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Title: LOFAR Discovery of a 23.5 s Radio Pulsar

Authors: C. M. Tan et al.

First Author’s Institution: Jodrell Bank Centre for Astrophysics, University of Manchester, UK

Status: Published in ApJ

Pulsar Rotation Rates

Neutron stars are formed from massive stars that undergo violent supernova explosions after they run out of nuclear fuel and collapse under their own gravity. Radio pulsars are highly magnetized, rotating neutron stars that emit beams of radiation from their magnetic poles. When these beams of radio emission sweep across our line of sight, they generate radio pulses that can be detected with radio telescopes on Earth. The surface magnetic field strength, age, and internal structure of these objects can be studied through measurements of their rotational rates. Astronomers have now discovered more than 2,700 pulsars in our galaxy, and they’re constantly on the lookout for rare breeds. In today’s astrobite, we cover the discovery of the slowest known spinning radio pulsar, PSR J0250+5854, which has a rotational period of 23.5 s. This exciting finding demonstrates that radio pulsars can rotate much slower than expected and still produce radio pulsations.

PSR J0250+5854: A Record-Setting Slow-Spinning Radio Pulsar

The authors discovered PSR J0250+5854 on 2017 July 30 using the LOw Frequency ARray (LOFAR) radio telescope (see Figure 1) as part of the LOFAR Tied-Array All-Sky Survey (LOTAAS). Additional follow-up radio observations were performed using the Green Bank, Lovell, and Nançay radio telescopes. Pulsations were detected between 120 and 168 MHz with LOFAR and at 350 MHz using the Green Bank Telescope (GBT), but no pulsed emission was detected at ~1.5 GHz using the Lovell and Nançay telescopes. The pulsar’s radio spectrum (spectral index of α = -2.6 ± 0.5, assuming its flux density follows a power-law as a function of frequency) is remarkably steep compared to the average pulsar population (<α> ≈ -1.8). This suggests that its radio emission is significantly brighter at lower frequencies (see Figure 2).

A Needle in a Haystack or a Haystack Full of Needles?

Based on measurements of the pulsar’s rotation spanning more than 2 years, PSR J0250+5854 has an inferred surface dipole magnetic field strength of 26 trillion Gauss, characteristic age of 13.7 million years, and a spin-down luminosity of 8.2 x 10erg s, assuming a dipolar magnetic field configuration. PSR J0250+5854’s radio beam is very narrow according to the measured width of its pulse profile (the pulse duty cycle is < ~1% below 350 MHz, see Figure 3). Individual single pulses were routinely detected from the pulsar at low radio frequencies, except during brief periods of “pulse nulling” when the pulsar stopped emitting radio pulses. This occurred 27% of the time on average. The pulsar’s slow rotation period of 23.5 s is similar to other classes of pulsars. In particular, magnetars have high magnetic fields, spin periods ranging between roughly 2 and 12 s, and often produce X-ray emission, and X-ray Dim Isolated Neutron Stars (XDINs) have spin periods ranging between 3.4 and 11.3 s. However, no X-ray emission was detected from PSR J0250+5854 during follow-up observations with the Neil Gehrels Swift Observatory X-ray Telelescope.

The P–Ṗ diagram is a key diagnostic tool for characterizing how pulsars evolve in time. Using pre-discovery LOTAAS data of PSR J0250+5854 from 2015, the authors measured a spin period derivative of Ṗ = 2.7 x 10-14 s s-1. The pulsar’s rotational parameters place it in the right region of the P–Ṗ diagram (see Figure 4) — an area where few pulsars have been found to reside. In particular, PSR J0250+5854 falls near/below many of the so-called “pulsar death lines,” beyond which pulsars are not expected to emit coherent radio emission. These models are based on assumptions about the conditions in the pulsar’s magnetosphere, such as pair production, which is thought to be essential for the generation of radio emission. Since the radio-emission mechanism in pulsars is not fully understood, searching for additional pulsars near these death regions will help to inform us about how pulsars produce radiation.

The discovery of PSR J0250+5854 begs the question: Is this a special kind of pulsar, or are there more to be found? The authors argue that more of these slow-rotating pulsars may be lurking around our galaxy, but we simply haven’t been sensitive to detecting them because commonly used Fast Fourier Transform (FFT)-based periodicity search algorithms are not well-suited to detecting slow pulsars with small duty cycles. The authors also point out that the radio emission observed from PSR J0250+5854 was much more erratic at higher frequencies. Therefore, if other slow rotating pulsars are similar to PSR J0250+5854, then this suggests that low-frequency radio telescopes, like LOFAR, may prove to be excellent observatories for searching for these slow rotators.