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

R. M. Ludlam*, P. M. Bult, K. C. Gendreau, Z. Arzoumanian, C. B. Markwardt, T. E. Strohmayer, S. Mahmoodifar, P. S. Ray, J. Neilsen, W. Iwakiri, S. Guillot, D. Altamirano, G. K. Jaisawal, T. Enoto, A ~L. Stevens, S. Eikenberry, J. Homan, J. Steiner

*Corresponding author for this work

Research output: Other contributionNet publication - Internet publicationResearch


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.
Original languageEnglish
Publication date6 Jun 2018
Publication statusPublished - 6 Jun 2018
SeriesThe Astronomer's telegram
NumberATel #11689


  • Request for Observations
  • Transient
  • X-ray

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