SAX J1750.8–2900 May Harbor Hottest, Most Luminous Known Neutron Star

Neutron stars represent the stellar remnants of the core-collapse of massive stars not quite big enough to become as cataclysmically space-time warping as a singularity, but are not as gravitationally strong that they are unable to overcome neutron degeneracy* (Haensel, Potekhin & Yakovlev 2007; Kiziltan, Kottas & Thorsett  2010; Kiziltan 2011). Recent observational analysis conducted by Lowell et al. (2012) has possible revealed the hottest and most luminous neutron star (NS) yet known.

N.B. Degenerate matter is matter that has such extraordinarily high density that the dominant contribution to its pressure is attributable to the Pauli exclusion principle, which states that no two fermions (in this case, our fermion particles are neutrons) can exist in the same quantum state. This degeneracy provides a natural and reactionary pressure against crushing it down further. The only way to overcome such degeneracies is to have so much matter that it overwhelms this law and becomes something exotic: a singularity.

SAX J1750.8-2900 was first detected by the Wide Field Cameras (WFCs) aboard the BeppoSAX satellite in 1997 (Natalucci et al. 1999). The WFCs detected nine separate Type I X-ray bursts from SAX J1750.8-2900 with intensities ranging from 0.4 to 1.0 Crab. From these bursts, a 3σ upper limit of 7 kpc was estimated for the source distance. A separate observation by Kaaret et al. (2002) in 2001 with the Rossi X-ray Timing Explorer’s (RXTE) Proportional Counter Array (PCA) revealed Type I X-ray bursts accompanied by millisecond quasi-periodic oscillations, allowing Kaaret et al. (2002) to estimate the NS spin to be 601 Hz. Two of the four bursts from the 2001 RXTE/PCA observation were found to display evidence of a phenomenon known as photospheric radius expansion (PRE) (Kaaret et al. 2002; Galloway et al. 2008) thus allowing for an estimation of the distance to the source. Galloway et al. (2008) suggested a distance of approximately D = 6.00±0.91 kpc.

Lowell et al.’s (2012) observations and analysis provided a measured value of the temperature of the NS in the SAX J1750.8-2900 system (the transient NS LMXB J1750) as 1718275K (148± 4eV) for the NS surface temperature. This is higher than that seen for the four, quasi-persistent NS LMXB systems where a quiescent base temperature has been identified: 123 eV for XTE J1701-462 (Fridriksson et al. 2011), <70 eV for KS 1731-260 (Cackett et al. 2010), 54 ± 2 eV for MXB 1659-29 (Cackett et al. 2008), and 109.4 ± 2.0 eV for EXO 0748-676 (Degenaar et al. 2011).

N.B.In astrophysics, the “hotness” of an object is not often measured in temperature. Instead, it is measured in something called “electron-volts” (eV) which is an energy itself equivalent to a certain amount of Joules (J): 1eV = 1.602×10^-19J. As some temperatures in astrophysics could often be very small, it is easier to use eV in some instances, but where temperatures are very large, it is often then suited to use J. To convert an energy into a temperature, use of the Bolztmann constant is needed.

Lowell et al.’s (2012) state that comparisons between J1750 and other quiescent LMXB (qLMXB) systems (see Tomsick et al. 2005; Heinke et al. 2010) suggests that J1750 could be the most luminous known NS LMXB in quiescence. Lowell et al. (2012) have measured the X-ray transient in  SAX J1750.8-2900  to have a luminosity of . The two results, of high luminosity and high surface temperature suggests, for unknown reasons, that the neutron star within  SAX J1750.8-2900 is unusually hot at its core.

Journal Reference:

• Kaarat, P. et al. (2008) Discovery Of Millisecond Variability In The Neutron Star X-Ray Transient SAX J1750.8-2900. The Astrophysical Journal, 575 (2), pp. 1018-1024.

• Cackett, E.M. et al. (2010) Continued Cooling Of The Crust In The Neutron Star Low-Mass X-ray Binary KS 1731-260. The Astrophysical Journal: Letters, 722 (2), pp. L137-L141.

• Lowell, A.W. et al. (2012) XMM-Newton Finds That SAX J1750.8–2900 May Harbor The Hottest, Most Luminous Known Neutron Star. The Astrophysical Journal, 749 (2), Article I.D.: 111.