Brightening of MAXI J0903-531 Seen with Fermi GBM and Swift BAT, and a Candidate Orbital Period

The transient X-ray binary system MAXI J0903-531 (ATel #14555) contains an X-ray pulsar with a period of approximately 14 s (ATel #14559, #14604). It has an optical counterpart (ATel #14557) that has been classified as a Be star (ATel #14564). Optical photometry has revealed periods of 3.84 days (ATel #14564) and 0.79 days (ATel #14568, #14569).

Hard X-ray light curves are presently available from the Swift/BAT transient monitor (15 - 50 keV; https://swift.gsfc.nasa.gov/results/transients/index.html) with measurements currently provided since MJD 59,323 (2021-04-19), and measurements of the pulsed flux are provided by the Fermi GBM (12 - 50 keV; https://gammaray.nsstc.nasa.gov/gbm/science/pulsars.html) for times since MJD 58,850 (2020-01-02).

Both the BAT and the GBM indicate that MAXI J0903-531 has recently experienced another outburst. The GBM has detected pulsed emission starting approximately MJD 59,375 (2021-06-10) at a pulsed flux level of ~0.1 keV cm^-2 s^-1. The determination of the time of an increase in the flux in the BAT is hampered by the large error bars, but the daily average shows an increase after about this time reaching ~40 mCrab, which may now be returning to a lower level.

Although the MAXI 2 - 20 keV light curves of this source (http://maxi.riken.jp/star_data/J0905-535/J0905-535.html) do not clearly show a strong recent brightening, this may be due to different instrument sensitivities. The Fermi GBM pulsed flux light curve also shows detections between time ranges of MJD 59,204.5 - 59,207.6, 59,255.6 - 59,261.5, and 59,318.4 - 59,324.4.

Large frequency changes are observed during some outbursts but with a possible longer term spin-down trend. We note that the intervals between the central times of these detections are ~53, 63, and 57 days. This may indicate that the activity is connected to periastron passage in an eccentric system.

If MAXI J0903-531 has an orbital period of ~57 days, then the source's location in the pulse period/orbital period diagram (e.g. Corbet 1986, MNRAS, 220, 1047) would be typical for a Be star HMXB, while a 3.84 or 0.79 day period would be exceptionally short. In this case the shorter period could, as noted in ATel #14568, be a pulsation or rotation period of the Be star.

With this ephemeris, another outburst would be expected around August 6, although with several days uncertainty.