Chandra Identification of the 2012 Transient in Terzan 5

ATel #4302; Jeroen Homan (MIT), David Pooley (Sam Houston State U., Eureka Scientific)
on 9 Aug 2012; 19:11 UT
Credential Certification: Jeroen Homan (jeroen@space.mit.edu)

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We triggered a Chandra ToO observation of the ongoing outburst of a new neutron star transient in the globular cluster Terzan 5 (ATel #4242, #4249, #4264). The 10.0 ks observation was taken with the ACIS S3 chip in imaging mode and began on on 2012-07-30 at 09:39:30 UTC.

Inspection of the new Chandra data (ObsID 13708) reveals the bright transient (which suffers from severe pile up) and a few low-luminosity X-ray sources that are also seen in a deeper Chandra observation (ObsID 3798) from 2003 (Heinke et al. 2006, ApJ, 651, 1098). The positions of these low-luminosity sources (CX3, CX4, CX7, and CX11) are in good agreement and allow us to identify the transient currently in outburst as CXOGlb J174805.4-244637 (CX2 in Heinke et al. 2006) at J2000 coordinates 17:48:05.413 (0.001), -24:46:37.67 (0.02) from their Table 1. This is in agreement with the analysis of Swift/XRT data in ATel #4242, which suggested the currently active source is CX2.

We used several of the long ACIS-S observations of Terzan 5 in the Chandra archive to study the quiescent properties of CX2. These observations cover the period of July 2003 to April 2011. No indications for previous outbursts can be seen in these observations. We performed simultaneous fits to five spectra, with exposures ranging from 10 ks to 40 ks. Good fits were obtained using a neutron star atmosphere model (nsatmos in XSPEC) plus a power-law; single component fits did not perform as well. Both components were multiplied by an absorption model (tbabs in XSPEC) with the column density fixed to 1.6e22 atoms/cm^2 (Heinke et al. 2006). The power-law index was tied between the five observations, whereas the neutron star temperature and power-law normalization were left free for each observation. Assuming a distance of 5.5 kpc, a neutron star mass of 1.4 solar mass, and a neutron star radius of 10 km, we find effective temperatures (at infinity) between 86+/-3 eV and 90+/-4 eV. The index of the power-law is 1.6+/-0.3 and its contribution to the 0.5-10 keV flux varies between ~60% and ~80%. The total unabsorbed 0.5-10 keV flux varies between 2.7e-13 ergs/s/cm^2 and 4.2e-13 ergs/s/cm^2 (corresponding to luminosities between 9.8e32 ergs/s and 1.5e33 erg/s for 5.5 kpc). These properties fall well within the broad range observed for quiescent neutron stars.

We thank Harvey Tananbaum and the entire Chandra staff for their help in executing this observation.