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

Abstract

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 ∼ 4 × 10 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. We use Swift/BAT, MAXI/GSC, Chandra/ACIS, and Swift/XRT data to study the spectral changes during the outburst, identifying a clear hard-to-soft state transition. We use a Chandra/ACIS observation during outburst to identify the transient's position. Seven archival Chandra/ACIS observations show evidence for variations in Terzan 5 X-3's non-thermal component, but not the thermal component, during quiescence. The inferred long-term time-averaged mass accretion rate, from the quiescent thermal luminosity, suggests that if this outburst is typical and only slow cooling processes are active in the neutron star core, such outbursts should recur every ∼10 years.11 Note that the true identity of the transient seen by EXOSAT in the 1980s (leading to the EXO name) is not known.

Limits on thermal variations in a dozen quiescent neutron stars over a decade

“…We start with the sample of candidate qLMXBs from Heinke et al (2006a), which identified thermally-dominated qLMXBs by spectral fitting, or by hardness ratio plus inferred X-ray luminosity. Analyses of the quiescent behaviour of the outbursting sources Ter 5 X-1 (EXO 1745-248), X-2 (IGR J17480-2446), and X-3 (Swift J174805.3-244637) have been reported in Degenaar, Brown & Wijnands 2011;Degenaar et al 2013;Bahramian et al 2014;Degenaar et al 2015, so we exclude those sources here.…”

“…4 In many previous studies, abundances from Anders & Grevesse (1989) have been used for absorption models. Using these abundances, Bahramian et al (2014) Table 6. Tests of hydrogen (NSATMOS) and helium (NSX) atmosphere models for Terzan 5 CX9.…”

“…A nearly identical soft spectrum can also be created in the upper layers of the NS atmosphere by low-level accretion (Zampieri et al 1995;Zane, Turolla & Treves 2000), though the most detailed model predicts, in addition, an optically thin high-temperature bremsstrahlung component (Deufel, Dullemond & Spruit 2001), which may be identified with the powerlaw component (e.g. Bahramian et al 2014). However, low-level accretion at sufficiently high rates (corresponding to L ∼ 10 33 erg/s) could maintain metals in the atmosphere at abundances sufficient to soften the spectrum, leading to an overestimate of R (Rutledge et al 2002b).…”

Unbeamed tidal disruption events at hard X-rays

“…The flare, with a position consistent with the centre of the globular cluster Terzan 5, was detected by INTEGRAL (Bordas et al 2010) and allowed the discovery of the new accreting pulsar IGR J17480-2446 (Papitto et al 2011). The second brightest flare in Terzan 5 peaked on 2012 July 18 and corresponds to a Type-I burst, which led to the discovery of the new accreting neutron star Terzan 5 X-3 (Bahramian et al 2014). …”

Numerical Experiments for Nuclear Flashes toward Superbursts in an Accreting Neutron Star

“…On the other hand, it is proposed from the analysis of the photospheric radius expansion that an accretion rate may change during the burst by some factors [21]. Changes in accretion rates are also considered from recent study of the outburst of a transient X-ray binary [22]. Therefore, it is reasonable to include accretion rate variations in numerical calculations because the observations indicate that the luminosity of accreting neutron stars is variable.…”