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Pulsar Highlights - A Dozen new MSPs in Globular Clusters

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).

Pulse profiles of new Arecibo globular cluster pulsars

Pulse profiles of new Arecibo globular cluster pulsars

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).