SIMBAD references

We report on preliminary results of ongoing Swift observations of the Be/X-ray binary GX 304-1. First activity of this source was reported on 2010 December 9 (ATEL #3075). Since December 12 several flares were detected, with a flux increasing to 500mCrab. This might indicate a further giant outburst as seen in August this year (ATEL #2779), which was the sixth outburst after 28 years of quiescence with a Swift/BAT flux of 1.4Crab. The time between each outburst of 132.5 days might indicate the orbital period of the binary, with the peak expected for December 23. Currently the peak flux as seen in Swift/BAT is around 1.18Crab. However, the outburst is eventually decaying already.

The 2.5-9keV quick look data of the first three Swift/XRT pointings performed on MJD 55547.9, 55549.3, and 55551.1, can be well described by an absorbed power law (reduced chi^2 of 1.35). The equivalent hydrogen column, N_H, seems to be constant at 4.7±0.3 10^22 cm^-2. This is also found for the photon index of 1.77±0.06. The 2.5-9keV source flux was 1.05±0.06, 1.51±0.07 and 2.19±0.09 times 10^-9 erg/cm^2/s.

The source shows pulsations with a pulse period of 275.38±0.06 s. The pulse profile exhibits a broad double peaked pulse with a small trailing shoulder. This period is in agreement with the period found by Fermi/GBM of 275.40±0.05 s (Finger at al.), and corresponds to a slight spin down from the ∼272.3s reported by McClintock et al. (1977). Pulse arrival time analysis shows indications of arrival time differences due to orbital motion, however, the data do not yet allow a determination of the orbital parameters. We are continuing to monitor the outburst with RXTE and Swift. Observations at other wavebands are encouraged.