2018-21

A new discovery reported in the Astrophysical Journal Letters might lay claim to title of the most unusual extreme class of astronomical object: very high energy (VHE) gamma-ray emitting, neutron star-massive star binary pairs. Of the one-hundred billion stars in our galaxy, fewer than ten are in known to be in gamma-ray binary systems, with this discovery being only the second with an identified neutron star. The gamma-ray emission was discovered during an event that will not happen again until 2067.

A neutron star is the dense remains of a type of supernova, the explosive death of a star that started its life more massive than about eight solar-masses. Containing as much material as the sun but in an object only the diameter of a city, neutron stars are so dense that most of their matter is in the form of neutrons, the uncharged atomic particles found in atomic nuclei. Neutron stars spin rapidly and generate powerful magnetic fields, fast winds and narrow beams that sweep like a lighthouse across the sky as the star rotates. If the Earth happens to lie in the path of one of these beams as it passes, astronomers can detect the radiation as regular pulses at radio and other wavelengths. There are a few thousand of these "pulsars" known, beating at a variety of rates from more than a thousand times a second to less than about once a second.

It is common for massive stars to form in binary pairs, and so it is not surprising that some pulsars have an orbiting companion that has survived its partner's explosive death. Both the pulsar and its companion are likely to have disks of material around them. The rapidly spinning pulsar and its wind can in some cases slam into the disk and wind of the companion star as the two periodically approach in their orbital dance. The energetic collision can produce intense shocks that accelerate charged particles to energies high enough to produce very high energy (VHE) gamma ray radiation by accelerating the particles to nearly the speed of light. When light scatters off such energetic particles it too becomes energized and becomes VHE gamma ray photons each one of which can pack a billion times more energy than a photon of optical light. The precise timing of the radio pulses allows astronomers to use the radio signals to deduce some parameters of the stars and their orbit. Although there are plenty of pulsars, until now most of the explanation was speculation, with only one known case of a binary pulsar system exhibiting VHE gamma-ray emission.

An international team of astronomers began intensively tracking a second, possible VHE gamma-ray pulsar system in 2016. Located about five thousand light-years away in a massive stellar nursery in the direction of the constellation Cygnus, the pulsar was identified as having a massive stellar companion that orbited it every 50 years in an extreme elliptical orbit. At their closest approach the two were expected to come within a mere one astronomical unit of each other (one AU is the average distance of the Earth from the sun), and the scientists had calculated that this would happen on November 13, 2017 – exactly one year ago.

CfA astronomers Wystan Benbow, Gareth Hughes, and Michael Daniel direct VERITAS operations and enabled the VERITAS collaboration’s participation in the program to monitor the behavior of this bizarre object before, after and during its expected closest approach. VERITAS is an array of four 12 m diameter optical telescopes located at the SAO's Fred Lawrence Whipple Observatory near Tucson, Arizona. VERITAS detects gamma rays via the extremely brief flashes of blue “Cherenkov” light created when gamma rays are absorbed in the Earth’s atmosphere. The VERITAS Collaboration consists of about 80 scientists from 20 institutions in the United States, Canada, Germany and Ireland. VERITAS scientists were joined by a team using the two 17 m MAGIC Cherenkov telescopes located at El Roque de Los Muchachos on the island of La Palma, Spain.

As the binary system is embedded in a larger, diffuse region of VHE gamma-ray emission, the international team of astronomers anxiously awaited the event to see whether the VHE gamma-rays emission brightened near the pulsar. According to Alicia López Oramas, a researcher with MAGIC at the Instituto de Astrofísica de Canarias (IAC), and one of the corresponding authors of the study, "such a unique system was expected to emit very-high-energy gamma rays during this approach, and this opportunity could not be missed." Graduate student Tyler Williamson and his advisor Professor Jamie Holder, both from University of Delaware’s Department of Physics and Astronomy, played leading roles in the VERITAS campaign, together with Ralph Bird, a post-doctoral researcher at the University of California, Los Angeles.

Initial observations, in 2016, revealed weak gamma-ray emission, consistent with earlier results. "This low-level, steady emission is most likely from a nebula which is being continuously powered by the pulsar," explains Dr. Bird. Starting in September 2017, the results became much more exciting. "The gamma-ray flux we observed in September was twice the previous value," says Williamson. But the fireworks were just beginning. "During the closest approach between the star and the pulsar, in November 2017, the flux increased 10 times in just a single night."

In an attempt to explain not only the strength of the gamma rays, but also their gradual variability and then sudden flaring, the team tried to match a recent theoretical model to their observations. The model contains the latest ideas about pulsars, the binary disk and wind environment, the nature of the ionized nebulosity around the object, the spectrum of emission and tries to refine the orbital parameters of the binary. It was unsuccessful and so the scientists conclude that significant revision is needed to the models in order to fit the observations, including better information about the geometry of the encounter. Since information about the structure of disks and winds around pulsars depends on many diverse yet key parameters like magnetic field strength and environmental history, this object – if it can be successfully modeled - offers to be a potential Rosetta Stone about the birth and evolution of compact objects, and so includes all compact objects produced in supernovae, pulsars without companions, and even many black hole binary systems. In the coming years the scientists plan to continue to monitor this and other pulsars to monitor the exotic behavior of these most unusual and extreme cosmic characters. Wystan Benbow from the CfA states that "continued investment in the operation of unique, leading edge facilities like VERITAS is critical and will ensure further opportunities to achieve transformative science."