Likely new outburst of the accretion-powered millisecond pulsar SAX J1808.4-3658 indicated by increased optical and X-ray intensity

ATel #12993; Adelle J. Goodwin (Monash University), David M. Russell (NYU Abu Dhabi), Duncan K. Galloway (Monash University), Jean J. M. in 't Zand (SRON), Craig Heinke (U. of Alberta), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU), and Maria Cristina Baglio (NYU Abu Dhabi)
on 7 Aug 2019; 02:26 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Duncan K. Galloway (duncan.galloway@monash.edu)

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Regular optical (i'-band) and X-ray (0.3-10 keV) monitoring of the accretion-powered 401 Hz pulsar SAX J1808.4-3658 (RA = 18:08:27.54; Dec = -36:58:44.3) has revealed an abrupt increase in intensity beginning August 5, likely indicating the onset of a new transient outburst. This activity is the first seen from the source since the last outburst in 2015 (ATel #7371, #7376, #7379, #7380, #7469).

In observations carried out with the 2-m Faulkes Telescope South (at Siding Spring, Australia) and the Las Cumbres Observatory (LCO) network of 1-m robotic telescopes, the optical i'-band magnitude fluctuated between 0.7-1.0 mag brighter than the quiescent level (typically approx. 19.8 mag) between July 25 - August 3 (i' = 18.8-19.5). Between August 3.85 and 5.98 the i-band intensity increased to approx. 2 mag brighter than the quiescent level (i' = 18.16 +/- 0.06 and i' = 17.88+/-0.01, respectively).

We triggered monitoring observations of SAX J1808.4-3658 with the Neil Gehrels Swift Observatory (Swift) X-Ray Telescope (XRT) beginning 2019 July 17, with a cadence of 2 days. Following the optical brightening reported on July 25 (ATel #12964), Swift/XRT did not detect the source through August 4. A 0.6 ks photon counting mode observation on August 6.44 detected 6 counts from a region 16 arcsec in radius surrounding the source position, with an estimated background contribution of 0.3 counts. The measurement corresponds to a detection significance of 5-sigma.

The X-ray detection on August 6 likely confirms that the July 25 optical brightening of the source was a precursor to a new transient outburst. The estimated unabsorbed X-ray flux (0.5-10 keV) of 1E-12 erg/cm^2/s (for an absorbed power law spectrum with photon index 1.5 and neutral column of 1E21 cm^-2, and including a correction for the flux outside the extraction radius) is significantly higher than the typical quiescent flux of 5E-14 erg/cm^2/s (0.5-10 keV, Heinke et. al 2007, ApJ 660, 1424). For a distance of 3.5 kpc (Galloway & Cumming 2006, ApJ, 652, 559) the inferred X-ray luminosity is roughly 1.5E33 erg/s, which may be the lowest the source has been detected at in the early part of any outburst.

Future observations with Swift and NICER have already been scheduled, and further observations are encouraged. The 12-d delay between the first optical activity and the subsequent X-ray brightening indicates that optical brightening may be used in the future as an early warning sign for the onset of outburst.

We thank the Swift team for the scheduling of the reported observations. The LCO observations are part of an on-going monitoring campaign of ~ 40 low-mass X-ray binaries (Lewis et al. 2008, arXiv:0811.2336) with LCO and the Faulkes Telescopes. This work makes use of observations from the Las Cumbres Observatory (LCO). We acknowledge the support of the NYU Abu Dhabi Research Enhancement Fund under grant RE124.