Eclipse-like Absorption in MAXI J0556-332

ATel #3110; T. E. Strohmayer (NASA/GSFC)
on 15 Jan 2011; 03:48 UT
Credential Certification: Tod Strohmayer (tod.strohmayer@nasa.gov)

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Additional RXTE observations of the new transient MAXI J0556-332 (ATel #3102, #3103, #3104, #3106) were obtained beginning on January 14, beginning near 17:02:00 UTC. The flux of the source has declined only slightly and the X-ray spectrum is also largely unchanged. The two orbit PCA exposure approximately brackets an absorption feature that is strongly suggestive of an eclipse. The ingress and egress are approximately symmetric, but absorption is not total, nor "sharp," as might be the case when some fraction of flux remains visible due to scattering. In hindsight, the drop in flux reported from the initial RXTE observation (ATel #3106) is quite similar to the apparent ingress evident in the present observations. Assuming these flux drops are indeed partial eclipses, then we obtain a constraint on the orbital period. The ingress to ingress time is approximately 1.175 days. This gives possible orbital periods of 1.175, 0.5875, 0.392, and 0.294 days. The eclipse duration is approximately 0.075 days, and would tend to rule out the shorter possible periods simply because an eclipse is unlikely to span so much of the orbit. The midpoint of the eclipse occurs on January 14 at ~17:56:58.004 UTC. I note that the high inferred column depth reported in ATel #3106 is likely unphysical, and resulted from the very steep power-law resulting from that model fit. The spectrum can also be modeled as the sum of a disk black body (diskbb in XSPEC) and a regular black body (bbodyrad in XSPEC). With this model the RXTE data do not require any significant absorption. Additional longer RXTE observations are planned.