An X‐Ray Variable Millisecond Pulsar in the Globular Cluster 47 Tucanae: Closing the Link to Low‐Mass X‐Ray Binaries

Abstract

We report the discovery of peculiar X-ray spectral variability in the binary radio millisecond pulsar PSR J0024À7204W in the globular cluster 47 Tuc. The observed emission consists of a dominant nonthermal component, which is eclipsed for a portion of the orbit, and a thermal component, which appears to be persistent. We propose that the nonthermal X-rays originate in a relativistic intrabinary shock, formed due to interaction between the relativistic particle wind from the pulsar and matter from the main-sequence companion star, while the thermal photons are from the heated magnetic polar caps of the millisecond pulsar. At optical wavelengths, the emission exhibits large-amplitude variations at the orbital period, which can be attributed to heating of one side of the tidally locked secondary star by the pulsar wind. The observed X-ray and optical properties of PSR J0024À7204W are remarkably similar to those of the low-mass X-ray binary and X-ray millisecond pulsar SAX J1808.4À3658 in quiescence. This supports the conjecture that the nonthermal X-ray emission and optical modulations seen in the SAX J1808.4À3658 system in a quiescent state are due to interaction between the wind from a reactivated rotationpowered pulsar and matter from the companion star. The striking similarities between the two systems provide support for the long-sought connection between millisecond radio pulsars and accreting neutron star systems.

XMM-Newtonobservations of the Galactic globular clusters NGC 2808 and NGC 4372

“…The brightest MSPs have luminosities around our flux detection limit for NGC 2808. However, we note that MSPs, which are hard X-ray sources, are difficult to distinguish from CVs at this luminosity (Bogdanov et al 2005). Therefore, our observations are not constraining for the presence of MSPs in NGC 2808.…”

OPTICAL IDENTIFICATION OF He WHITE DWARFS ORBITING FOUR MILLISECOND PULSARS IN THE GLOBULAR CLUSTER 47 TUCANAE

“…Again, at odds with other redback systems, this lies in a region more similar to that occupied by the two black widow companions identified so far in GCs (Pallanca 2014, Cadelano et al 2015. The X-ray counterpart to 47TucW shows a variability that is likely due to an intra-binary shock between the PSR wind and the matter lost by the companion (Bogdanov et al 2005). Interestingly, as discussed by Bogdanov et al (2006), the minimum of the X-ray light curve is displaced with respect to the optical one.…”

X-ray studies of the black widow pulsar PSR B1957+20

“…The pulsar wind electrons and positrons are accelerated and randomized by the shock and emit the X-rays via the synchrotron process. Such shock emission has also been suggested to explain the variable X-ray flux as a function of orbital phase in 47 Tuc W (Bogdanov et al 2005) and PSR J1740-5340 in NGC 6397 (Huang & Becker 2010) and PSR J1023+0038 (Archibald et al 2009(Archibald et al , 2010Tam et al 2010;Bogdanov et al 2011).…”

“…Close binary systems with MSPs are a subject of special interest since they are thought to be the missing link between low-mass Xray binaries (LMXBs) and isolated MSPs. Discoveries of the eclipsing binary pulsar systems, such as PSR B1957+20, (Fruchter et al 1988b), PSR J2051-0827 (Stappers et al 1996) and PSR J1023+0038 (Archibald et al 2009(Archibald et al , 2010 in the Galactic plane and 47 Tuc W (PSR J0024-7204W; Bogdanov et al 2005) and PSR J1740-5340 (D'Amico et al 2001) in globular clusters, gave support to this formation scenario. Studying these binary systems provides a wealth of information not only on the evolutionary history of isolated MSPs but also on the physical details of the pulsar's high energy emission properties.…”