Double (Neutron Star) Trouble

Title: The Green Bank Northern Celestial Cap Pulsar Survey. VI. Timing and Discovery of PSR J1759+5036: A Double Neutron Star Binary Pulsar

Authors: Gabriella Agazie, Michael Mingyar, Maura McLaughlin, Joseph Swiggum, David Kaplan, Harsha Blumer, Pragya Chawla, Megan DeCesar, Paul Demorest, William Fiore, Emmanuel Fonseca, Joseph Gelfand, Victoria Kaspi, Vladislav Kondratiev, Malcolm LaRose, Joeri van Leeuwen, Lina Levin, Evan Lewis, Ryan Lynch, Alexander McEwen, Hind Al Noori, Emilie Parent, Scott Ransom, Mallory Roberts, Ann Schmiedekamp, Carl Schmiedekamp, Xavier Siemens, Renée Spiewak, Ingrid Stairs, Mayuresh Surnis

First Author’s Institution: Center for Gravitation, Cosmology, and Astrophysics, Dept. of Physics, University of Wisconsin-Milwaukee, P.O. Box 413, Milwaukee, WI 53201, USA

Status: Submitted to ApJ

Pulsars, rapidly rotating neutron stars that emit radio beams like a lighthouse creating pulses of radio emission as the beam crosses our line of sight, are some of the most astrophysical interesting objects in the Universe. Astronomers can predict when these pulses will arrive at Earth very accurately through a process called pulsar timing. Studying these variations in these pulse times of arrival allow astronomers to do experiments from testing general relativity, to the detection of low frequency gravitational waves, and beyond! At the time of this writing we have found 2872 pulsars, and more are being discovered all the time.

Searching for new pulsars often leads to the discovery of new and interesting systems that expand our knowledge of things from supernovae to the neutron star equation of state (which describes how the incredibly dense matter neutron stars are made of behaves). The authors of today’s paper have found just such an interesting system.

A Heavy Binary System

The pulsar that is the focus of this paper is PSR J1759+5036, which was first discovered accidentally while a different pulsar was being observed by the 100-m Green Bank Telescope. However, in follow up observations of PSR J1759+5036, the authors noticed that not only did the period of the pulsar – how fast it’s rotating – change, but also that it was very difficult to detect. Out of 109 different observations of this pulsar over seven years, it was only detected 50 times, shown by the blue points in Figure 1. The red lines in Figure 1 show represent each observation where PSR J1759+5036 was not detected. Luckily, these detections were enough to determine what was causing its period to change: a binary companion.

Figure 1: Measured spin period of PSR J1759+5036 as a function of it’s orbital phase, where the span of 0 to 1 describe one full orbit of the pulsar and its companion. The blue points show the measured period of PSR J1759+5036 for a particular observation, and the red lines indicate observations where the pulsar was not detected. The black line shows the model fit to the period variations based on the orbital parameters. Figure 2 in the paper.

From the data the authors obtained, they were able to determine all of the Keplerian orbital parameters that describe the orbit of PSR J1759+5036 and its unknown companion, including an orbital eccentricity of 0.3, much larger than most known pulsars in binary systems.

What is particularly interesting is that the authors were also able to measure one post-Keplerian orbital parameter, in this case the advance of periastron. Post-Keplerian parameters describe relativistic orbital effects from general relativity and are dependent on the mass of both objects in the binary system. So if one or more of these parameters is measurable, you can start to constrain the masses of the objects in the system, which would otherwise be almost impossible.

The authors were able to measure the total system mass to be 2.62 solar masses (or 2.62 times mass of the Sun), which is quite heavy for a binary pulsar system. They were then able to determine the likelihood of the two separate masses, shown in Figure 2, and found it is likely that both objects in this system are neutron stars. This is particularly exciting because of the 2872 known pulsars, only 15 of them are double neutron star systems.

Figure 2: Limits on the allowed values of the masses of PSR J1759+5036 and its companion based on the advance of periastron measurement, shown in red, and the mass function shown in green, which describes the ratio of pulsar to companion masses. The histograms on the top and right hand side show the likelihood of each object in the system having a particular mass, with a larger probability density indicating a larger likelihood. Figure 7 from the paper.

Weighing the Options

But since the authors only measure a single post-Keplerian parameter, they cannot rule out that this binary system might still be a pulsar with a white dwarf (or even main sequence star) companion, which are much more common than double neutron star systems. To narrow down the nature of PSR J1759+5036’s companion might be, the authors did some optical observing. PSR J1759+5036 has an estimated distance of between 500-700 parsecs (fairly close as pulsars go), so if its companion is a main sequence or white dwarf star, it is likely that it would show up in optical observations, whereas neutron stars typically do not emit at optical wavelengths.

Figure 3: Optical observation of PSR J1759+5036 taken with the Las Cumbres Observatory 2-meter telescope on Haleakala. A 5 arcsecond area around the pulsar is shown by the black circle. No apparent optical counterparts, shown by the white dots in the image, are found within the circle, indicating the pulsar’s companion is not visible at optical wavelengths. Figure 9 from the paper.

The authors observed PSR J1759+5036 with the Las Cumbres Observatory 2-meter telescope on Haleakala, but found no apparent optical companion near the pulsar, shown in Figure 3. When combined with the large eccentricity measurement of 0.3, which is consistent with known double neutron star binary system eccentricities, this non-detection further reinforces the idea that PSR J1759+5036 is a double neutron star system.

Tipping the Scales

A possible new double neutron star system is exciting because it can teach us about supernovae geometry and how these systems form and evolve. If tighter limits can be placed on the masses of these neutron stars it may allow further constraints on the neutron star equation of state too. But while the evidence so far points to a double neutron star system, it has yet to be confirmed. Measuring more post-Keplerian parameters will be critical in constraining the masses, which means further observations of PSR J1759+5036 are necessary.

The authors are already conducting weekly observations of this new system with the Canadian Hydrogen Intensity Mapping Experiment, which should help them get to the true nature of PSR J1759+5036 and its companion, so be on the lookout for news of this intriguing system in the future!

Astrobite updated on September 3, 2021

Astrobite edited by: Mitchell Cavanagh

Featured image credit: NASA/Dana Berry, Sky Works Digital

About Brent Shapiro-Albert

I'm a fourth year graduate student at West Virginia University studying various aspects of pulsars. I'm a member of the NANOGrav collaboration which uses pulsar timing arrays to detect gravitational waves. In particular I study how the interstellar medium affects the pulsar emission. Other than research I enjoy reading, hiking, and video games.

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