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Swift/XRT and RXTE/PCA results from SWIFT J1756.9-2508

ATEL # 1111; H. A. Krimm (GSFC/USRA), S. Campana (INAF-OAB) , J. Kennea (PSU), C. Markwardt (GSFC/UMD), P. Romano (Univ Bicocca&INAF-OAB), J. Swank (GSFC), J. Tueller (GSFC)
on 14 Jun 2007; 23:48 UT
Password Certification: Hans A. Krimm (krimm@milkyway.gsfc.nasa.gov)

Subjects: X-ray, Binaries, Pulsars, Transients

We report on further analysis of the newly discovered millisecond pulsar SWIFT J1756.9-2508 (ATel #1105, #1107, #1108), including Swift XRT and RXTE PCA observations.

Based on about 5 ks of Swift XRT data, the revised position for SWIFT J1756.9-2508 is:
RA = 17h 56m 57.1s (269.23794)
Dec = -25d 06m 26.7s (-25.10743)
with an estimated error of 3.5" radius (90% confidence).

We analyzed 1.1 ksec of XRT data from the first Swift observation of the source. After correction for pile-up and the removal of scattered light from GX 5-1, we find that the data is well fit by an absorbed power law with a photon index of 2.2 +/- 0.4 and an absorption of nH = (6.2 +/- 1.4) X 10^{22} cm^2. The unabsorbed / absorbed flux in this model is (1.15 / 0.20) X 10^-9 erg/cm^2/s for 0.2-10 keV. Fitting to an absorbed black body yields kT = 1.2 +/- 0.1 keV and nH = (2.9 +/- 0.8) X 10^{22} cm^2. The unabsorbed / absorbed fluxes in this model for 0.2-10 keV are (2.51 / 1.84) X 10^{-10} erg/cm^2/s. Fitting the 0.3-10 keV pile-up corrected light curve with a constant yields a mean count rate of 2.1 +/- 0.1 cps. Quoted uncertainties are at the 90% confidence level.

Preliminary analysis of the Swift observations from June 14 shows similar results.

The RXTE PCA observed the source starting at 13 June 2007 at 19:00 UTC for about 1700 seconds. Because of the nearby bright source GX 5-1, an offset pointing was performed. The source was detected up to energies ~30 keV, and the spectrum is consistent with an absorbed power law with photon index 1.9 +/- 0.1 (with absorption fixed to 5 x 10^{22} cm^{-2}). Contamination from the galactic ridge was present at about the 2 mCrab level (Raymond-Smith model, kT = 3 keV). After subtracting the estimated contamination, the fluxes in the 2-10, 10-20 and 20-40 keV bands were 3.3, 2.2 and 2.4 in units of 10^{-10} erg/cm^2/s. The PCA and XRT spectral results are in reasonable agreement.

PCA bulge scan monitoring observations are not possible in this region of the sky for most of the month of June, due to RXTE spacecraft slewing constraints. The most recent observation was June 6. The source has not been detected in the past month, with a 95% upper limit of about 1 mCrab (2-10 keV).


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R. E. Rutledge , Editor
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