SWIFT J1749.4-2807: X-ray decay, refined position and optical observation

ATel #2579; Y. J. Yang, D. M. Russell, R. Wijnands, M. van der Klis, D. Altamirano, A. Patruno, A. Watts , M. Armas Padilla, Y. Cavecchi, N. Degenaar, M. Kalamkar, R. Kaur (University of Amsterdam); M. Linares (MIT), P. Casella (Southampton), N. Rea (IEEC-CSIC), P. Soleri (Groningen), F. Lewis (Faulkes Telescope Project, University of Glamorgan) and A. K.H. Kong (NTHU, Taiwan)
on 22 Apr 2010; 14:15 UT
Credential Certification: David M. Russell (D.M.Russell@uva.nl)

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We analyzed seven, target ID 31686, Swift follow-up observations of the neutron-star X-ray transient Swfit J1749.4-2807 (Wijnands et al. 2009) currently in outburst and which was found to be an accreting millisecond X-ray pulsar (ATel #2565). The observations span from April 11 to April 20. We obtained a refined X-ray position (enhanced position by UVOT field astrometry, Goad et al. 2007, A&A, 476, 1401) at R.A = 17:49:31.80 (267.38249), Dec. = -28:08:04.9 (-28.13470) with an estimated error of 1.9 arcsec (90% confidence radius) which is in agreement with Wijnands et al. (2009). The Swift long-term lightcurve shows that the source brightness is declining steadily, which is consistent with the RXTE PCA lightcurve (ATel #2576). The latest RXTE-PCA pointing on 2010-04-21 shows the source at ~8 mCrab (2-16 keV; without correcting for Galactic diffuse emission), and a 1.6 ks Swift observation taken on April 21 only marginally detected the source with ~8 counts. PIMMs estimated unabsorbed flux is 1.11x10^-12 ergs/cm/cm/s, the lowest intensity measured during the current outburst.

The following XRT photon-counting mode count rates (0.3-10 keV) were measured:


2010-04-11 4.42 +/- 0.10 c/s

2010-04-12 2.63 +/- 0.06 c/s

2010-04-16 2.02 +/- 0.07 c/s

2010-04-17 1.45 +/- 0.06 c/s

2010-04-18 0.85 +/- 0.03 c/s

2010-04-20 0.10 +/- 0.01 c/s
2010-04-21 0.005 +/- 0.002 c/s

We also analyzed the energy spectra of the first six observations. All the spectra can be well fitted with a simple power-law model with absorption. The average column density NH we obtained is 2.8 +/- 0.2 x 10^22 /cm^2, which is slightly lower but still in agreement with the NH obtained from the first Swift ToO observation (Atel #2548). The spectral fitting result shows that the source spectrum is hardening. As the brightness of the source has faded, the power law photon index has changed from 2.64 +/- 0.2 to 1.64 +/- 0.9.


Optical follow-up observations were also performed with the Faulkes Telescope South on 2010-04-15, 17 hours after the detection of coherent millisecond pulsations (ATel #2565). Images in the V (2 x 200 sec), R (2 x 200 sec) and i' (200 sec) bands showed no counterpart detection within the 1.9 arcsec XRT error circle. Conditions were photometric at Siding Spring during the night; using standard stars we derive a 3 sigma upper limit of i' > 19.6 for the optical counterpart within the error circle (the V and R images are less constraining). Just to the north and south-east of the error circle there are two stars (both 4 arcsec from the centre of the error circle), which have magnitudes i' = 17.64 +- 0.16 and i' = 18.40 +- 0.17, respectively. Column density towards the source is high. We may expect an optical extinction of A_v ~ 16 mag (Predehl & Schmitt, 1995, A&A, 293, 889), which corresponds to A_i' ~ 10 mag. Observations in the near-IR are therefore strongly encouraged.

This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester (Evans et al. 2009, MNRAS, 397, 1177).