Unusual spectroscopic (SALT) and photometric (OGLE) behaviour observed in the potential optical counterparts of RX J0032.9-7238.
ATel #16904; I. M. Monageng (UCT/SAAO), M. J. Coe (Southampton), D. A.H. Buckley (SAAO/UCT/UFS), A. Udalski (Warsaw University Observatory)
on 12 Nov 2024; 14:58 UT
Credential Certification: Itumeleng Monageng (itu@saao.ac.za)
Subjects: Optical, Binary, Star
Referred to by ATel #: 16930
The optical counterpart of the candidate Be X-ray binary system RX J0032.9-7238, which recently experienced an X-ray brightening detected by the Einstein Probe mission (ATel#16880), was observed using the Southern African Large Telescope (SALT) on November 7, 2024, starting at 20:59:25 UTC. This observation was conducted with the Robert Stobie Spectrograph and utilized the PG1800 grating, with a total exposure time of 1200 seconds. The grating was set to an angle of 36.875 degrees, covering a wavelength range from 6000 to 7250 Angstroms. The slit was oriented to observe the two proposed optical counterparts (Object 1 and Object 2, as shown in Figure 1a of Stevens et al. 1999).
The spectrum of Object 2 shows the Ha line in absorption, while Object 1 displays the Ha line in strong single-peak emission, with an equivalent width of -33.31 ± 0.90 Angstroms. This measurement is similar to the observations reported by Stevens, Coe, and Buckley in 1999. However, Haberl et al (ATel #16901) report X-ray observations with XMM that makes Object 2 the most likely optical counterpart to RX J0032.9-7348 based upon an accurate X-ray position. The optical behavior of both sources appears to be at odds with the increased X-ray emission. While the SALT spectrum reveals the Ha line in emission, the OGLE lightcurve of Object 1 reveals a long term diminishing of the source's brightness from I=16.80 to I=16.92 over the 23 years. The OGLE lightcurve of Object 2 reveals that the source is currently in its baseline state, which, together with an absence of Ha emission, indicates the absence of a Be disc.
Additionally, a 23-year analysis of OGLE I-band data was performed on the counterpart to Objects 1 and 2 to search for any periodic behaviour. The large majority of the data that excluded the two large outbursts in Object 2 were used. However, no periodic behaviour was found within the search range of 2 to 200 days in both objects. The 7s pulse period reported by Haberl et al would suggest a 20-30d binary period based upon the Corbet diagram.
An attachment of the error circles of the different X-ray missions (Einstein Probe, S-CUBED, XMM and ROSAT) on an optical image of the field is included. The ROSAT position and error are from Kahabka & Pietsch (1996; A&A 312, 919K).
RX_J0032_error_circles
