NICER / ISS Science Nugget
for August 18, 2022
NICER Tracks a Faint, Distant, Short-lived Pulsar Outburst
NICER has been monitoring the latest outburst of XTE J1829-098, a binary system consisting of a neutron star that accretes matter from a massive companion star. Discovered in 2004, XTE J1829 is notable for exhibiting pulsed X-ray emission with a repetition period of 7.8 seconds (corresponding to the spin rate of the neutron star), and week-long outbursts that recur approximately every 246 days (the likely orbital period of the binary system). XTE J1829 is challenging to study because it is very distant, and the amount of absorption of X-rays in the interstellar gas between Earth and the pulsar is high - even at the peak of its outbursts, the system is rather faint. Distance estimates range from 15,000 to 60,000 light years; the upper half of this range would place XTE J1829 on the other side of the Galactic Center from Earth. Prior studies have suggested classifying the system as a Supergiant Fast X-ray Transient (SFXT), with the companion being either a supergiant or main-sequence star of spectral type 'B'. The apparent regularity of outbursts with a 246-day cadence suggests that the orbit is elliptical and that the neutron star periodically comes close to enough to the companion to interact with an outflow disk of material surrounding it.
Following up on a recent detection by JAXA's MAXI payload, NICER detected X-ray pulsations with a period of 7.847089±0.000015 seconds, and has resolved the shape and energy-dependence of the pulsation brightness profile. The overall X-ray spectrum is consistent with that of other known accreting X-ray pulsar binaries, with magnetically channeled accretion flow onto at least one of the neutron star's magnetic polar caps. The luminosity of the current outburst is at least twice that seen during an outburst in 2018. NICER will continue monitoring through the end of the outburst. Preliminary results of initial NICER observations, led by M. Wolff (Naval Research Laboratory) were published this week in Astronomer's Telegram #15556.
As the neutron star in XTE J1829-098 spins every 7.847 seconds, we observe the accreting channel of hot plasma from different viewing angles, and the result is the pulsed character of the received X-rays. The top panel shows the pulsation profile across 0.5-10 keV of photon energy; lower panels distinguish the contributions of three distinct energy sub-bands. Two full phases of the neutron star's rotation are shown so that the pulsation can be clearly seen. Figure courtesy Gaurava Jaisawal (DTU Space).