Swift does not detect a source near H 1743-322

ATel #3295; S. Motta, T. Belloni, S. Campana, T. Munoz-Darias (INAF - Brera Observatory)
on 19 Apr 2011; 10:45 UT
Credential Certification: Sara Elisa Motta (sara.motta@brera.inaf.it)

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A low-frequency oscillation with a period of approximately 91 s was visible in the RXTE/PCA light curve of the black-hole candidate H1743-322 (ATel #3277), in outburst since April 6, 2011 (ATel \#32763) and currently in hard state. The oscillation was detected only in the first RXTE observation (performed 16:05:01 (UTC) on April 12, 2011). The hypothesis that the oscillations are due to a second active source in the PCA field of view was put forward. We triggered a Swift ToO in Photon Counting mode to confirm or refute this possibility. Swift XRT observed H1743-322 starting from 2011-04-17 05:42:29.00UT for 1536 s (divided into two consecutive orbits). A single very bright source (more than 5 c/s uncorrected for pile-up) is detected at a position consistent with that of H1743-322 within the ~12 arcmin (radius) field of view. Timing analysis of data collected during the two orbits separately failed to detect any significant feature in the power spectra. The oscillation is also not visible in the RXTE observations that followed the first one nor in the Swift observation performed on April 10 (ATel \#3280). It is thus possible that either a pulsing source in the RXTE FOV was active on April 12 (but not subsequently), or the oscillation is intrinsic to the source. A third possibility is that a pulsing source is active outside the Swift/XRT field of view. Since no instrument detected an additional transient, pulsing source close to H1743-322, we consider the second possibility more reliable. We analyzed the April 12 RXTE data and find that indeed the oscillation is consistent with being coherent. Given the duration of the observation, around 2400 s, this translates in a quality factor larger than 26. Flaring behavior, however not strictly periodic or quasi-periodic, was observed in the hard state of GX 339-4 in January 2010.