X-ray brightening and softening of RS Ophiuchi monitored with NICER
ATel #14864; Teruaki Enoto (RIKEN), Marina Orio (University of Wisconsin and INAF-Padova), Andrew Fabian (IoA, Cambridge), Michael Parker (IoA, Cambridge), Jon M. Miller (Univ. of Michigan), Pragati Pradhan (MIT Kavli Institute for Astrophysics and Space Research), Keith Gendreau (NASA/GSFC), Zaven Arzoumanian (NASA/GSFC), Hiroyuki Maehara (National Astronomical Observatory of Japan), Elizabeth C. Ferrara (University of Maryland, CRESST, NASA/GSFC), Richard Ignace (East Tennessee State University)
on 17 Aug 2021; 22:02 UT
Credential Certification: Teruaki Enoto (teruaki.enoto@gmail.com)
The NICER X-ray observatory continued daily-cadence monitoring of the ongoing outburst of the recurrent nova RS Opiuchi (RS Oph) for four additional days (see ATels #14834, #14838, #14840, #14846, #14850, #14851, #14852, #14855, #14857, #14858, #14860, #14863). As of August 15, there are 47 snapshots with a few hundred exposures for each. The accumulated exposure was 19.6 ks in total.
The NICER 1-10 keV count rate increased from ~54 cps (MJD 59436.5, August 10) to ~144 cps (MJD 59441.9, August 15), whereas the spectral hardness, the 2-10 keV rate relative to the 1-2 keV band, is continuously decreased from 7.1 to 1.3. The absorbed 1-10 keV flux increased from 6e-10 erg/s/cm2 to a maximum of 1.1e-9 erg/s/cm2 at around MJD 59440.3 (August 14), and then started to decline. The 2-10 keV flux peaked around MJD 59440.3, while the 1-2 keV flux was continuously increasing.
The X-ray spectra can be approximated with two components of collisionally ionized plasma, with absorption. The temperatures of the two components are ~1.1 and ~4-6 keV. The temperature of the former component was almost constant during the monitoring, while the temperature of the latter gradually decreased. Preliminary spectral analysis suggests that the velocity of the emission components remained around a few hundred km/s after the initial decrease at MJD 59436.5-59438.0 (ATels #
14850). The photon intensity of the He-like iron emission increased until MJD 59440.3 (August 14), and then decreased again. At the onset of our observation, we measured the line center was at 6.71 keV, and at 6.69 keV after MJD 59438.
Preliminary timing analysis on the NICER lightcurves has not revealed any coherent pulsations at this stage.
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.
The preliminary NICER light curve
