A millisecond pulsar discovery in a survey of unidentified Fermi $\gamma$-ray sources with LOFAR

TL;DR

This paper reports the discovery of a millisecond pulsar at a low radio frequency using LOFAR, highlighting a potentially overlooked population of steep-spectrum MSPs relevant for future low-frequency surveys.

Contribution

First detection of an MSP at <200 MHz frequency, demonstrating the effectiveness of low-frequency surveys with LOFAR for uncovering steep-spectrum pulsars.

Findings

Discovered PSR J1552+5437 with a 2.43 ms period

The pulsar has a very steep spectral index (< -2.8)

Radio and gamma-ray profiles are nearly aligned

Abstract

Using LOFAR, we have performed a very-low-frequency (115-155 MHz) radio survey for millisecond pulsars (MSPs). The survey targeted 52 unidentified Fermi $\gamma$-ray sources. Employing a combination of coherent and incoherent dedispersion, we have mitigated the dispersive effects of the interstellar medium while maintaining sensitivity to fast-spinning pulsars. Toward 3FGL J1553.1+5437 we have found PSR J1552+5437, the first MSP to be discovered (through its pulsations) at a radio frequency $<$ 200 MHz. PSR J1552+5437 is an isolated MSP with a 2.43 ms spin period and a dispersion measure of 22.9 pc cm$^{-3}$. The pulsar has a very steep radio spectral index ($\alpha < -$2.8 $\pm$ 0.4). We obtain a phase-connected timing solution combining the 0.74 years of radio observations with $\gamma$-ray photon arrival times covering 7.5 years of Fermi observations. We find that the radio and $\gamma$-ray pulse profiles of PSR J1552+5437 appear to be nearly aligned. The very steep spectrum of PSR J1552+5437, along with other recent discoveries, hints at a population of radio MSPs that have been missed in surveys using higher observing frequencies. Detecting such steep spectrum sources is important for mapping the population of MSPs down to the shortest spin periods, understanding their emission in comparison to slow pulsars, and quantifying the prospects for future surveys with low-frequency radio telescopes like SKA-Low and its precursors.