NICER follow-up observation of the transient magnetar XTE J1810-197

Referred to by ATel #: 12600, 12689 Tweet Following recent reports of re-brightening of the transient anomalous X-ray pulsar XTE J1810-197 (ATels #12284, #12285, #12288, #12291, #12297), we report Neutron Star Interior Composition Explorer (NICER) observations, performed on 2019 February 6 at 23:59:40 UTC, as soon as the Sun avoidance criterion allowed.

The exposure time of the initial observation is 3189 s. Individual pulses from the source are clearly detected with NICER. Our timing analysis in the 1.0-10 keV range, yields a barycentric spin frequency of 0.18045714(6) Hz (consistent with the extrapolation of the timing solution presented by Pintore et al. 2019).

The corresponding RMS pulsed fraction in this energy band is 0.205+/-0.003. We extracted an X-ray spectrum from this initial data set (grouped to have at least 50 counts per channel), and modeled it with an absorbed (tbabs model with Wilms et al. 2000 abundances) blackbody plus a power-law model in the 0.5-10 keV range.

The best fit parameters we found are NH = (1.35 +/-0.04) x10^22 cm^-2, kT = 0.669 +/-0.003 keV and normalization of 86.6 +/-1.9 (R^2/D_10^2) and the slope of the power-law is \Gamma=2.97 +\-0.10. The hydrogen column density we find is somewhat larger than what has been reported for this source earlier.

The resulting reduced Chi2 is 1.0449 for 475 degrees of freedom. All uncertainties are 1-sigma statistical uncertainties. The absorbed and unabsorbed 0.5-10 keV fluxes of the source with such a model are (1.67 +/-0.01) x10^{-10} erg/s/cm^2 and (3.34 +/-0.03) x10^{-10} erg/s/cm^2, respectively. For comparison, the 2-10 keV absorbed flux is (1.27 +/-0.01) x10^{-10} erg/s/cm^2, which is almost 2.3 times larger than the flux reported by Ibrahim et al. (2004) as 5.5 x10^{-11} erg/s/cm^2 from RXTE/PCA data based on observations obtained on 2003 July 18, more than 6 months after the start of that outburst.

Our flux measurement also shows that the source decayed to ~43% of the flux reported by MAXI (ATel #12291) and to about 63% of the flux observed by NuSTAR (ATel #12297). In this initial NICER dataset, there is no indication of the line feature at around 1.2 keV detected in the previous outburst (Bernardini et al. 2009, Alford & Halpern 2016, Coti Zelati et al. 2018, Pintore et al. 2019, Vurgun et al. 2019).

The NICER team will continue to monitor XTE J1810-197. 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. References Alford & Halpern 2016, ApJ 818, 122 Bernardini et al. 2009, A&A 498, 195 Coti Zelati et al. 2018, IAU Symposium, 337, 326 Ibrahim et al. 2004, ApJL, 609, L21 Pintore et al. 2019, MNRAS, 483, 3832 Vurgun et al. 2019, NewA 67, 45 Wilms et al. 2000, ApJ, 542, 914