The new X-ray transient Swift J174805.3-244637 in Terzan 5 is a neutron star LMXB

ATel #4264; D. Altamirano, R. Wijnands (U. of Amsterdam), C. O. Heinke, G. R. Sivakoff (U. of Alberta), D. Pooley (Sam Houston State U., Eureka Scientific)
on 18 Jul 2012; 16:27 UT
Credential Certification: Rudy Wijnands (rudy@space.mit.edu)

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We report on the ongoing monitoring Swift observations of the X-ray transient Swift J174805.3-244637 (ATEL #4249, #4242) in the globular cluster Terzan 5. During an observation taken on July 17th, 2012, we detect a Type-I X-ray burst in a ~950 seconds long observation which started at 20:54:00 UT.

The burst shows a rapid rise (~3 sec) and exponential decay with an e-folding time of ~22 seconds. Preliminary analyses show that the spectrum is well fitted with an absorbed black-body model. Time-resolved spectra reveals that the black-body temperature decreases from ~2.3 keV at the peak, to 1.2 keV as the burst evolves from peak to persistent emission. The bolometric luminosity at the peak is about 1e38 erg s^-1 as estimated from the black-body normalization (assuming a distance of 5.9+/-0.5 kpc, see, Valenti et al. 2007, AJ, 133, 1287).

The shape, duration, and spectral characteristics of this X-ray burst is typical of thermonuclear (Type-I) X-ray bursts occurring on the surface of a neutron star, allowing us to identify Swift J174805.3-244637 as the 3rd conclusively identified neutron star low-mass X-ray binary in Terzan 5 showing a transient bright outburst in the last ~16 years.

We extracted spectra from Swift/XRT observations on July 16 and July 17 (the latter excluding the burst and tail), choosing only grade 0 data. Swift/XRT data of bright sources in WT mode have known calibration problems at low energies, so we exclude data below 1.1 keV and fix the N_H to 1.8e22 (as found in ATEL #4249). Both datasets can be fit by comptonization models (COMPTT in XSPEC) with typical parameters (electron kT >50 keV, tau~1). The inferred 0.5-10 keV unabsorbed fluxes are 1.2e-9 and 1.8e-9 ergs cm^-2 s^-1, translating (for a 5.9 kpc distance) to luminosities of 5e36 erg s^-1 and 7e36 erg s^-1, respectively.

Swift/BAT transient analysis by Hans Krimm (http://swift.gsfc.nasa.gov/docs/swift/results/transients/) indicates detection of a rising flux from Terzan 5, reaching 100 mCrab by July 14. Our preliminary comptonization model predicts a luminosity of 8e36 ergs/s in the 15-50 keV band on July 16, which is consistent with the Swift/BAT estimate on that day.

Analyses of the power density spectrum in the 0.5-2 keV, 2-10 keV and 0.5-10 keV ranges find no evidence of burst oscillations during the burst, nor any evidence of broad band noise or QPOs in the persistent emission.

Observations of Swift J174805.3-244637 by Swift are on-going.