Radio Pulsars: How Slow Do They Go?

Title: LOFAR Discovery of a 23.5 s Radio Pulsar

Authors: C. M. Tan, C. G. Bassa, S. Cooper, T. J. Dijkema, P. Esposito, J. W. T. Hessels, V. I. Kondratiev, M. Kramer, D. Michilli, S. Sanidas, T. W. Shimwell, B. W. Stappers, J. van Leeuwen, I. Cognard, J.-M. Grießmeier, A. Karastergiou, E. F. Keane, C. Sobey, and P. Weltevrede

First Author’s Institution: Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester

Status: Published in The Astrophysical Journal; open access on arXiv

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 the Galaxy, and they’re constantly on the look out 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).

Figure 1: An aerial view of the LOFAR Superterp, part of the core of the extended telescope located in the Netherlands. Image Credit: LOFAR/ASTRON.

Figure 2: Radio spectrum of PSR J0250+5854 using LOFAR and GBT observations. The black line shows the fitted spectral index, with 1σ uncertainties indicated by the shaded gray region. The circle corresponds to the measured flux density from LOFAR Two-meter Sky Survey imaging observations, and the triangles correspond to upper limits on the flux densities from LOFAR Low Band Antenna, Nançay, and Lovell radio telescope observations, respectively. Image Credit: Figure 5 from the paper.

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, a characteristic age of 13.7 million years, and a spin down luminosity of 8.2 x 1028 erg s-1, 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.

Figure 3: Integrated pulse profiles of PSR J0250+5854 at observing frequencies of 350 MHz (GBT), 168 MHz (LOFAR), and 129 MHz (LOFAR). Here, only 5% of the rotational phase is shown. The inset figure shows the pulse profile across the whole LOFAR High Band Antenna band over a full rotation period. Image Credit: Figure 6 from the paper.

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

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.

Figure 4: P–Ṗ diagram of pulsars derived from their measured rotational periods and rotational period derivatives. The positive sloped gray lines indicate characteristic ages of 1 kyr, 100 kyr, 10 Myr, and 1 Gyr. The negative sloped gray lines correspond to inferred surface magnetic field strengths of 10 GG, 100 GG, 10 TG, and 100 TG. Magnetars (green), XDINSs (orange), RRATs (yellow), and the 8.5 s radio pulsar PSR J2144–3933 are indicated on the plot. The colored lines show the various death line models, where pulsars below these lines are not expected to produce radio emission. Image Credit: Figure 4 from the paper.

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 the 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.

About Aaron Pearlman

I am a Ph.D. candidate in Physics at Caltech. My research focuses on searching for new pulsars near the center of the Galaxy using JPL's Deep Space Network radio dishes in the southern hemisphere. I am also interested in studies of magnetars, FRBs, gravitational-wave searches, and high-energy observations of compact objects. When I'm not hunting for pulsars, I can usually be found hanging out with dogs or trying the latest vegetarian cuisine Los Angeles has to offer!

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