Chandra non-detection of the newly discovered Millisecond X-ray Pulsar SWIFT J1756.9-2508

ATel #1133; A. Papitto (INAF-OARoma/Univ. Roma Tor Vergata), T. Di Salvo (Univ. Palermo), L. Burderi (Univ. Cagliari), E. Gallo (UCSB), M. T. Menna (INAF-OARoma), A. Riggio (Univ. Cagliari), M. I. Krauss (MIT), D. Chakrabarty (MIT), A. Antonelli (INAF-OARoma), E. Bozzo (INAF-OARoma/Univ. Roma Tor Vergata), M. Burgay (INAF-OACagliari), N. D'Amico (INAF-OACagliari), R. Iaria (Univ. Palermo) G. L. Israel (INAF-OARoma), A. Possenti (INAF-OACagliari), V. Testa (INAF-OARoma)
on 7 Jul 2007; 03:53 UT
Credential Certification: Alessandro Papitto (papitto@oa-roma.inaf.it)

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We report on the non detection of the neutron star transient SWIFT J1756.9-2508 during a 11ks Chandra/ACIS-I DDT observation performed on 2007-07-06 (starting at 01:25:11 UT). This recently discovered X-ray source, which harbors the eighth known Millisecond X-ray Pulsar, was found in outburst on 2007 June 7. Based on the observed decay rate (ATels #1105, #1107, #1108, #1111, #1114, #1117), the source is expected to have reached quiescence at the time of the observation reported here.

Only one photon was recorded over the energy range 0.3-8.0 keV inside a circular region centered at the Swift/XRT position (ATel 1111) with a 3.5 arcsec radius. Applying the prescription for small number statistics by Gehrels (1986), we derive a conservative upper limit of 4.3e-4 count/sec (95%) for the quiescent count rate. Depending on the assumed spectral shape, we can thus set upper limits on the source unabsorbed flux. For a blackbody model with kT = 0.2-0.3 KeV, we derive F(0.3-8) = 0.4-1.4E-13 erg/cm2/s, while F(0.3-8)= 0.2-1.3E-13 erg/cm2/s for a power law model with photon index varying in the range 1-3. In both cases, the nH was fixed to the respective best fitting values for the first Swift/XRT observation (ATEL #1111).

We thank the Chandra X-ray Center Team and in particular the Director Dr.Harvey Tananbaum for having made this DDT observation possible.