Last summer, the Fermi Gamma-ray Space Telescope discovered new pulsars through gamma-ray detection alone and found that millisecond pulsars do emit gamma rays. At the Fermi Symposium in Washington, DC, this week, Scott Ransom of the National Radio Astronomy Observatory announced that Fermi has located new millisecond pulsars based on gamma-ray emissions alone. Fermi may come to greatly assist the radio astronomy community in locating millisecond pulsars, which may eventually assist in the detection of gravitational waves.
“Fermi is giving something back to the radio-astronomy community that I don’t think anyone expected,” said Ransom after his talk at the Fermi Symposium on Monday.
From the Fermi telescope’s frequent scans of the night sky, the collaboration generated a list of noticeably bright sources, which many observatories then investigated in greater detail. Some of these sources (though not the very brightest) turned out to be pulsars, and upon further investigation, NRAO has found three new millisecond pulsars. These discoveries have been made within the last month and have not yet been reported in any academic journals.
Millisecond pulsars are very old pulsars that are thought to increase their rotation speed over the years. While these pulsars are born in the plane of our galaxy, over time they drift out into the darker regions. Astronomers only know of about 80 millisecond pulsars in our galaxy, but they expect that tens of thousands of them could be floating in the darkness all around us.
Because millisecond pulsars tend to drift into isolated areas, they become very difficult to find with radio telescopes. A quick scan of the sky won’t reveal their locations because their radio signals are rather faint, and it would take long exposure times to see them. Ransom says that NRAO can’t dedicate enough time to watching every point in the sky to hope to discover faint pulsars, but Fermi’s frequent scans of the sky eventually make these pulsars visible in gamma rays. With this capability, Ransom says he expects that over time the observatory’s contribution to the search for millisecond pulsars could be significant.
What is perhaps most thrilling about these new discoveries is that the millisecond pulsars can be used to search for gravitational waves. Millisecond pulsars are very accurate time keepers. They pulse in extremely regular, reliable intervals. If gravitational waves pass near them and distort space-time, it could delay the arrival of their signals on Earth. If astronomers are keeping careful watch on a large number of pulsars simultaneously, they might observe these delays, and find evidence for gravitational waves.
There is currently a project called NANOGrav, or the North American Nanohertz Observatory for Gravitational Waves, which is leading this gravity wave project. Ransom says the three new millisecond pulsars are particularly valuable to NANOGrav because they are actually quite bright, and a lack of bright pulsars has so far been NANOgrav’s biggest hindrance.
