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ATCA radio observations of Swift J1756.9-2508 in outburst

ATel #2138; V. Tudose (ASTRON), A. Tzioumis (ATNF), A. Patruno, M. Linares, P. Soleri, D. Russell (UvA)
on 28 Jul 2009; 10:52 UT
Credential Certification: Valeriu Tudose (vtudose@science.uva.nl)

Subjects: Radio, Binary, Neutron Star, Transient, Pulsar

Swift J1756.9-2508 was discovered by the X-ray satellite Swift during an outburst in 2007 (ATEL #1105) and recognized immediately as an accretion-driven millisecond X-ray pulsar (ATEL #1108). No radio pulsations (ATEL #1129) and no continuum radio emission (ATEL #1128) were detected then. While the radio pulsation search was performed during the outburst, the continuum observations were carried out when the X-ray flux levels already decreased below the detection limit of Swift.

The object has recently undergone another major outburst (ATEL #2130). The peak of activity was around 2009 July 14 when the RXTE-PCA X-ray flux level reached 22 mCrab. This new active state offered the possibility to probe the radio continuum emission of the system during an ongoing X-ray outburst.

We observed Swift J1756.9-2508 with the Australia Telescope Compact Array (ATCA) simultaneously at the median frequencies of 5.5 and 9.0 GHz on 2009 July 15 between 08-14 UT. The observations were carried out in the H75 array configuration (generating a beam size of 10 X 5 arcsec^2) using the Compact Array Broadband Backend (CABB) which features bandwidths of 2 GHz.

At 5.5 GHz we do not detect the target. The 3 sigma upper limit to the flux density is 0.060 mJy.

At 9.0 GHz we do not detect any radio source within the 3.5 arcsec radius (90 percent confidence) Swift-XRT error box (ATEL #1111). The 3 sigma upper limit to the flux density is 0.075 mJy.

We note that we detect a compact radio source with a flux density of 1.5 +/- 0.3 mJy at the position (J2000) RA: 17h56m59.087s; Dec: -25d06'38.13", which is 30 arcsec away from the location of the target as determined by Swift-XRT. The estimated ATCA positional uncertainty is around 1.5 arcsec. Therefore it is almost sure that this is not the radio counterpart of Swift J1756.9-2508.