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Astrosat broadband characterization of the 2018 outburst of Swift J1756.9-2508

ATel #11566; Manoneeta Chakraborty (IIT Indore), Sudip Bhattacharyya (TIFR)
on 24 Apr 2018; 03:12 UT
Credential Certification: Manoneeta Chakraborty (mono.phy@gmail.com)

Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar

Referred to by ATel #: 11581, 11603

Following the reports of a new outburst of the accreting millisecond X-ray pulsar Swift J1756.9-2508 (ATel #11497, #11502, #11505, #11523), we performed ToO observations of this source with Astrosat for 21.4 ks on April 10, 2018 between 14:27:48 UTC and 15:41:39 UTC. We here report the detection of this latest outburst of this source simultaneously with Astrosat LAXPC and SXT providing a broadband coverage. A source is detected significantly using both LAXPC and SXT over a wideband range of 0.7-25 keV. The position was consistent with that reported by NICER and INTEGRAL. We fitted the combined spectrum with an absorbed multi-colored blackbody and a powerlaw (tbabs*(diskbb+powerlaw) in Xspec). We performed a joint fit using LAXPC and SXT, and the neutral hydrogen column density, the disk temperature and the powerlaw index were tied for the spectral fit of the two instruments. We found the neutral hydrogen column density to be 4.29(8)E22 cm^(-2) which is slightly lower than the value obtained from the NICER observation. The value of the inner disk temperature and the powerlaw index were obtained to be 1.71(+/-0.02) keV and 2.31(+/-0.08) respectively. For the SXT spectrum, the powerlaw component was not needed and hence the normalization was kept frozen to zero. The total 1-25 keV unabsorbed flux is 7.47E-10 erg/s/cm^(-2). No evidence of any broad spectral line was present in the data (Patruno et al 2010). The power spectra constructed from the high time resolution LAXPC data were searched and no detection of any significant presence of the accretion powered millisecond pulsations as reported in ATel #11502 could be obtained in the preliminary analysis. Further multi-wavelength observations of this recent outburst of the source are encouraged. We are grateful to the Astrosat team for the rapid scheduling of this ToO observation.