Discovery of the Third Transient X-ray Binary in the Galactic Globular Cluster Terzan 5

Understand

We report and study the outburst of a new transient X-ray binary (XRB) in Terzan 5, the third detected in this globular cluster, Swift J174805.3-244637 or Terzan 5 X-3.

• We find clear spectral hardening in Swift/XRT data during the outburst rise to the hard state, thanks to our early coverage (starting at L_X ~ 4x10^{34} ergs/s) of the outburst.
• This hardening appears to be due to the decline in relative strength of a soft thermal component from the surface of the neutron star (NS) during the rise.
• We identify a {Type I X-ray burst} in Swift/XRT data with a long (16 s) decay time, indicative of {hydrogen burning on the surface of the} NS.