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The First Supergiant Fast X-Ray Transient: XTE J173911.58–30237.6

30/04/2012

On August the 12th, 1997 at 16:58 UT one of the most unusual X-ray emissions was discovered emanating from within the Galactic centre, only to completely disappear sometime in the next 48 hours from the first scan by the Rossi X-Ray Timing Explorer (RXTE). Designated XTE J173911.58-30237.6, this highly irregular X-ray transient (XRT) was seen in outburst for just a fleeting day: leveling off at a rate over twice that expected for the nearest hard X-ray source ‘1E 1740.722942’ (Smith et al. 1998). This was a rather perplexing set of observations. To make matters worse, at the time of the discovery there was no known counterpart at radio or optical wavelengths to XTE J1739-302: where such an identification of a counterpart was hampered by its proximity to the Galactic centre causing source confusion and extinction due to dust and gas obscuration.

Before introducing more complex variable star properties, I think it is just right to explain qualitatively the term “X-ray transient”. Emission in the X-ray energy range occurs, for many different reasons, from many different cosmic objects, some of which exist only as a transient astronomical event: events which occur for a very short, cosmologically speaking, period of time (Lewin et al. 1993; Sidoli 2008; Vedrenne & Atteia 2009). Most bright X-ray transients last for weeks, and come from two kinds of systems: low-mass X-ray binaries where the compact object is either a black hole or a neutron star with a low magnetic field (X-ray novae” (e.g. [4U 0614+091: Stella & Vietri 1998]) or a neutron star binary with a high-mass, Be-type companion* (Be/NS) (e.g. [A0535+26: Clark et al. 1998]; [IGR J16393-4643: Bodaghee et al. 2006]; [20+ in SMC: Antoniou et al. 2009; [RX J1037.5-5647: La Palombara et al. 2009]).

N.B.* Be-type stars are stars is a Harvard Spectral Class [see: http://www.star.ucl.ac.uk/~pac/obafgkmrns.html%5D B star with prominent emission lines in its light spectrum.

Assuming a thermal bremßtrahlung spectral model, Smith et al. (1998) estimated an unabsorbed peak flux for XTE J173911.58-30237.6 (2–25 keV) of . They first proposed that the source was a binary system comprising a Be-type star and a neutron star based upon its spectral shape but noted that its brief, yet luminous, outburst was uncharacteristic for such objects.

In Be/NS binary systems, the NS is usually in an eccentric orbit, where the outbursts are quasi-regular (Smith et al. 2006) close to the periastron†  of ~0.4882AU; [figure calculated assuming an orbital eccentricity of e ~ 0.8 and a semi-major axis of a = 175 as quoted by Drave et al. (2010)]. These two classes can be distinguished from each other by neutron star pulsations, by optical observations of the companion to determine the compact object’s mass, by regularity of recurrence (which indicates a Be/NS binary), or by their X-ray spectra. However, work by Negueruela et al. (2006) has shown that the optical companion to the X-ray emitter (cataloged as USNO-B1.0 0596-0585865) identifying it as a O8 Iab(f) supergiant at a distance of ~2.3 kpc. In layman’s terms, this star is a intermediate luminous supergiant: big, slightly reddened but predominately white in the visible spectrum with peculiar emissions due to abundances of N [III] and He [II]. In essence, this star is not a Be-type companion to its compact binary sibling: ruling out this as the Be/NS class but ruling it in a different class of supergiant O-NS class binaries that Smith et al. (2006) designate as supergiant fast X-ray transients (SFXTs).

N.B.† The periastron is the point where the Be-type companion and the neutron star are at the closest distance during their mutual gravitational orbit, separated by the orbital period of the system.

As outlined by Smith et al. (2006), there are more SFXTs. Two X-ray transients discovered with INTEGRAL also have very fast timescales: IGR J17544-2619 (Sunyaev et al. 2003b) and IGR J16465-4507 (Lutovinov et al. 2004). Both have blue supergiant companions similar to the companion as seen in the XTE J173911.58–30237.6 system, estimated as spectral types O9 Ib and B0.5 I, respectively (L. J. Pellizza et al. 2006; Negueruela et al. 2005). However, although the general properties of such objects remain well documented the exact mechanism which causes such short lived outbursts remains elusive, with many differing hypotheses (e.g. [spherically symmetric clumpy winds: Walter & Zurita-Heras 2007]; [equatorially enhanced wind: Sidoli et al. 2007]; [gated mechanisms:  Grebenev & Sunyaev 2007]) are still being hotly debated within the astronomical research community.

Journal References:

  • Smith, D.M.; et al. (1998) XTE J1739-302: An Unusual New X-Ray TransientThe Astrophysical Journal: Letters, 501: pp.L181–L184.
  • Smith, D.M.; et al. (2003) XTE J1739-302: A New Kind Of Transient? American Astronomical Society: HEAD Meeting #7, #17.33; American Astronomical Society Bulletins, 35: pp.629.
  • Smith, D.M.; et al. (2006) XTE J1739-302 As A Supergiant Fast X-Ray TransientThe Astrophysical Journal, 638 (2): pp. 974-981.
  • Negueruela, I. et al. (2006) The Optical Counterpart To The Peculiar X-Ray Transient XTE J1739-302The Astrophysical Journal, 638 (2): pp. 982-986.
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