AstroSat detection of two Type-I X-ray bursts from the accreting neutron star XTE J1739-285
ATel #13538; S. Chakraborty (TIFR, Mumbai), S. Banerjee (TIFR, Mumbai)
on 2 Mar 2020; 19:26 UT
Credential Certification: Sudip Chakraborty (sudipchakraborty93@gmail.com)
Subjects: X-ray, Binary, Neutron Star, Transient
Referred to by ATel #: 13656
XTE J1739-285 is a transient neutron star low mass X-ray binary that was discovered with RXTE in 1999 (IAUC 7300 ). This source was suggested to harbor the fastest spinning neutron star based on the measurement of burst oscillation frequency (Kaaret et. al. 2007, ApJL 657, 2, L97-L100). Recently, INTEGRAL detected this source in outburst states twice, in September 2019 (ATel #13138) and February 2020 (ATel #13474). However, no thermonuclear (Type-I X-ray) bursts were reported so far for these two outbursts.
This source was observed with AstroSat (Obs ID: T03_180T01_9000003524) from 19th February (22:20 UTC) to 20th February (23:20 UTC), 2020. In this period, the source has exhibited two thermonuclear bursts (20th February 7:37:10 UTC and 15:36:51 UTC), with 3-12 keV peak count-rates of about 1121 cps (555 mCrab) and 1162 cps (575 mCrab), respectively. However, the observation of the second thermonuclear burst had an overlap with the SAA passage time interval. So the tail part of the second thermonuclear burst could not be observed properly. The persistent source count-rate was found to be about 45 cps (22 mCrab) in 3-12 keV energy band. We report here the time-resolved spectroscopic analysis of these two type-I X-ray bursts using LAXPC on board AstroSat.
The 3-12 keV LAXPC20 spectra of both the X-ray bursts were well described by an absorbed blackbody model, with the column density fixed at 2.5e22 cm^-2 (ATel #13148). The blackbody temperature for the first X-ray burst (duration of about 40 seconds) decreases from the peak value of 2.17+/-0.09 keV to 1.31+/-0.08 keV. The corresponding 3-12 peak keV flux was found to be 7.76e-9 erg/cm^2/s. For the second X-ray burst (with a duration of about 30 seconds), the black body temperature falls from 2.16+/-0.09 keV to 1.58+/-0.06 keV, with a 3-12 peak keV flux of 8.04e-9 erg/cm^2/s. The 3-30 keV persistent LAXPC20 spectrum was reasonably well fitted by an absorbed blackbody with a temperature of about 0.72+/-0.08 keV plus a power-law with a photon index near 1.74+/-0.02 with the column density fixed at the above-mentioned value, and a Gaussian to take care of the complex residuals near 6.7 keV. The corresponding 3-30 keV persistent flux was found to be 5.91e-10 erg/cm^2/s.
We acknowledge the Astrosat team for the rapid scheduling of this ToO observation and prompt availability of the data. We are also thankful to Prof. S. Bhattacharyya (TIFR, Mumbai) and Prof. H. M. Antia (TIFR, Mumbai) for their useful suggestions.
