Chandra studies of the globular cluster 47 Tucanae: a deeper X-ray source catalogue, five new X-ray counterparts to millisecond radio pulsars and new constraints to r-mode instability window

Abstract

We combined Chandra ACIS observations of the globular cluster 47 Tucanae (hereafter, 47 Tuc) from 2000, 2002, and 2014-15 to create a deeper X-ray source list, and study some of the faint radio millisecond pulsars (MSPs) present in this cluster. We have detected 370 X-ray sources within the half-mass radius (2 .79) of the cluster, 81 of which are newly identified, by including new data and using improved source detection techniques. The majority of the newly identified sources are in the crowded core region, indicating cluster membership. We associate five of the new X-ray sources with chromospherically active BY Dra or W UMa variables identified by Albrow et al. (2001). We present alternative positions derived from two methods, centroiding and image reconstruction, for faint, crowded sources. We are able to extract X-ray spectra of the recently discovered MSPs 47 Tuc aa, 47 Tuc ab, the newly timed MSP 47 Tuc Z, and the newly resolved MSPs 47 Tuc S and 47 Tuc F. Generally, they are well fit by black body or neutron star atmosphere models, with temperatures, luminosities and emitting radii similar to those of other known MSPs in 47 Tuc, though 47 Tuc aa and 47 Tuc ab reach lower X-ray luminosities. We limit X-ray emission from the full surface of the rapidly spinning (542 Hz) MSP 47 Tuc aa, and use this limit to put an upper bound for amplitude of r-mode oscillations in this pulsar as α < 2.5 × 10 −9 and constrain the shape of the r-mode instability window.

Search for multiwavelength emission from the binary millisecond pulsar PSR J1836-2354A in the globular cluster M22

“…The X-ray spectrum, indeed, could be equally described by a black-body with temperature of 0.8 ± 0.4 keV. It is perfectly consistent with the temperatures of other samples of X-ray pulsars (see, for instance, Bogdanov et al (2006) and Bhattacharya et al (2017) for a spectral analysis of the MSPs of the GC 47 Tucanae).…”

“…Thermal emission is often observed from faint MSPs, where the total power generated is log 10 (L x )=30-32 erg s −1 (Bogdanov et al 2006, Forestell et al 2014, Bhattacharya et al 2017) and the magnetic field is low, typically B 10 9 G (Zavlin et al 1996. The intensity of the magnetic field at the surface of the NS, in the simple case of a magnetic dipole, is given by B surf = 3.2 × 10 19 (P P) 1/2 G (Manchester & Taylor 1977), where P and P are respectively the spin period and the spin-down rate of the NS.…”

Signatures for quark matter from multi-messenger observations

“…There are two ways to account for the observation of stars within this region: (1) there are additional damping mechanisms that shift the instability curves upwards; (2) although small-amplitude r-modes are unstable inside the Vshaped regions, non-linear mechanisms saturate their amplitude at a low value α sat . Spectral X-ray observations of LMXBs and millisecond pulsars have put a bound on the saturation amplitude α sat 10 −9 − 10 −8 [82,83,84]. This bound is obtained by looking for the heating that would be produced by the dissipation required to saturate the mode.…”

Understanding the γ -ray emission from the globular cluster 47 Tuc: Evidence for dark matter?

“…This conservative approach allows us to account for all MSPs within 47 Tuc, even those below the detection sensitivity threshold of the Fermi-LAT. In addition, we considered a second scenario where we determined the number of γ-ray bright MSPs by considering Chandra x-ray observations, which have resolved 23 MSPs within 47 Tuc [16,17,19]. The γ-ray flux expected from these 23 x-ray bright MSPs was calculated using the x-ray-to-γ-ray MSP flux ratio of hlogðG 100 =F X Þi ¼ 2.31 derived in the 2PC [3].…”