Swift and SMARTS observations of the 2015 outburst of V1118 Ori

ATel #8548; Marc Audard (University of Geneva), Kenji Hamaguchi (CRESST NASA/GSFC and UMBC), Joel Kastner (Rochester Institute of Technology), Nicolas Grosso (Observatoire Astronomique de Strasbourg), Frederick M. Walter (Stony Brook University)
on 15 Jan 2016; 11:23 UT
Credential Certification: Marc Audard (Marc.Audard@unige.ch)

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Swift observed V1118 Ori on 2015 Dec 11-12 (5.17 ksec, obs ID 00034203001) and on Dec 16 (2.45ks, obs ID 00034203002) during the latest mass accretion outburst reported by Lorenzetti et al. (ATel #8100). We also followed V1118 Ori with SMARTS/ANDICAM to obtain its optical/infrared photometry.

V1118 Ori was not detected in either of the Swift observing runs. Combining the data sets for a total exposure time of 7.62ks, we detect 6 events within a radius of 10 pixels (~23.5") in the 0.2-10 keV range (80% PSF). This extraction radius is smaller than the 90% PSF at 1.5 keV (20 pixels, i.e., 47") to avoid contamination by the nearby (46.6" separation) Herbig Ae star V372 Ori. In contrast, within a nearby background region of radius 33 pixels that is free of point sources (based on a deep earlier XMM-Newton observation, see Audard et al. 2010, A&A, 511, A63), we obtain 38 events. Scaled to our 10 pixel extraction radius, this corresponds to 3.49 events. We used the Kraft et al. (1991, ApJ, 374, 344) approach to determine confidence limits (CL) in the low-count regime. We conclude that the source is undetected. The 95% CL upper limit for the flux is 2.66E-14 erg/s/cm2, adopting an absorbed plasma model parametrized by NH=3E21 cm-2, logT=7.1 (1.08 keV), Z=0.2, which converts to an X-ray luminosity of 1.22e30 erg/s for a distance of 400 pc, similar to the X-ray luminosity measured from detections of V1118 Ori in the early phases of its 2005 outburst.

This comparison indicates that V1118 Ori was, if anything, a weaker X-ray source in 2015 December than when observed during the peak of its 2005 outburst. The X-ray fluxes during the decay phase of the 2005 outburst were lower, i.e., between 0.5 and 0.9E-14 erg/s/cm2, consistent with the 2015 December upper limit. Overall, these results indicate that, as in the 2005 event, V1118 Ori has not exhibited a significant increase in X-ray flux during its latest outburst. This behavior stands in contrast to that observed during the 2003 and 2008 outbursts of V1647 Ori (Kastner et al. 2006, ApJ, 648, L43; Teets et al. 2011, ApJ, 741, 83).

We obtained BVRIJHK photometry with SMARTS/ANDICAM starting MJD 57360.71 and continuing through JD 57398.65. Mean magnitudes range from 14.96+/-0.20 (B) through 9.84+/-0.10 (K), where the uncertainties are the standard deviation of the means of the 24 observations; uncertainties on individual observations range from <0.01 mag in the optical through 0.04 mag at J. No temporal trends are seen in the fluxes. The magnitude of the variability ranges from 0.74 mag (B,V) through 0.39 mag (H,K); the target is redder when fainter. This photometry is similar to the level observed during the early decaying phase of the 2005 outburst (e.g., MJD 53727 in Audard et al. 2010). The values are, however, still about 2.5-3 mag brighter than the quiescent level measured before the 2005 outburst.