NICER monitoring of the outburst from the transient X-ray pulsar XTE J1829-098

XTE J1829-098 is a transient X-ray pulsar discovered by RXTE in 2004 with a pulse period of 7.8 s (ATel #317, #319) and a probable infrared counterpart (Halpern & Gotthelf 2007, ApJ, 669, 579). RXTE/PCA Galactic Bulge source monitoring found outbursts with a recurrence time of ~246 days lasting for ~7 days. A cyclotron line was detected at ~15 keV (Shtykovsky et al. 2019, MNRAS, 482, L14). The distance of the source is uncertain, and estimates range from 4.5 to 18 kpc (Halpern & Gotthelf 2007; Sguera et al. 2020, MNRAS, 491, 4543) depending on the spectral type of the companion. There is ongoing discussion that the source may be a Supergiant Fast X-ray Transient (SFXT) with a supergiant primary, or, have a main sequence primary and fall at the lower end of the Corbet Luminosity Gap comparable to 4U 0115+63 and V 0332+53 (Christodoulou et al. 2022, ApJ, 929, 137).

Recent activity from the source detected during the third eRosita/SRG survey was followed up by MAXI/GSC and Swift/XRT (ATel #14521, #14554, #15551, #15555).

In response to ATel #15551, NICER started monitoring the source on 2022 August 16, having accumulated >9 ks of data thus far. A preliminary timing analysis in the 0.3-12 keV range confirmed the detection of X-ray pulsations with a period of 7.847089(15) s with at least two significant harmonics. The fundamental, second, and third harmonics have fractional amplitudes of 29.4 +/- 0.7%, 8.9 +/- 0.7%, and 3.5 +/- 0.7%, respectively.

The 0.5-10 keV NICER unabsorbed flux is 3.9e-10 erg/cm^2/s. This implies a luminosity of 4.7e36 erg/s at a distance of 10 kpc. The 0.5-10 keV NICER spectrum is reasonably well described (reduced Chi^2 = 1.04) by an absorbed cutoff power law with a spectral index of 0.36(+/-0.11) with a cutoff energy of 14 keV plus a Gaussian Fe line at 6.61(+/-0.04) keV. Extrapolating to the 3-79 keV range suggests the current luminosity would be a factor of two higher than the 2018 NuSTAR measurement (Shtykovsky et al. 2019). The absorbing column density is N_H=7.0(+/-0.2)e22 cm^-2, which is consistent with previous measurements of strong absorption (e.g. Halpern & Gotthelf 2007).

Previous outbursts of this source suggest the duration of a typical outburst is of order 7 days. Thus, NICER observations are planned over at least the next week. A detailed schedule can be found at: https://heasarc.gsfc.nasa.gov/docs/nicer/schedule/nicer_sts_current.html.

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.