New outburst from the X-ray pulsar RX J0440.9+4431 and discovery of a possible ~155 days orbital period
ATel #3137; S. Tsygankov (MPA, IKI), A. Lutovinov (IKI), R. Krivonos (MPA, IKI)
on 31 Jan 2011; 11:42 UT
Credential Certification: lutovinov elbrus (aal@hea.iki.rssi.ru)
Subjects: X-ray, Binary, Pulsar
On the basis of recent SWIFT/BAT observations we report a detection of the third consequent outburst from the X-ray pulsar RX J0440.9+4431, which was started on 2011-01-29.
The persistent X-ray source RX J0440.9+4431 has been found during the ROSAT Galactic plane survey with the optical companion of BSD 24-491 / LS V +44 17) classified as a Be star (Motch et al., A&A, 323, 853, 1997). Later Reig, Roche (MNRAS, 306, 100, 1999) detected X-ray pulsations from this source with the period of ~202.5 s. First evidence of the outburst activity of the pulsar in X-rays has been found by Morii et al. (Astron. Telegram #2527, 2010) in late March 2010 with the MAXI all-sky monitor. The next outburst was repeated ~5 months later in September 2010 (Krivonos et al., Astron. Telegram #2828, 2010).
Peak luminosities of the flares are lower then ~10^{37} erg/s (assuming the distance to the source of 3.3 kpc), that allow us to consider them as Type I outbursts, typical for Be binary systems. It is believed that such events are separated by one orbital period of the system. Taking this into account we can estimate the orbital period of RX J0440.9+4431 as ~155 days (a mean time between starts of the outbursts). This value is in a good agreement with the period of about 150 days derived from the Corbet diagram of "P_{spin} vs P_{orbit}" (Corbet, MNRAS, 220, 1047, 1986).
Thus, this discovery makes RX J0440.9+4431 only the second (after 4U 0352+30) persistent Be/X-ray pulsar with the known orbital period. We encourage follow-up observations in other wavebands.
