NICER X-ray observations of the recurrent nova RS Ophiuchi
ATel #14850; Teruaki Enoto (RIKEN), Hiroyuki Maehara (National Astronomical Observatory of Japan), Marina Orio (University of Wisconsin and INAF-Padova), Andrew Fabian (IoA, Cambridge), Michael Parker (IoA, Cambridge), Keith Gendreau (NASA/GSFC), Zaven Arzoumanian (NASA/GSFC), Pragati Pradhan (MIT Kavli Institute for Astrophysics and Space Research), Elizabeth C. Ferrara (University of Maryland, CRESST, NASA/GSFC), Jon M. Miller (Univ. of Michigan)
on 11 Aug 2021; 20:52 UT
Credential Certification: Teruaki Enoto (teruaki.enoto@gmail.com)
A new outburst of the recurrent nova RS Opiuchi (RS Oph) was reported on 2021 August 8 (ATels #14834, #14838, #14840, #14846), 15 years after the last outburst in 2006 (ATel #737). The Neutron Star Interior Composition Explorer (NICER) observed RS Oph on 2021 August 10, from 11:57:00 to 12:14:20 UTC, with an exposure of 942 sec. This observation was coordinated simultaneously with optical monitoring of the 3.8-meter SEIMEI Telescope to study the maximum of the optical light curve.
During the NICER observation, the 1-10 keV count rate was stable at around 55 cts/s. The X-ray spectrum can be approximated with a model of collisionally ionized gas with a temperature of ~5 keV and the absorption column density of ~6e+22 cm-2 (implying large intrinsic absorption in addition to the interstellar value of about 2.0E+21 cm-2). A prominent iron emission line was observed at 6.7 keV, along with several other possibly unresolved Sulphur lines in the soft X-ray band around 2.5 keV. The absorbed 1-10 keV flux was 5e-10 erg/s/cm2.
Two NICER observations taken 17 hours later showed a slightly lower absorption (~3e+22) and an increase in flux especially below 2 keV, reaching ~ 7e-10 erg/s/cm2 in the 1-10 keV band with ~ 68 cts/s. Preliminary spectral modeling suggests that the velocity of the shock-heated ejecta evolved from 3100 km/s to 1050 km/s between the first two observations, further slowing down to 980 km/s in the third observation.
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.
We are planning to continue NICER observations of RS Oph. NICER observations planned for RS Oph and other targets can be found at https://heasarcdev.gsfc.nasa.gov/docs/nicer/schedule/nicer_sts_current.html.
