Chandra and HST Studies of Six Millisecond Pulsars in the Globular Cluster M13

TL;DR

This study analyzes X-ray and optical observations of six millisecond pulsars in M13, identifying counterparts and characterizing their properties to enhance understanding of MSPs in globular clusters.

Contribution

It provides new X-ray and optical identifications of MSP counterparts in M13, including the first optical detection of some companions, and analyzes their spectral properties.

Findings

Confident X-ray counterparts detected for five MSPs.

Optical counterparts identified for two MSPs, both likely white dwarfs.

Spectral analysis shows MSPs are described by blackbody or power-law models.

Abstract

We analyse 55 ks of Chandra X-ray observations of the Galactic globular cluster M13. Using the latest radio timing positions of six known millisecond pulsars (MSPs) in M13 from Wang et al. (2020), we detect confident X-ray counterparts to five of the six MSPs at X-ray luminosities of $L_X$(0.3-8 keV)$\sim 3 \times 10^{30} - 10^{31}~{\rm erg~s^{-1}}$, including the newly discovered PSR J1641+3627F. There are limited X-ray counts at the position of PSR J1641+3627A, for which we obtain an upper limit $L_X<1.3 \times 10^{30}~{\rm erg~s^{-1}}$. We analyse X-ray spectra of all six MSPs, which are well-described by either a single blackbody or a single power-law model. We also incorporate optical/UV imaging observations from the Hubble Space Telescope (HST) and find optical counterparts to PSR J1641+3627D and J1641+3627F. Our colour-magnitude diagrams indicate the latter contains a white dwarf, consistent with the properties suggested by radio timing observations. The counterpart to J1641+3627D is only visible in the V band; however, we argue that the companion to J1641+3627D is also a white dwarf, since we see a blackbody-like X-ray spectrum, while MSPs with nondegenerate companions generally show non-thermal X-rays from shocks between the pulsar and companion winds. Our work increases the sample of known X-ray and optical counterparts of MSPs in globular clusters.