# Hubble Space Telescope non-detection of PSR J2144-3933: the coldest known neutron star

## TL;DR

This study reports non-detections of the old, slow pulsar PSR J2144-3933 with Hubble, setting an upper temperature limit of 42,000 K, making it the coldest known neutron star and providing insights into neutron star thermal evolution.

## Contribution

First to establish an upper temperature bound for PSR J2144-3933, the coldest known neutron star, informing models of neutron star thermal evolution.

## Key findings

- Upper temperature limit of 42,000 K for PSR J2144-3933.
- Consistent with models including Urca reactions and rotochemical heating.
- Constrains on neutron superfluid angular momentum excess.

## Abstract

We report non-detections of the $\sim 3\times 10^8$ yr old, slow, isolated, rotation-powered pulsar PSR J2144$-$3933 in observations with the Hubble Space Telescope in one optical band (F475X) and two far-ultraviolet bands (F125LP and F140LP), yielding upper bounds $F_{\rm F475X}< 22.7$ nJy, $F_{\rm F125LP}< 5.9$ nJy, $F_{\rm F140LP}< 19.5$ nJy, at the pivot wavelengths 4940 \AA, 1438 \AA\ and 1528 \AA, respectively. Assuming a blackbody spectrum, we deduce a conservative upper bound on the surface (unredshifted) temperature of the pulsar of $T<42,000$ K. This makes PSR~J2144--3933 the coldest known neutron star, allowing us to study thermal evolution models of old neutron stars. This temperature is consistent with models with either direct or modified Urca reactions including rotochemical heating, and, considering frictional heating from the motion of neutron vortex lines, it puts an upper bound on the excess angular momentum in the neutron superfluid, $J<10^{44}\,\mathrm{erg\,s}$.