NICER observations of X-ray transient LXT 221107A: an exceptional flare from the star HD 251108 (2RXS J060415.1+124554)

ATel #15755; Dheeraj Pasham (MIT), K. Hamaguchi (CRESST II NASA/GSFC & UMBC), J. M. Miller (Univ. of Michigan), T. Enoto (Kyoto Univ./RIKEN), G. K. Jaisawal (DTU Space), M. Ng, Hans Moritz Guenther (MIT), P. Bult (UMCP/NASA GSFC), A. Sanna (Univ. of Cagliari), D. J. K. Buisson (Independent), T. Strohmayer, Z. Arzoumanian, K. C. Gendreau (NASA GSFC), Thomas Wevers (ESO), Muryel Guolo (Johns Hopkins)
on 11 Nov 2022; 18:34 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Dheeraj Pasham (drreddy@mit.edu)

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The X-ray transient LXT 221107A was detected by the Lobster Eye Imager for Astronomy (LEIA)/Einstein Probe-WXT Pathfinder on 7 November 2022 (ATel #15748). Multiple exposures between 2022-11-07T06:26:36 and 2022-11-08T12:25:24 showed increasing 0.5-4.0 keV flux, from 7.7e-11 erg/s/cm^2 to 3.6e-10 erg/s/cm^2. As reported in ATel #15748, a cataloged ROSAT source lies 2.5 arcmin from the nominal coordinates of LXT 221107A, within the 3-arcmin-radius positional uncertainty derived from the LEIA measurements. The ROSAT source, 2RXS J060415.1+124554, has previously been identified (Kiraga 2012) as the X-ray counterpart of a bright (V = 9.9 mag) and nearby (distance of 505+/-5 pc from Gaia parallax; Bailer-Jones et al. 2021) late-type (K) star, HD 251108, but at a flux level 10 times lower than the LEIA transient. Low-level chromatic optical variability (~0.1 mag amplitude, not periodic) from HD 251108 has been reported by both ASAS-SN and Gaia.

To test the possible association of LXT 221107A with the X-ray-bright star, NICER observed the ROSAT coordinates directly, and also carried out a 9-point grid search of and around the LEIA source error region. The grid search yielded no additional candidate sources. The first observation of the ROSAT source, beginning at 2022-11-09T18:41 UT (< 7 hours after ATe l#15748), detected emission with a mean count rate of 90 s^-1; subsequent observations through 2022-11-10T13:00 UT show a declining trend, with the most recent observation at a countrate of 62 s^-1. Power-spectral analyses reveal no evidence for periodic or quasi-periodic variability in the frequency range of 0.005-2500 Hz.

The 0.3-8 keV time-averaged NICER spectrum (3.7 ks exposure) is well fit by a collisionally ionized plasma (APEC models; vapec in XSPEC) with cool (0.8 keV) and hot (5.3 keV) components, where the emission measure of the hot component is about 20 times larger than the emission measure of the cool component. The abundance pattern shows a Ne/O ratio around 3 and Ne/Fe ratio around 10; this abundance pattern is consistent with the “inverse first ionization potential (IFIP)” distribution discussed in coronally active dwarfs (Testa 2015). The 6.7 keV Fe line is clearly present. The 0.3-8 keV observed flux is estimated to be 1.5 × 10^-10 erg/s/cm², consistent with a recent Swift/XRT measurement (ATel #15754).

These observations combined with the prior ROSAT detection suggest that LXT 221107A is from a coronally active K giant, likely with large active areas on the surface that provide the magnetic field for this long-lasting X-ray outburst; magnetic fields and chromospheric/coronal activity on K giants has been known for decades (Huensch 1996), but a flare of this magnitude is exceptional. NICER plans to continue observing this flare into quiescence. NICER’s observing schedule can be found here: 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.

References: Kiraga 2012 - https://ui.adsabs.harvard.edu/abs/2012AcA....62...67K/abstract Testa 2015 - https://ui.adsabs.harvard.edu/abs/2015RSPTA.37340259T/abstract Huensch 1996 - https://ui.adsabs.harvard.edu/abs/1996A%26A...310..801H/abstract