NICER confirms a new outburst from IGR J17498-2921

ATel #15998; A. Sanna (Univ. of Cagliari), M. Ng (MIT), S. Guillot (IRAP/CNRS), D. Altamirano (Univ. of Southampton), K. Gendreau (NASA GSFC), Z. Arzoumanian (NASA GSFC), E. C. Ferrara (Univ. of Maryland / NASA GSFC), D. Chakrabarty (MIT), T. E. Strohmayer (NASA/GSFC), P. S. Ray (NRL), S. Bogdanov (Columbia), P. M. Bult (RIVM)
on 20 Apr 2023; 20:16 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Andrea Sanna (andrea.sanna@dsf.unica.it)

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On April 13-15, 2023, INTEGRAL detected a possible new outburst from the accreting millisecond X-ray pulsar IGR J17498-2921 (ATel #15996). We performed targeted follow-up observations with NICER starting on April 19, 00:44 UTC. Our first observation collected a total of 3.4 ks of exposure. We detect the source at a constant count rate of ~80 ct/s in the 0.5-10 keV band, well above the ~1 ct/s background level.

To search for the coherent pulsation at ~401 Hz, we barycenter-corrected the photon arrival times and folded the data using the binary ephemeris measured during the 2011 outburst (Papitto et al. 2011, A&A, 534, L4). We detected the pulse at >15 sigma significance. The local time of passage through the ascending node (TASC ~ 60053.92544(1) MJD) occurred about 38 seconds later than predicted from the 2011 orbital solution (with the difference between the two values within two sigma), assuming a constant pulse frequency. The comparison between the current spin frequency value and that measured at the end of the 2011 outburst does not reveal a significant spin evolution.

The pulsed emission was also independently detected (at ~401 Hz) in an acceleration search over the 395-405 Hz frequency range in the 2-10 keV band, with a single-trial significance of 4 sigma. The detection of pulsations at ~401 Hz confirms that IGR J17498-2921 started its second outburst. No significant aperiodic variability or broad-band noise is detected from the power density spectrum of the source.

The 1-10 keV energy spectrum is well described using an absorbed blackbody and a Comptonization continuum (nthcomp), with a best-fit chi-square of 144 for 124 degrees of freedom. We find an absorption column density of 2.0(1) x 10^22 cm^-2, a blackbody temperature of 0.30(3) keV, and a Comptonization power-law index of 1.76(2). The unabsorbed (0.5-10 keV) flux is estimated at ~1 x 10^-9 erg/s/cm^2, which matches the 2011 peak flux (Papitto et al. 2011, A&A, 534, L4).

Further NICER observations of this source are underway. Additional multiwavelength follow-up is 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.