The MAVERIC survey: a hidden pulsar and a black hole candidate in ATCA radio imaging of the globular cluster NGC 6397

Abstract

Using a 16.2 hr radio observation by the Australia Telescope Compact Array (ATCA) and archival Chandra data, we found > 5σ radio counterparts to 4 known and 3 new X-ray sources within the half-light radius (r h ) of the Galactic globular cluster NGC 6397. The previously suggested millisecond pulsar (MSP) candidate, U18, is a steep-spectrum (S ν ∝ ν α ; α = −2.0 +0.4 −0.5 ) radio source with a 5.5 GHz flux density of 54.7 ± 4.3 µJy. We argue that U18 is most likely a "hidden" MSP that is continuously hidden by plasma shocked at the collision betwen the winds from the pulsar and companion star. The nondetection of radio pulsations so far is probably the result of enhanced scattering in this shocked wind. On the other hand, we observed the 5.5 GHz flux of the known MSP PSR J1740-5340 (U12) to decrease by a factor of >2.8 during epochs of 1.4 GHz eclipse, indicating that the radio flux is absorbed in its shocked wind. If U18 is indeed a pulsar whose pulsations are scattered, we note the contrast with U12's flux decrease in eclipse, which argues for two different eclipse mechanisms at the same radio frequency. In addition to U12 and U18, we also found radio associations for 5 other Chandra X-ray sources, four of which are likely background galaxies. The last, U97, which shows strong Hα variability, is mysterious; it may be either a quiescent black hole low-mass X-ray binary, or something more unusual.

A Deep Search for Faint Chandra X-ray Sources, Radio Sources, and Optical Counterparts in NGC 6752

“…A mag- gravitational potential well, and thus a neutron star or black hole accretor. Alternatively, "redback" millisecond pulsars (with nondegenerate, mass-losing companions of >0.1 𝑀 ) can also match all the data; several candidate redbacks have recently been identified in other globular clusters without the detection of radio pulsations (Zhao et al 2020a;Urquhart et al 2020). Our ATCA radio observations allow a (3𝜎) 5 GHz radio luminosity limit of 1.1 × 10 27 erg s −1 , which along with the 1-10 keV 𝐿 𝑋 of 8.7 × 10 31 erg s −1 , are not deep enough to rule out a black hole (e.g.…”

A Census of X-ray Millisecond Pulsars in Globular Clusters

“…For instance, Bogdanov et al (2010) suggested an X-ray source in the cluster NGC 6397 (source ID U18 in their work) as a strong MSP candidate, given its similar X-ray and optical properties to those of the known MSP, PSR J1740−5340, in the cluster. Later, Zhao et al (2020a) found the radio counterpart to U18 using the Australia Telescope Compact Array, while Pichardo Marcano et al ( 2021) reported the optical modulation of the companion star to U18, and both of their studies provided strong evidence that U18 is a "hidden" redback MSP. The radio pulsations from this source have been detected recently by the Parkes radio telescope (Lei Zhang et al 2021, in prep.…”

“…), verifying it as a redback MSP (PSR J1740−5340B). The reason of previous non-detection of radio pul- sations can be interpreted as scattering of the radio pulsations by wind from the companion (Zhao et al 2020a).…”

“…Those MSPs coupled with non-degenerate companion stars are usually referred to as 'spider' MSPs, and based on the companion masses, they are further grouped as redbacks (𝑀 𝑐 ∼ 0.2 M , hereafter RBs) and black widows (𝑀 𝑐 ∼ 0.02 M , hereafter BWs), respectively. In addition, it is common that Eclipses of the radio emission commonly occur in observations of spider pulsars, which can be explained as the radio emission from the MSP being absorbed ★ E-mail: jzhao11@ualberta.ca † E-mail: heinke@ualberta.ca and/or scattered by the plasma produced at the collision between the relativistic pulsar wind and material from the companion (Fruchter & Goss 1992;Thompson et al 1994;Stappers et al 2001;Polzin et al 2018;Zhao et al 2020a). On the other hand, the companion stars of MSPs may also be compact objects, and particularly white dwarfs (WDs) are the most common companions among these MSP binaries (e.g.…”

Exploring the Mass Segregation Effect of X-Ray Sources in Globular Clusters. IV. Evidence of Black Hole Burning in ω Centauri

“…Observationally, there are a growing number of BH candidates that have been identified in Galactic and extragalactic GCs (Maccarone et al 2007;Strader et al 2012;Chomiuk et al 2013;Miller-Jones et al 2015;Bahramian et al 2017;Shishkovsky et al 2018;Giesers et al 2018Giesers et al , 2019Zhao et al 2020). Some clusters, such as M 22 (Strader et al 2012) and NGC 3201 (Giesers et al 2018(Giesers et al , 2019, are found to host more than one BH candidate, strongly supporting the existence of a BHS in these systems.…”