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Swift J1658.2-4242 as observed by AstroSat LAXPC

ATel #11375; Aru Beri (U. Southampton), Tomaso Belloni (INAF-OAB), Federico Vincentelli (INAF-OAB and U. Insubria), Poshak Gandhi, Diego Altamirano (U. Southampton)
on 1 Mar 2018; 21:43 UT
Credential Certification: Poshak Gandhi (p.gandhi@soton.ac.uk)

Subjects: X-ray, Black Hole, Neutron Star, Transient

We report on preliminary analysis of an AstroSat observation of the newly discovered X-ray transient, Swift J1658.2-4242 [J1658] (GCN #22416, #22417, #22419; ATel #11306, #11307, #11310, #11311, #11318, #11321, #11336). The observation of J1658 was performed between 2018-02-20 18:21:45 UT and 2018-02-21 04:25:38 UT, covering 7 satellite orbits, for a total of 20 ks exposure. We analyzed the data from the LAXPC instrument, covering the energy range 3-80 keV (Antia et al. 2017 ApJS 231 10).

Power density spectra show the presence of band-limited noise plus a strong (rms of ~16%) and sharp Quasi-Periodic Oscillation (QPO), whose centroid frequency increased with time from ~1.6 to 2 Hz. The total 0.1-20 Hz fractional rms is ~34% over 3-80 keV. The presence of such a QPO indicates that the source was in the hard intermediate state during these observations. The power density spectrum combined from all three LAXPC cameras is shown at the link below.

Spectral fitting was performed over the energy range of 4-60 keV for the cameras LAXPC10 and 20, with LAXPC30 excluded due to a suspected gas leak leading to loss of efficiency. The continuum spectra can be approximately described by an absorbed disk blackbody and cutoff power law. A systematic error of 4% was included while performing spectral fitting. We obtained an N(H) = 1.6(+/-0.2)x10^{23} cm^{-2} (consistent with a NuSTAR observation carried out ~4 days prior to ours; ATel #11321), an inner disk temperature of 1.4+/-0.1 keV, a power law photon index of 1.76+/-0.06 and a high-energy cutoff of 46+/-3 keV. We obtained a reduced Chi^2 = 0.82 for 292 degrees of freedom. All uncertainties are stated at a 1-sigma confidence level. The 3-80 keV unabsorbed flux was 3.6x10^{-9} ergs cm^{-2} s^{-1}, corresponding to a luminosity of 3x10^{37} ergs s^{-1} at 8 kpc. The power law to disk flux ratio is ~8 over 3-80 keV. Analysis of the other AstroSat instruments is underway.

Acknowledgment: The authors would like to thank all the AstroSat operations team members for scheduling the ToO observation, and for prompt provision of the data.

Click here for AstroSat/LAXPC Power Spectrum