An Ultracompact X-Ray Binary in the Globular Cluster M15 (NGC 7078)

Abstract

We have used the Advanced Camera for Surveys on board the Hubble Space Telescope to image the core of the globular cluster M 15 in the far-ultraviolet (FUV) waveband. Based on these observations, we identify the FUV counterpart of the recently discovered low-mass X-ray binary M 15 X-2. Our time-resolved FUV photometry shows a modulation with 0.062 ± 0.004 mag semi-amplitude and we clearly detect a period of 22.5806 ± 0.0002 min. We have carried out extensive Monte Carlo simulations which show that the signal is consistent with being coherent over the entire observational time range of more than 3000 cycles. This strongly suggests that it represents the orbital period of the binary system. M 15 X-2 is FUV bright (FUV ≃ 17 mag) and is characterized by an extremely blue spectral energy distribution (F λ ∝ λ −2.0 ). We also find evidence for an excess of flux between 1500Å and 1600Å and probably between 1600Å and 2000Å which might be due to C IV 1550Å and He II 1640Å emission lines. We also show that M15 X-2's X-ray luminosity can be powered by accretion at the rate expected for gravitational-wave-driven mass transfer at this binary period. The observed FUV emission appears to be dominated by an irradiated accretion disk around the neutron star primary, and the variability can be explained by -2irradiation of the low-mass white dwarf donor if the inclination of the system is ≈ 34 • . We conclude that all observational characteristics of M15 X-2 are consistent with it being an ultracompact X-ray binary, only the third confirmed such object in a globular cluster.

Evolution of Intermediate-Mass X-Ray Binaries Driven by The magnetic Braking of Ap/Bp Stars. I. Ultracompact X-Ray Binaries

“…(1) Galloway et al (2010); (2) Harris (2010); (3) Stella et al (1987); (4) (2011); (7) Wang & Chakrabarty (2004); (8) Homer et al (1996); (9) Dieball et al (2005); (10) Chakrabarty (1998); (11) Yoshida (1993); (12) Walter et al (1982); (13) Iaria et al (2015); (14) Brandt et al (1992); (15) Shahbaz et al (2008); (16) Galloway (2006); (17) Statistically, only a small fraction of A/B stars (about 5%) stars have anomalously strong magnetic fields and appear as Ap/Bp stars (Landstreet 1982;Shorlin et al 2002). However, the populations formed via our evolutionary route should appear as UCXBs for a much longer time than in the circumbinary disk model (Ma & Li 2009b).…”

Stability of helium accretion discs in ultracompact binaries

“…However, the distance to the peculiar globular cluster NGC 6712 first believed to be 6.8 kpc (Cudworth 1988) is now measured to be ∼8 kpc by Paltrinieri et al (2001). Also in the case of M 15 X-2, the mass-transfer rate based on the X-ray luminosity used in Table 1 could be an underestimate anḋ M > ∼ 4 × 10 −10 M y −1 (Dieball et al 2005). At first, the second possibility seems to be more promising since the dependence of stability criterion on the outer disc radius is very strong.…”

Demonstrating the likely neutron star nature of five M31 globular cluster sources withSwift-NuSTAR spectroscopy

“…This value is slightly above the highest luminosity seen from Sco X-1 of 4.5×10 38 erg/sec (Barnard et al 2003), which is robust given the geometric parallax distance (Bradshaw et al 1999), but the discrepancy can be explained if the neutron star in Bo 375 is a bit more massive than the neutron star in Sco X-1, or if the neutron star in Bo 375 is accreting hydrogen-poor gas. Given that ultracompact X-ray binaries represent a substantial fraction of the X-ray binaries in Milky Way globular clusters (Stella et al 1987;Dieball et al 2005;Zurek et al 2009), this latter interpretation would not be surprising. Barnard et al (2008) had previously found with XMMNewton data that a single power law could not fit the spectrum of that source, suggesting that it is a neutron star.…”