The MAVERIC Survey: Simultaneous Chandra and VLA observations of the transitional millisecond pulsar candidate NGC 6652B

Abstract

Transitional millisecond pulsars are millisecond pulsars that switch between a rotation-powered millisecond pulsar state and an accretion-powered X-ray binary state, and are thought to be an evolutionary stage between neutron star low-mass X-ray binaries and millisecond pulsars. So far, only three confirmed systems have been identified in addition to a handful of candidates. We present the results of a multi-wavelength study of the low-mass X-ray binary NGC 6652B in the globular cluster NGC 6652, including simultaneous radio and X-ray observations taken by the Karl G. Jansky Very Large Array and the Chandra X-ray Observatory, and optical spectroscopy and photometry. This source is the second brightest X-ray source in NGC 6652 (LX ∼ 1.8 × e1034 erg s−1 and is known to be variable. We observe several X-ray flares over the duration of our X-ray observations, in addition to persistent radio emission and occasional radio flares. Simultaneous radio and X-ray data show no clear evidence of anti-correlated variability. Optical spectra of NGC 6652B indicate variable, broad H α emission which transitions from double-peaked emission to absorption over a time-scale of hours. We consider a variety of possible explanations for the source behaviour, and conclude that based on the radio and X-ray luminosities, short time-scale variability and X-ray flaring, and optical spectra, NGC 6652B is best explained as a transitional millisecond pulsar candidate that displays prolonged X-ray flaring behaviour. However, this could only be confirmed with observations of a change to the rotation-powered millisecond pulsar state.

A Census of X-ray Millisecond Pulsars in Globular Clusters

“…We found a total of 87 unidentified X-ray sources with 𝐿 𝑋 > 10 32 erg s −1 in 29 GCs, while we did not find any unidentified sources with 𝐿 𝑋 > 10 33 erg s −1 in those GCs. However, we note a special case, the X-ray source B in NGC 6652 (see Heinke et al 2001;Stacey et al 2012), which is suggested as a transitional MSP (tMSP) in a recent work by Paduano et al (2021). It is considered to be in an accretion-powered state currently, with an average X-ray luminosity of ∼ 1.8 × 10 34 erg s −1 .…”

The MAVERIC survey: A catalogue of radio sources in southern globular clusters from the Australia Telescope Compact Array

“…Despite the limited statistical signal in many of the sample clusters, our ATCA survey has already been successful in identifying new compact binaries in globular clusters, such as the ultracompact black hole X-ray binary candidate X9 in 47 Tuc (Miller-Jones et al 2015;Bahramian et al 2017) and the transitional millisecond pulsar candidate NGC6652B (Paduano et al 2021). This highlights the need and opportunity for detailed follow-up of each central radio source in our catalogue-many are likely to be true cluster sources, even if the individual cluster does not formally show a statistical excess of sources.…”

“…Apart from pulsars, we have also detected some well-known X-ray binaries-in Terzan 5, EXO 1745-248, which was in outburst at the time of the observations (Tetarenko et al 2016b), here catalogued as Ter5-ATCA3; in NGC 6624, 4U 1820-30, here catalogued as N6624-ATCA1 (see Russell et al 2021 andPanurach et al 2021); and in Terzan 6, GRS 1747-312, here catalogued as Ter6-ATCA1 (see Panurach et al 2021). We have previously published two X-ray binaries discovered as part of this ATCA survey: in 47 Tuc, the candidate black hole X9 (Miller-Jones et al 2015), here catalogued as 47Tuc-ATCA1; and in NGC 6652, the candidate transitional millisecond pulsar NGC6652B (Heinke et al 2001;Paduano et al 2021), here catalogued as N6652-ATCA3.…”

The MAVERIC Survey: The first radio and X-ray limits on the detached black holes in NGC 3201

“…This overabundance is due to the formation channels of XRBs There are currently three confirmed tMSPs and a handful of other candidates, of which one confirmed tMSP resides within the GC M28 (Papitto et al 2013). Two candidate tMSPs also reside within the clusters Terzan 5 and NGC 6652 respectively (Bahramian et al 2018b;Paduano et al 2021). Cataclysmic variables (CVs), white dwarfs accreting from low mass stars, make up a large fraction of X-ray emitting binaries in globular clusters (Grindlay et al 2001;Pooley et al 2002).…”

“…Further details about the observations of the cluster sample and the reduction of the radio data are presented in Tremou et al (2018) and Shishkovsky et al (2020). In addition to the identification of several BH candidates in Galactic GCs (see 1.1), the MAVERIC survey has also identified tMSP candidates (Bahramian et al 2018b;Paduano et al 2021), and placed limits on the existence of intermediate mass BHs in GCs (Tremou et al 2018). Accompanying the MAVERIC survey is a catalogue of over 1100 X-ray sources detected by the Chandra X-ray Observatory's ACIS detector from observations of 38 GCs (Bahramian et al 2020).…”