NICER observations of the new transient SWIFT J005139.2-721704 in the Small Magellanic Cloud

S. Guillot, G. Vasilopoulos, D. Pasham, G. K. Jaisawal, P.S. Ray*, M. T. Wolff, K. C. Gendreau, T. E. Strohmayer, Z. Arzoumanian, M. Corcoran, D. Altamirano, C. Wilson-Hodge, V. Antoniou, A. Zezas, F. Haberl

*Corresponding author for this work

Research output: Other contributionNet publication - Internet publicationResearch


Referred to by ATel #: 12222, 12224


NICER observed the new transient Swift J005139.2-721704 located in the Small Magellanic Cloud (SMC), and discovered on 2018 Nov. 9 (ATel #12209). The source position identified using Swift XRT as RA=00:51:39.2 and DEC=-72:17:03.6 (with an uncertainty of 1.4″, ATel #12209).

We obtained 8.6 ksec of NICER data between 2018 November 14 at 22:11:16 UTC and 2018 November 15 at 15:36:13 UTC. The average NICER count rate is 22.4 c/s (5.4 mCrab) in the 0.5-8 keV range (after background subtraction of ~0.5 c/s in that range).

The spectrum can be described by an absorbed blackbody+powerlaw model in 0.5-8.0 keV range, with chi2=1.2 (399 d.o.f.), and with best fit parameters NH = (0.56±0.04) ×1022cm-2, kT = 0.120±0.005 keV and a powerlaw photon index Gamma = 1.07±0.03 (all uncertainties are at the 90% confidence level). The flux in that energy range is 1.1×10-10erg s-1cm-2, which corresponds to a luminosity (corrected for absorption) of LX ~7×1037erg s-1, assuming a distance of 60 kpc to the SMC.

A timing analysis on these initial NICER exposures finds no sign of short period coherent pulsations. However, we find significant variability (~30% fractional amplitude) on a timescale of ~600-650 seconds. The current NICER snapshots are too short for a definitive conclusion on a periodicity on such timescale, and additional observations are needed to decipher the nature of this variability. We encourage follow-up observations with other facilities. The short-term schedule of additional NICER observations is as follows:

Start Time-----------------End Time-------------Exposure (in seconds)
2018-319T22:49:20 2018-319T23:20:20 1860
2018-320T06:32:20 2018-320T07:04:40 1940
2018-320T14:15:20 2018-320T14:48:40 2000
2018-320T15:48:00 2018-320T16:21:20 2000
2018-320T21:58:20 2018-320T22:32:20 2040
2018-320T23:31:00 2018-321T00:05:00 2040
2018-321T07:14:00 2018-321T07:49:00 2100
2018-321T08:46:40 2018-321T09:21:40 2100
2018-321T13:24:20 2018-321T14:00:00 2140

NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.
Original languageEnglish
Publication date15 Nov 2018
Publication statusPublished - 15 Nov 2018
SeriesThe Astronomer's telegram
NumberATel #12219


  • Neutron Star
  • Transient
  • X-ray


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