Einstein Probe detects the possible X-ray brightening of the High Mass X-ray Binary RX J0032.9-7348 in the Small Magellanic Cloud
ATel #16880; S. Q. Jiang (NAO, CAS), Q. C. Liu (THU), H. Q. Cheng, W. D. Zhang (NAO, CAS) on behalf of the Einstein Probe team
on 29 Oct 2024; 09:17 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)
We report the possible X-ray brightening of the high mass X-ray binary (HMXB) RX J0032.9-7348 in the Small Magellanic Cloud, as detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe mission (EP). The WXT observation started at 2024-10-27T19:36:51(UTC) with an exposure of about 5.6 ks. During the observation, WXT detected a point source with coordinates of R.A. = 8.232 deg, DEC = -73.807 deg (J2000) with an uncertainty of 2.2 arcmin (radius, 90% C.L. statistical and systematic). To pin down the X-ray source position, we performed two target-of-opportunity observations with the Follow-up X-ray Telescope (FXT) on board EP, which were taken at 2024-10-28T10:04:53 and 2024-10-28T16:29:24, respectively, about 14.5 and 20.9 hours after the WXT detection. An X-ray source was clearly detected by FXT within the WXT error circle, at a position of R.A. = 8.2249, DEC = -73.8094 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The FXT position is consistent with the HMXB RX J0032.9-7348 (R.A. = 8.2342, DEC = -73.8056 (J2000) with an uncertainty of 12.9 arcsec, Haberl & Sturm 2016), indicating a possible association with the HMXB. Variability with a timescale of a few hundreds of seconds is clearly visible in the lightcurves of both FXT observations.
The spectrum of the first FXT observation can be fitted by an absorbed power-law model with a neutral hydrogen column density of 4.77 (-2.40, +2.50) x 10^20 cm^2 and a photon index of 0.81 (+/-0.05). The unabsorbed flux in 0.5-10 keV is 7.95 (-0.30/+0.31) x 10^-11 erg/s/cm^2. The spectral parameters and flux derived from the second FXT observation do not exhibit significant difference compared with those derived from the first one. Uncertainties are quoted at a 90% confidence level for the spectral parameters.
RX J0032.9-7348 stayed in a low-flux state with a flux of ~2 x 10^-12 erg/s/cm^2 in 0.2-2 keV after its discovery by the Rosat All-Sky Survey (RASS) in 1993 (as estimated by using the XMM-Newton upper limit server; Saxton et al. 2022, http://xmmuls.esac.esa.int/upperlimitserver/, assuming an absorption column density of 3 x 10^20 cm^-2 and a photon index of 2). X-ray observations taken between 2010 and May 2024 with other X-ray instruments, such as XMM-Newton or Swift, did not achieve a significant detection of the source, but rather derived upper limits on the X-ray flux of several times 10^-12 erg/s/cm^2 in 0.2-12 keV (also estimated by using the XMM-Newton upper limit server) . The fluxes measured by EP are thus at least a few tens times higher, indicative of a recent brightening/outburst activity of this object if the source detected by EP is indeed RX J0032.9-7348.
EP will continue monitoring this source in the next few days. Further follow-up observations are encouraged. For queries on more information about the X-ray source and data analysis, or coordination of multi-wavelength observations, please contact us at EP science center (ep_ta@bao.ac.cn). The contact persons are QiChun Liu and HuaQing Cheng.
Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE and CNES.
Reference:
Haberl & Sturm 2016, Astronomy & Astrophysics, Volume 586, id.A81, 19 pp.
Saxton et al. 2022, Astronomy and Computing, Volume 38, article id. 100531
