-1 shows thermonuclear bursts now but not in the past, when it was extensively observed and accreting at a similar rate.

DISCOVERY OF AN ACCRETING MILLISECOND PULSAR IN THE ECLIPSING BINARY SYSTEM SWIFT J1749.4-2807

We report on the discovery and the timing analysis of the first eclipsing accretion-powered millisecond X-ray pulsar (AMXP): SWIFT J1749.4-2807. The neutron star rotates at a frequency of similar to 517.9 Hz and is in a binary system with an orbital period of 8.8 hr and a projected semimajor axis of similar to 1.90 lt-s. Assuming a neutron star between 0.8 and 2.2 M-circle dot and using the mass function of the system and the eclipse half-angle, we constrain the mass of the companion and the inclination of the system to be in the similar to 0.46-0.81 M-circle dot and similar to 74 degrees.4-77 degrees.3 range, respectively. To date, this is the tightest constraint on the orbital inclination of any AMXP. As in other AMXPs, the pulse profile shows harmonic content up to the third overtone. However, this is the first AMXP to show a first overtone with rms amplitudes between similar to 6% and similar to 23%, which is the strongest ever seen and which can be more than two times stronger than the fundamental. The fact that SWIFT J1749.4-2807 is an eclipsing system that shows uncommonly strong harmonic content suggests that it might be the best source to date to set constraints on neutron star properties including compactness and geometry.