XSPECT/XPoSat detection of a thermonuclear burst followed by a superburst from the neutron star LMXB 4U 1608-52
ATel #17118; Rwitika Chatterjee, Vivek K. Agrawal, Koushal Vadodariya, Radhakrishna V., Ramadevi M. C., Anurag Tyagi, Vaishali Sharan, Kiran M. Jayasurya, Shyam Prakash V. P. (U. R. Rao Satellite Centre) on behalf of the XSPECT team
on 29 Mar 2025; 08:38 UT
Credential Certification: M. C. Ramadevi (mc.ramadevi@gmail.com)
Subjects: X-ray, Binary, Neutron Star, Variables
To study the recent outburst of 4U 1608-52 (ATels #17013, #17036, #17037), XSPECT triggered a long ToO observation spanning 14 days and has been monitoring the outburst. On 19th March 2025, at 23:02:41 UT, XSPECT detected a Type-I (thermonuclear) X-ray burst from the source with a peak flux (0.8 - 15 keV) of ~2.5 Crab increasing from a quiescent rate of ~360 mCrab, with a possible second peak within 5 - 6 seconds of the main peak.
The burst was fitted with an absorbed blackbody model, with a blackbody temperature of 1.84 ± 0.06 keV and a blackbody radius of ~9.3 km (assuming a distance of 4 kpc to the source, Guver et al. 2010). The burst lasted around 20 seconds and the average unabsorbed flux of the burst was found to be ~6.3 × 10-8 ergs s-1 cm-2.
Within ~16 minutes of the burst decay, at 23:19:30 UT, the source started brightening again and the flux abruptly increased by a factor of 2. Subsequently, the decay of this 'burst' was detected over 6 orbits of XPoSat before reaching earlier quiescent levels, with an e-folding time of ~2.9 hours, making this a superburst as reported by MAXI/GSC (ATel #17097). Due to data gaps owing to operational constraints, it is not certain whether the peak of the superburst was detected by XSPECT. The decaying phase of the superburst was divided into 5 segments and fitted with an absorbed nThcomp model in XSPEC, with the unabsorbed flux declining from 4.5 × 10-8 ergs s-1 cm-2 to 1.3 × 10-8 ergs s-1 cm-2.
Detailed analysis of these observations is underway. Light curves of the event can be found at this link.
XSPECT (X-ray SPECtroscopy and Timing) instrument on-board XPoSat is a soft X-ray spectrometer operating in the energy band 0.8 - 15 keV. XSPECT employs Swept Charge Devices (SCDs) and has a net geometric area of 60 cm2. XSPECT, with a 5σ sensitivity of ~0.2 mCrab in 10 ks, carries out long-term, pile-up free observations of bright X-ray sources.
The XPoSat project is managed and facilitated by the Indian Space Research Organisation.
