NuSTAR X-ray Observation of the Outburst of Magnetar XTE J1810-197

ATel #12297; E. V. Gotthelf, J. P. Halpern (Columbia U.), B. W. Grefenstette, F. A. Harrison, K. C. Madsen, H. Miyasaka (Caltech)
on 14 Dec 2018; 18:21 UT
Credential Certification: Eric Gotthelf (eric@astro.columbia.edu)

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Based on reports of renewed radio activity from XTE J1810-197 (ATel #12284, #12285, #12288), we initiated a NuSTAR Director's Discretionary Time observation of the magnetar from UT 2018 Dec 13 03:59:08 to Dec 13 23:49:24 resulting in 21.9 ks of usable data. A bright source was detected with a steady count rate of 4.8 counts/s with flux up to at least 30 keV in the hard (3-79 keV) NuSTAR bandpass. The source location is consistent with XTE J1810-197, given the large coordinate uncertainty (~2') associated with a NuSTAR observation close (10 degrees) to the Sun. The high count rate is also consistent with the X-ray flux enhancement for XTE J1810-197 observed by MAXI/GSC (ATel #12291). A preliminary analysis reveals that the spectrum is similar to that found during the discovery outburst of 2003 (Ibrahim et al. 2004, ApJ, 609, L21), with a blackbody temperature of kT = 0.73+/-0.01 keV and a non-thermal power-law tail with a steep photon index of 4.28+/-0.11. The absorption column, characterized by the TBABS model in XSPEC, is not well-constrained in the NuSTAR bandpass and is held fixed in the fit to NH = 1.0E22 cm^-2. The absorbed 2-10 keV flux is 2.0E-10 erg/s/cm^2, a factor of 2 greater than the 2003 projected maximum outburst flux of (0.8-1.1)E-10 erg/s/cm^2, determined by fitting an exponential decay model to the historic light curve (Gotthelf & Halpern 2007, Ap&SS, 308, 79). A strong pulsed signal is recovered at a barycentric period of 5.5414468(43) s at MJD 58465.2, where the uncertainty on the least significant digits, given in parentheses, is computed from delta Z^2_1 = -1 from the peak period in the Z^2_1 periodogram. The pulse profile is close to a pure sine curve with a pulsed fraction of ~29%, similar in shape to that recorded by XMM-Newton during the previous outburst, but less modulated. There is no evidence for bursts on second timescales, as detected by RXTE during the earlier outburst (Woods et. al. 2005, 629, 985); however the chance of catching a burst during the short NuSTAR observation is low.