NICER detection of the new X-ray transient MAXI J1727-203

ATel #11689; R. M. Ludlam (U. Michigan), P. M. Bult, K. C. Gendreau, Z. Arzoumanian, C. B. Markwardt, T. E. Strohmayer, S. Mahmoodifar (NASA/GSFC), P. S. Ray (NRL), J. Neilsen (Villanova), W. Iwakiri (Chuo U.), S. Guillot (IRAP/CNRS/CNES), D. Altamirano (U. of Southampton), G. K. Jaisawal (DTU Space), T. Enoto (Kyoto U.), A. L. Stevens (Michigan State U.), S. Eikenberry (U. Florida), J. Homan (Eureka Scientific & SRON), J. Steiner (MIT)
on 6 Jun 2018; 04:12 UT
Credential Certification: Renee Ludlam (rmludlam@umich.edu)

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Following the MAXI/GSC nova alert of a possible new transient (ATel #11683), NICER attempted to observe the source at 2018-06-05T15:40 (UTC), within 75 minutes of the alert. Starting at the nominal coordinates reported by MAXI (with ~20 arcmin error radius), NICER performed a series of offset pointings, resulting in a preliminary improved source position of
R.A. = 262.112 deg
Dec = -20.385 deg
(J2000), with 1.5 arcmin radius uncertainty set by the NICER field of view. There are no previously known X-ray sources in this region, confirming that MAXI J1727-203 is indeed a new transient. The source is clearly detected at a count rate of ~1500 ct/s (0.35-10 keV) at this improved position, well above the typical background level of < 2 ct/s in this band. The light-curve shows significant variability on timescales of seconds, the detailed analysis of which is ongoing.

For the two offset-pointed observations (where MAXI J1727-203 was detected), only a portion of the data was available in real-time telemetry; the remainder is scheduled for later download. Initial spectra were extracted from these real-time datasets. The first observation, with a real-time exposure of 475 s, was collected at a ~2.5 arcmin offset relative to the source position reported above. The continuum spectrum (0.5-10 keV), after applying the Crab correction of Ludlam et al. (2018, ApJL, 858, L5), could be approximated with an absorbed power-law plus disk-blackbody model (chisq = 1098.2 for 944 d.o.f.). The absorption column density was measured at n_H = 4.6(1)E21 cm^-2, the power-law photon index at Gamma = 2.47(4), and the disk temperature kT = 0.20(1) keV. The second observation, approximately 90 minutes later, used the improved position and provided a real-time exposure of 340 s. The Crab-corrected spectrum for this observation can be described using the same continuum model (chisq = 1346.5 for 944 d.o.f.), with similar results: n_H = 4.6(1)E21 cm^-2, Gamma = 2.51(4), and kT = 0.24(1) keV. The photon index suggests steepening but is consistent with the earlier observation within uncertainties; the change in the spectral shape between observations may be due in part to the difference between the on-axis and off-axis response of the instrument. The absorbed 0.5-10 keV flux was 2.60(1)E-9 erg cm^-2 s^-1. We did not detect a prominent Fe line feature in either spectrum.

Further NICER observations of this source are underway. Additional multi-wavelength observations are strongly encouraged.

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.