Arecibo Radio Telescope Finds a Dozen New Millisecond Pulsars
A Canadian and American team of Astronomers have used the 305-m wide Arecibo
radio dish, the world's largest and most sensitive radio telescope, to
uncover a dozen new super-fast-spinning pulsars. The team directed their
search for these rare and exotic stars by pointing the Arecibo radio
telescope towards massive star clusters, called Globular Clusters, orbiting
our Milky Way galaxy. The pulsars they have discovered are all spinning
hundreds of times a second and all but one has a binary companion. Three of
the new pulsars are eclipsed as they pass behind their bloated companion
star. These results will be published in the Astrophysical Journal
(preprint).
Figure 1: Pulse profiles of the new globular cluster
millisecond pulsars discovered with the Arecibo telescope. Those with
asterisks still require confirmation.
With ages often greater than 10 billion years, Globular clusters are the
oldest stellar groups in our galaxy. They scrunch hundreds of thousands of
stars so close together that the stars can collide or exchange their binary
companions. This environment can create exotic stellar systems that are much
harder, and sometimes impossible, to form in the Galactic disk. One of the
most exotic residents in the Globular cluster system are the millisecond
pulsars.
Millisecond pulsars are formed when a slowly-rotating neutron star accretes
matter from a companion star and is slowly spun-up by the extra spin that
this added material carries with it. This process is commonly referred to
as "pulsar recycling" and creates the most rapidly rotating stars we know
of, rotating up to 700 times a second. "These stars are rotating so fast
that their surface is moving up to 15 percent of the speed of light."
says Dr. Jason
Hessels of the University of Amsterdam who conducted this survey as part of
his doctoral thesis at McGill University in Montreal, Canada. He adds:
"Globular clusters are an excellent place to find millisecond pulsars
because the dense environment of the cluster's core can create binary
systems capable of recycling neutron stars to rapid rotation rates."
The Astronomers surveyed 22 Globular Clusters visible with the Arecibo radio
telescope to search for the pulsations indicative of radio pulsars. Their
survey is the deepest survey for such objects ever undertaken. "Many of
these pulsars are so weak and distant that Arecibo is the only telescope in
the world that could have discovered them." says team member Dr. Scott
Ransom of the National Radio Astronomical Observatory, in Charlottesville
Virginia. In addition to the 15 previously known pulsars in these clusters,
all discovered with Arecibo, the team confirmed 11 new pulsars and found 2
promising candidate pulsar signals, almost doubling the known population in
these clusters. What they found is shedding new light on the nature of
these objects.
The new pulsars are on average spinning faster and are in tighter orbits
than the previously know pulsars in these clusters. This shows that the
known population has been significantly biased towards isolated and more
slowly spinning pulsars. "We've taken an important step in this survey
towards determining what fraction of millisecond pulsars still have a binary
companion, and how fast they typically spin." says team member Dr. Paulo
Freire of the Arecibo Observatory. Three of the new pulsars they have found
are especially exotic as they are in orbits of only a few hours and show
eclipses as the pulsar passes behind its bloated companion star. "At least
two of the eclipsing pulsars we have found have subsequently been identified
as X-ray sources in data taken with the Chandra X-ray Observatory" says team
member Prof. Ingrid Stairs of the University of British Columbia in
Vancouver, Canada. She adds: "This X-ray emission may be created as the
pulsar's strong wind shocks with material being ripped off of it's companion
star. The pulsar is slowly destroying its companion."
The team also considered the luminosities of all known globular cluster
pulsars to show that isolated and binary pulsars have statistically
consistent luminosities. "It's still unclear why some millisecond pulsars
become isolated after their formation in a binary system; there is no
obvious difference in the properties of isolated and binary pulsars." says
team member Prof. Vicky Kaspi of McGill. The team hopes that this and other
outstanding mysteries about the nature of these extreme stars will be
clarified as more millisecond pulsars are discovered.
Located near Arecibo, Puerto Rico, the Arecibo Observatory is part
of the National Astronomy and Ionosphere Center (NAIC) and is operated by
Cornell University under a cooperative agreement with the National Science
Foundation (NSF).
