The Low-mass X-ray binary Swift J1658.2-4242 returned to quiescence

ATel #12072; Aru Beri, Diego Altamirano (U. Southampton)
on 2 Oct 2018; 16:12 UT
Credential Certification: Aru Beri (aru.beri8@gmail.com)

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The newly discovered X-ray transient Swift J1658.2-4242 (J1658) was first detected in outburst by the BAT instrument aboard the Neil Gehrels Swift observatory on February 16, 2018. Since then, Swift performed extensive monitoring of this source from February through August of 2018 (ATel #11307, ATel #11311). We used the online tools provided by the UK Swift Science Data Centre (http://www.swift.ac.uk) to extract light curves and energy spectra of all Swift/XRT available observations.

Light curve created using the XRT observations of J1658 showed a peak of the outburst on 02-03-2018 12:16:06 UT(MJD 58179.52) (see https://sites.google.com/view/swiftj16582-4242). To fit the spectrum extracted using the observation made on this date, we began with an absorbed blackbody, however, we observed some residuals indicating the presence of a high energy component. Adding a power law component to the absorbed blackbody did not improve the fit. The broadband observations made with NuSTAR and AstroSat showed the presence of high energy cut-off (ATel #11321, ATel #11375). Thereafter, we added a cut-off power law to the absorbed blackbody for fitting the XRT spectrum. As the cut-off energy is much beyond the energy band of XRT, therefore, we fixed the energy cut-off at 46 keV as observed with the AstroSat observation (ATel #11375). We obtained an N(H) = 1.1(+/-0.3) x 10^{23} cm^{-2} and a blackbody temperature of 1.22+/-0.03 keV. It was difficult to constrain the power law index, therefore, we fixed the power law index to the value obtained ~ 1.1. We obtained a Chi^2 = 753 for 652 degrees of freedom. All uncertainties are stated at a 1-sigma confidence level. We also tried to replace the blackbody component with the disk blackbody, however, we noticed the increase in the Chi^2 to 789 from 753 for 652 degrees of freedom. Using the best-fit obtained with an absorbed blackbody plus cut-off power law we estimated the absorbed flux to be 5 x 10^{-9} ergs cm^{-2} s^{-1} in the 0.3-10 keV band. From the XRT light curve, we notice that after this peak, the flux of J1658 showed a gradual decrease.

Swift/XRT observed J1658 for the last time on 10-08-2018 08:56:37 UT (MJD 58340.37). Due to the limited statistics, we could not use the Chi^2 statistics but used W-statistics which is background subtracted Cash statistics. The spectrum was well modelled with an absorbed power law of index 1.2 +/- 0.4 with an absorption fixed to NH= 1.8 x 10^{23} cm^{-2}. The absorbed flux measured in the 0.3-10 keV band using this observation is ~ 3.5 x 10^{-11} erg cm^{-2} s^{-1}. Given the flux trend, J1658 should be in quiescence. To confirm that, we requested another observation of J1658 which was done on 28-09-2018 02:50:54 UT (MJD 58389.1172, i.e. about a month after the previous Swift/XRT observation). We detected only 2 counts in a 2000 s long observation after background subtraction. We also determined upper limits on the count rate (95 per cent confidence level) using the prescription of Gehrels (1986). This turns out to be ~ 1.5e-3 c/s and using WEBPIMMS we converted this count-rate into 0.3-10 keV flux. By assuming an absorbed power law of index 1.5 and an absorption NH= 1.8 x 10^{23} cm^{-2} , we estimated a flux of ~ 2.54 x 10^{-13} erg cm^{-2} s^{-1}. Assuming a source distance of 8 kpc, this observed flux translates into a 0.3-10 keV luminosity of ~ 9 x 10^{32} erg s^{-1}. This is consistent with the quiescence luminosities observed from these type of systems, and therefore conclude that the source has transitioned into quiescence.

Acknowledgement: We thank the Swift team members for scheduling the ToO observation. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester.

Swift J1658.2-4242