The gamma-ray millisecond pulsar deathline, revisited - New velocity and distance measurements

TL;DR

This study revisits the gamma-ray pulsar deathline by using new velocity and distance measurements of millisecond pulsars, discovering four gamma-ray MSPs and analyzing their emission limits and contributions to galactic gamma-ray background.

Contribution

It provides updated proper motion, distance, and parallax data for MSPs, discovers four new gamma-ray MSPs, and investigates the gamma-ray emission deathline with improved measurements.

Findings

Discovered four new gamma-ray MSPs, including the least energetic one.

Provided new proper motion and distance measurements for several MSPs.

The relationship between spin-down luminosity and gamma-ray luminosity remains unclear at low energies.

Abstract

Millisecond pulsars (MSPs) represent nearly half of the more than 160 currently known $\gamma$-ray pulsars detected by the Large Area Telescope on the \textit{Fermi} satellite, and a third of all known MSPs are seen in $\gamma$ rays. The least energetic $\gamma$-ray MSPs enable us to probe the so-called deathline for high-energy emission, i.e., the spin-down luminosity limit under which pulsars (PSRs) cease to produce detectable high-energy radiation. Characterizing the MSP luminosity distribution helps to determine their contribution to the Galactic diffuse $\gamma$-ray emission. We made use of the high-quality pulsar timing data recorded at the Nan\c{c}ay Radio Telescope over several years to characterize the properties of a selection of MSPs. For one of the pulsars, the dataset was complemented with Westerbork Synthesis Radio Telescope observations. The rotation ephemerides derived from this analysis were also used to search the LAT data for new $\gamma$-ray MSPs. For the MSPs considered in this study, we obtained new transverse proper motion measurements or updated the existing ones, and placed new distance constraints for some of them, with four new timing parallax measurements. We discovered significant GeV $\gamma$-ray signals from four MSPs, i.e., PSRs J0740+6620, J0931$-$1902, J1455$-$3330, and J1730$-$2304. The latter is now the least energetic $\gamma$-ray pulsar found to date. Despite the improved $\dot E$ and $L_\gamma$ estimates, the relationship between these two quantities remains unclear, especially at low $\dot E$ values.