Detection of Type-II bursts from the Rapid Burster by XSPECT on-board XPoSat

ATel #17388; Rwitika Chatterjee, Koushal Vadodariya, Vivek K. Agrawal, Radhakrishna V., Piyush Kumar Gaurav, Sachin Narang, Madhu K V, Shyam Prakash V P, Vaishali Sharan, Valluru Uma Maheshwara Rao, Jayashree Jana Bhattacharya, Thameemunnisha M, Anandraj R, Shivam Asati (U. R. Rao Satelllite Centre; ISRO Telemetry, Tracking and Command Network)
on 12 Sep 2025; 12:32 UT
Credential Certification: Rwitika Chatterjee (rwitika@ursc.gov.in)

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MXB 1730-335, more popularly known as the Rapid Burster, is a transient neutron star low mass X-ray binary (NS-LMXB), located in the globular cluster Liller 1 at a distance of ~7.9 kpc. It is one of only two known neutron star sources, which shows type-II bursts, and the only source which shows both type-I and type-II bursts. The source goes into an active phase with a typical recurrence time of ~100 days (Masetti 2002). Anticipating such a phase, as well as monitoring MAXI fluxes, XSPECT (X-ray SPECtroscopy and Timing) instrument (Vatedka et al. 2025, Chatterjee et al. 2025) on-board XPoSat triggered a ToO observation to observe this source.

XPoSat observed the Rapid Burster on 2025 September 5-6, and between 05:30 UT to 14:15 UT of September 5, XSPECT detected four type-II bursts. For two of these, the complete burst profile was traced (see Figure at this link), while for the other two, only partial burst profiles could be captured due to gaps in observing window owing to mission constraints. The bursts show the typical profile of type-II bursts, with a somewhat raised 'hump' before the burst, and a slight dip just preceding a rapid increase. This is followed by an almost flat top, a decline to an intermediate level and finally reaching the pre-burst level.

XSPECT consists of co-aligned square collimators of 2° × 2° and 3° × 3° FOVs. Due to the proximity of this source with another source (4U 1728-34, ~0.5° away), this observation was carried out in offset pointing mode, ensuring that only the rapid burster is present in the 2° × 2° FOV. Hence, we used the data from eight of the sixteen detectors having 2° × 2° collimators. After accounting for the collimator response, the background-subtracted pre-burst count rate is 2.1 ± 0.4 cps, and it rises to 10.7 ± 1.0 cps and 12.1 ± 1.1 cps (for the two fully traced profiles) during the flat top of the burst. These two bursts lasts for about 650 s and 250 s respectively, and are possibly mode-0 bursts, according to the classification scheme of Bagnoli et al. (2015).

We fitted the pre-burst spectrum with an absorbed powerlaw, which gave an unabsorbed 0.8 - 15 keV flux of 36 ± 3 mCrab. The burst spectrum is well-fitted by an absorbed blackbody model, yielding a best-fit blackbody temperature of ~ 1.4 keV, and a flux of 205 ± 5 mCrab. Follow-up observations of this source are ongoing.

The XPoSat project is managed and facilitated by the Indian Space Research Organisation.