NuSTAR observation of Swift J1357.2-0933 during its 2019 outburst

ATel #12821; Anjali Rao (University of Southampton), John A. Paice, Poshak Gandhi, Aru Beri
on 30 May 2019; 17:22 UT
Credential Certification: Poshak Gandhi (p.gandhi@soton.ac.uk)

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Renewed activity of the black hole candidate Swift J1357.2-0933 has recently been reported by the Zwicky Transient Facility (ATel #12796), following outbursts in 2011 and 2017. The source brightened to a magnitude (r_ZTF) of 17.8 from its quiescence mag of about 20.7 (ATel #12796). Russell et al. have also reported the date of the start of outburst in optical to be March 21, 2019 (ATel #12803) and I-band magnitude of 16.90+/-0.01 on May 22, 2019.

The source was detected in X-rays with a follow-up observation with NICER on May 23, 2019 (ATel #12801) and also with Swift/XRT (ATel #12816). The spectrum in soft X-rays can be fit with a power-law with Gamma~1-6-1.7 and flux ~<1e-11 erg/s/cm^2.

We report on the follow-up observation of Swift J1357.2-0933 with NuSTAR in hard X-rays performed on 2019-05-29T12:11:09, ~31 ks duration; ObsID: 90501325002. The source is detected and exhibited a count rate of ~0.09 cts/s in 3-80 keV.

The spectra were extracted from a source region of 40 arcsec radius from the quick look data for FPMA and FPMB. The joint spectra in 3-80 keV energy range are well explained with a simple power law giving a photon index of 1.92 +/- 0.05 (90% confidence interval) and flux of 1.0e-11 erg/s/cm^2 (3-80 keV), resulting in an X-ray luminosity L_X(3-80 keV) of 3.0x10^{34} (d/5 kpc)^2 erg/s. Extrapolating to soft X-rays, the flux in 0.3-10 keV is found to be 5.0e-12 erg/s/cm^2. The 90% upper limit to the equivalent width for any putative narrow iron K-alpha 6.4 keV fluorescence line is found to be 0.13 keV.

The source was also found to display a curiously simple power-law in hard X-rays lacking reflection or high-energy cutoff signatures during the 2017 outburst (Stiele & Kong 2018 ApJ 852 34, Beri et al. 2019 MNRAS 485 3064, Paice et al. 2019 arXiv:1905.09784v1). Our NuSTAR observation now suggests a similar evolution during the 2019 outburst. The exact nature of the source still remains to be elucidated.

We thank the NuSTAR team for rapid approval and completion of this observation request.