MAXI J0903-531: Detection of a 0.79d period with TESS
ATel #14568; Rivera Sandoval, L. E. (U. Alberta), Maccarone, T. (TTU), Strader, J. (MSU)
on 22 Apr 2021; 03:48 UT
Credential Certification: Liliana Rivera Sandoval (lriveras@ualberta.ca)
Subjects: Optical, Transient
MAXI J0903-531 is a newly discovered X-ray transient (ATel #14555, #14557, #14558, #14564). The optical counterpart has been classified as an early type star of the spectral type B1.5-2 III-V, with an average V-band magnitude from ASAS-SN of 13.3 mags (Atel #14564).
Due to its large brightness, the counterpart was detected by the Transiting Exoplanet Survey Satellite (TESS) on 2 different sectors, with start observation dates: 2019-02-02 and 2019-03-26. Each sector observed the target continuously for approximately 27 days and with a cadence of 30 min. A Lomb-Scargle analysis on the background subtracted light curves reveals a strong periodicity at 0.79d which is observed in both sectors. This period could correspond to non radial g-mode pulsations, which are commonly observed in early type stars. Though using the tables in Pecaut & Mamajek (2013, ApJS, 208, 9) we find that a B1.5-B2V star should undergo Roche lobe overflow for a period of 30-35 hours (and the period would be longer for an evolved Be star). Thus, it is possible that the TESS observed period could correspond to ellipsoidal variability, with the primary peak in the periodogram at half the orbital period for a binary just wider than Roche lobe overflow, but such a short period would be unprecedented for a Be X-ray binary. Then it is more likely that this period is a pulsation period or a rotation period, either of which is plausible for a Be star (Rivinius, 2005, Habilitationsschrift, Ruprecht-Karls Universitat). If the period is related to the orbit, it argues strongly against the evolved Be star possibilities. TESS observations in a third sector with start date 2019-02-28 seem to be affected by instrumental effects and therefore were discarded from the analysis.
We do not observe a peak in the periodogram at the 3.84d period reported in Atel #14564.
