X-ray activity of Swift J1357.2-0933 detected by NICER

ATel #12801; P. Gandhi (U. Southampton), J. A. Paice, K. Gendreau, Z. Arzoumanian, D. Altamirano, A. Beri, J. M. Miller, D. R Pasham, J. F. Steiner, P. A. Charles, D. A. Buckley, T. M. Belloni, P. Casella, T. J. Maccarone, D. M. Russell, G. R. Sivakoff, F. M. Vincentelli, T. Shahbaz
on 23 May 2019; 21:29 UT
Credential Certification: Poshak Gandhi (p.gandhi@soton.ac.uk)

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Swift J1357.2-0933 is a candidate Galactic black hole transient displaying (1) puzzling optical blue dipping activity, (2) a lack of significant X-ray dipping as well as any significant reflection features, and (3) a weak outburst peak luminosity L_X below approximately 10^{36} erg/s for distances D ranging between 2.3-6.3 kpc (e.g. Corral-Santana et al. 2013 Science 339 1048; Armas Padilla et al. 2013 MNRAS 428 3083; Shahbaz et al. 2013 MNRAS 434 2696; Mata-Sanchez et al. 2015 MNRAS 454 2199).

It was discovered in 2011, and then underwent a second outburst in 2017, displaying similar outburst evolution on both occasions (e.g. Stiele & Kong 2018 ApJ 852 34; Beri et al. 2019 MNRAS 485 3064; Paice et al. 2019 subm.).

Due to its faint X-ray nature, its 2017 outburst was first reported in the optical (ATel #10297). In a similar vein, the Zwicky Transient Factory (ZTF; Bellm et al 2019, PASP 131, 018002) has just reported a potential ongoing outburst with the source being several mags brighter than quiescence in the optical since 2019 March 31 or a few days earlier (ATel #12796).

We report here results of a quicklook analysis of a NICER follow-up observation carried out on 2019 May 23 starting at UT13:51 for a duration of 609 s with 52 working detectors.

The source is clearly detected with NICER at a full band count rate of 5.5 cts/s. Flares of duration ~10-50 s are observed with peak count rates increasing by factors of ~2-3. See light curve at link below.

The background begins to dominate the observed counts above ~6 keV. For this quicklook analysis, we restricted spectral fitting to an energy range of 0.3-5 keV. Using an absorbed power law (N_h fixed at 1.2x10^{20}/cm^2; Armas Padilla et al. 2013) yields an acceptable fit with a photon-index of 1.68+/-0.05 (C-stat/dof=146/169; confidence range for Delta C-stat=2.71). The modeled source flux is F(0.3-10 keV) =~ 1.1x10^{-11} erg/s/cm^2, which translates to an unabsorbed luminosity L(0.5-10) =~ 3.0x10^{34} (D/5kpc)^2 erg/s. This is about an order of magnitude below the peak X-ray flux seen during the 2017 outburst.

The current outburst appears to have been ongoing for almost two months (ATel #12796), so we cannot rule out the possibility that the peak of the outburst may have already passed in X-rays. Examining publicly available MAXI light curves is suggestive of weak X-ray activity around mid-March 2019 (~MJD 58558), but no currently detectable activity in MAXI. On the other hand, it has been only two years since the last outburst, so it is still unclear whether this phase of activity represents a weak phase of flaring or a full outburst.

Multiwavelength monitoring is being organised and further observations are encouraged. NICER will attempt further observations this week.

NICER view of Swift J1357.-0933 on 2019 May 23