VAR CAS 2006, A Nearby Microlens?

ATel #942; D. Spiegel et al. (Columbia U.)
on 12 Nov 2006; 19:55 UT
Credential Certification: Joseph Patterson (jop@astro.columbia.edu)

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D. Spiegel, J. Patterson, E. Gotthelf, J. Sokoloski, N. Zimmerman, N. Mirabal, Columbia U.; T. Krajci, CBA-New Mexico, R. Koff, CBA-Colorado; P. de Ponthiere, CBA-Lesve; A. Oksanen, CBA-Finland; S. Dong, S. Gaudi, L. Watson, Ohio State U.; R. Remillard, MIT Kavli Institute for Space Research. Time-series photometry of the new transient in Cassiopeia (GSC 3656-1328, see CBET #711) during November 1-10 with the telescopes of the Center for Backyard Astrophysics (CBA) reveals no variability other than the timescale of overall decay. Snapshot BVRI magnitudes show no discernible change in color, and continued spectral coverage is consistent with the A-star description given by Munari et al. (CBET #718), with no emission components. Study of the RXTE All-Sky-Monitor database shows no detections over the 10-year lifetime of RXTE, and a 5000 s observation with SWIFT on November 3 shows no 0.5-10 Kev flux to a limit of 10**-12 ergs cm-2 s-1. These observations are difficult to reconcile with any of the easy-to-imagine theories for the transient's origin: a dwarf nova, an X-ray transient, an erupting shell star. But the properties and light curve can be acceptably fit by a microlens interpretation (see also ATEL #931), despite the very low optical depth to microlensing for such a nearby star (1 Kpc). This may provide an opportunity to study a nearby microlens, with observable effects from parallax. Continued photometric and spectroscopic observation is vital to severely test this idea. Archival searches of this region during the pre-maximum phase (October 18-31) are especially critical, as the existing data are sparse. Since the transient appears to have reached V=7.5, even images from very small cameras may be quite helpful. Of course, any evidence that GSC 3656-1328 is an intrinsic variable star is even more crucial!