The US Naval Research Laboratory (NRL), in partnership with the international Fermi Large Area Telescope Collaboration, declared the finding of nearly 300 gamma ray pulsars in the publication of their Third Catalog of Gamma Ray Pulsars.
Gamma-Ray Pulsars/Milky Way. The positions of the cataloged pulsars shown in a top-down view of the Milky Way. The red and orange symbols indicate millisecond pulsars, while the green and blue symbols indicate young, unrecycled pulsars. Some radio-quiet pulsars (blue symbols) do not have well-measured distances, so their positions only indicate the direction towards these pulsars. Image Credit: US Naval Research Laboratory.
Fifteen years have passed since Fermi's 2008 launch, marking when the count of identified gamma-ray pulsars numbered less than ten.
Work on this important catalog has been going on in our group for years. Our scientists and postdocs have been able to both discover and analyze the timing behavior and spectra of many of these newfound pulsars as part of our quest to further our understanding of these exotic stars that we are able to use as cosmic clocks.
Paul Ray Ph.D., Head, High Energy Astrophysics and Applications Section, US Naval Research Laboratory
Pulsars emerge from massive stars that have exhausted their fuel, succumbing to their gravitational force. This collapse forms a dense, magnetized neutron star, rapidly spinning. These spinning magnetic fields emit gamma-ray beams, the most potent type of light.
As these beams traverse Earth, the incredibly sensitive Fermi gamma-ray telescope captures their rhythmic energy pulses. With over 15 years of data, Fermi has revolutionized pulsar research, fundamentally altering the field's landscape.
We have been very excited about how many millisecond pulsars (MSPs) we have been able to detect using these gamma rays. We are able to study these objects that began as young pulsars in a binary system. Like a spinning top, they eventually slowed down and became inert. Over the past hundreds of millions of years, their binary companions dumped matter on to them, causing their speed to increase again, very dramatically and far faster than before, “recycling” these pulsars into MSPs. These high speed MSPs are now some of Nature’s most precise timekeepers.
Matthew Kerr Ph.D., Astrophysicist, US Naval Research Laboratory
Researchers are utilizing these cosmic clocks in experiments called Pulsar Timing Arrays. The researcher can search for ripples in space-time by searching for tiny deviations during the arrival of the pulses. The ripples, also called gravitational waves, are created when massive objects, such as pulsars, speed up. Tough gravitational wave sources denote a cataclysmic crash of dense, compact objects such as neutron stars and black holes.
Various pulsar timing array collaborations, including researchers from NRL, unveiled the initial convincing proof of extremely low-frequency gravitational waves. These waves are believed to originate from the merger of supermassive black holes.
These are such exciting results. These low frequency gravitational waves allow us to peer into the centers of massive galaxies and better understand how they were formed.
Thankful Cromartie Ph.D., National Research Council Research Associate, US Naval Research Laboratory
The pulsar timing array outcome has significant practical uses as well. The space-time alterations fix a limit on the usage of pulsars for critical navigation and timing accurately. Spinning pulsars are the same as GPS satellites in pulsar-based navigation and can be utilized further than the Earth’s orbit.
Dr. Ray says, “Now we know where that ultimate stability limit is.”
There is an influence on pulsar timing array work using Fermi’s gamma ray detection abilities.
Dr. Kerr adds, “Previously, once we found an MSP we had to hand it off to radio astronomers to monitor with huge telescopes. What we have found is that Fermi is sensitive enough by itself to constrain these gravitational waves and, unlike radio waves, which are bent like the light in a prism as they travel to earth, the gamma rays shoot straight to us. This reduces potential systemic errors in measurements.”
Megan DeCesar, Ph.D., a Scientist from George Mason University working at NRL, finds the most captivating element of the recent work to be the significant surge in "spider" pulsars. “Spider pulsars are named after arachnids that eat their smaller mates. Something similar can happen when a neutron star and its binary companion are very close to each other and the MSP “recycling” process gets a little carried away. The intense radiation and particle wind from the pulsar eats away at the surface of the other star, resulting in a puffball of evaporated material.”
Fermi is predominantly proficient at finding these “spiders” compared to radio observations. In many cases, radio waves are eclipsed as the pulsar beam passes the remnants of the companion star. Gamma rays are efficient in penetrating through it.
DeCesar says, “While it may be that spider systems are also intrinsically brighter in gamma rays, studying them will help us to understand their origins and the bonanza of discoveries we have made with Fermi,”
The research was published in the Astrophysical Journal, Supplement. This gathering of the latest information on gamma ray pulsars, with its consistent form is invaluable to the scientific community.