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Optical monitoring of the milli-second X-ray pulsar IGR J00291+5934

ATel #7837; D. M. Russell (NYU Abu Dhabi), F. Lewis (Faulkes Telescope Project and Astrophysics Research Institute, LJMU)
on 24 Jul 2015; 23:01 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

Subjects: Optical, X-ray, Binary, Neutron Star, Transient

Referred to by ATel #: 7852, 7949

Prompted by the reports of X-ray (GCN #18051; ATel #7836) and optical (ATel #7835) flaring of the accreting milli-second X-ray pulsar (AMXP), IGR J00291+5934, we here report optical magnitudes from our long-term monitoring of the source. We have been observing IGR J00291+5934 with the 2-m Faulkes Telescope North at Maui, Hawaii (see ATel #1666; Lewis et al. 2010, A&A, 517, A72). Since the double-peaked outburst of the source in 2008 (Lewis et al. 2010), we have been monitoring the source in i'-band (200 sec exposures).

From images taken on 14 dates in 2015, the source was detected on three dates: 15 January (MJD 57037.24), 6 July (MJD 57209.49) and 23 July (MJD 57226.47). On all other dates the source was not detected (spanning 8 January - 29 June). On 15 January the magnitude was i' = 22.33 +- 0.46, consistent with being in quiescence. The magnitude was i' = 18.34 +- 0.08 on 6 July and i' = 17.51 +- 0.07 on 23 July. This confirms the new outburst of IGR J00291+5934, which is likely to have started between 29 June and 6 July, but which was not significantly detected by X-ray all-sky monitors. The observation on 23 July was by chance just 18 hours before the Swift BAT trigger (GCN #18051). The magnitude i' = 17.51 +- 0.07 is brighter than both outburst peaks of the 2008 outburst (Lewis et al. 2010) and comparable to the brightest it was seen in its first known outburst in 2004 (R ~ 17.4; ATel #354; Torres et al. 2008).

It is currently unclear at what stage this outburst is. It may have had a slow rise in flux over the last few weeks, or the recent flaring may be a second peak and the first one was missed. We encourage multi-wavelength observations of this outburst to better characterize its morphology, and to take broadband data while it is bright. AMXP outbursts typically last just a few weeks or so.

The Faulkes Telescope observations are part of an on-going monitoring campaign of ~ 40 low-mass X-ray binaries (Lewis et al. 2008). This work makes use of observations from the Las Cumbres Observatory Global Telescope Network.