AstroSat detects 3 Type-I bursts from EXO 0748-676

ATel #16753; Unnati Kashyap (TTU, USA), P Aromal (IITI, India), Manoneeta Chakraborty (IITI, India), Sudip Bhattacharyya (TIFR, India), Thomas J. Maccarone (TTU, USA)
on 6 Aug 2024; 14:59 UT
Credential Certification: Unnati Kashyap (ukashyap@ttu.edu)

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The neutron star (NS) Low Mass X-ray Binary (LMXB) source EXO 0748-676 recently went into an outburst, as reported by the Neil Gehrels Swift Burst Alert Telescope and X-ray Telescope on 10th June 2024 (D'Elia et al., 2024, GCN #36653) after 16 years of quiescence. The follow-up optical observations further confirm the source entering the outburst (ATel#16646, #16648, #16649). The previous outburst is estimated to have started between 1980-1984 (Degenaar et al., 2009a) and lasted more than 20 years, until 2008.

EXO 0748-676 exhibits Type-I bursts, indicating the NS nature of the central compact object (Boirin et al., 2007). Recent MAXI/GSC (ATel#16654) and Swift/BAT (ATel#16655) report the initial detections of Type-I X-ray burst from EXO 0748-676 during its current outburst. Following that, NuSTAR observed EXO 0748-676 on 2024 June 17 and reported clear detections of five eclipses and a Type I X-ray burst in the hard X-ray state (ATel#16673). NinjaSat reports the detection of an X-ray burst on 2024 June 23 with a duration of about 80 s (ATel#16678). More recently, XMM-Newton reports the detection of another Type-I burst from EXO 0748-676 on 2024 June 30 (ATel#16703).

We report the AstroSat ToO observation of EXO 0748-676 conducted between 2024 July 24 08:01:03.88 UTC and July 26 9:37:41.65.00 UTC for a total on-source exposure time of 30 ks. Three Type-I bursts were detected in the LAXPC light curve with durations of 39 s, 47 s, and 47 s starting at 2024 July 24 17:31:33.146 UTC, 2024 July 25 16:31:03.239 UTC, and 2024 July 26 14:45:53.329 UTC. The burst spectra were fitted with an absorbed blackbody radiation model with the estimated peak blackbody temperatures of 2.26 +/- 0.04 keV, 2.54 +/- 0.05 keV, and 2.58 +/- 0.04 keV and peak burst flux (4-12 keV) of approximately (3.30 +/- 0.04)e-8, (1.56 +/- 0.02)e-8, and (2.13 +/- 0.03)e-8 erg/s/cm^2. No significant detection was obtained from the burst oscillation search conducted for all three bursts. The LAXPC persistent spectrum in the 4.0-20.0 keV energy range is reasonably well described by an absorbed blackbody radiation and power law model. We find a blackbody temperature of 1.94 +/- 0.03 keV, and a power law photon index of 1.38 +/- 0.09. The total unabsorbed flux in the 4.0 to 20.0 keV range is estimated as (1.49 +/- 0.01)e-9 erg/s/cm^2. 5 eclipses has also been detected in the LAXPC light curve with an average eclipse separation time of ~ 3.8 hours. The source exhibits aperiodic variability, and the LAXPC power spectrum is observed to consist of low-frequency red noise and a broadband noise well represented by a zero-centered Lorentzian. Further multi-wavelength monitoring of EXO 0748-676 is recommended.

We would like to thank the AstroSat teams for scheduling this ToO observation, for rapid processing of the data, and for providing access to the data.