Web edition: August 21, 2012
BEIJING — A cast of exotic, and sometimes mysterious, astrophysical objects quietly took center stage during the first session of the pulsar symposium at the International Astronomical Union’s General Assembly meeting on August 20.
Presenters cautiously discussed several new — indeed, some very new — discoveries, including a millisecond pulsar parked in the heart of a triple system, another “planet”-pulsar pair similar to the (erroneously named) diamond planet, and a very distant, mysterious burst resembling the controversial Lorimer burst of 2007.
Pulsars are spinning neutron stars, the extremely dense remains of collapsed giants. Some, the millisecond pulsars, can spin up to 1,000 times per second. Scientists have been conducting vast searches for these objects, which are better timekeepers than atomic clocks, and whose spins readily reveal orbiting bodies.
“Every time we’ve looked for new pulsars in new surveys, we find things that are interesting,” said Ryan Lynch, a postdoctoral fellow at McGill University in Montreal who presented results from a survey using the Green Bank Telescope in West Virginia.
Among the objects Lynch presented was PSR J0337+1715, a millisecond pulsar forming the heart of a triple system, only the second one ever seen. Spinning once every 2.7 milliseconds, the pulsar hosts a white dwarf star in a 1.6-day orbit, as well as a larger, unknown object in a 327-day orbit. This system differs from the other known triple system, which comprises a millisecond pulsar orbited by a white dwarf and a Jupiter-mass object, Lynch said.
Jupiter-mass objects also made an appearance in a talk by Cherry Ng, a graduate student at the Max Planck Institute for Radio Astronomy in Bonn, Germany. Ng described a millisecond pulsar — spinning once every 3.4 milliseconds — orbited by an object weighing 0.8 Jupiter masses. The system was discovered in July.
Ng called the system “Number Two,” a reference to its similarity to the “diamond planet” announced last year. That system, PSR J1719-1438, comprises a millisecond pulsar and a Jupiter-mass object. But the object is way too dense to be a proper planet, and is instead the probable collapsed core of a former white dwarf star that has lost as much as 99 percent of its mass.
Ng didn’t present a density for the object circling “Number Two,” noting only that the two systems “are quite similar.”
But astronomer Scott Ransom suggests that Number Two’s friend is likely similar in density to the known diamond “planet.”
“It looks like it’s the remnant of some type of old white dwarf,” says Ransom, from the U.S. National Radio Astronomy Observatory. “These things — whatever you call these outer things — are rare.... Some weird thing is happening to these systems.”
Rounding out the discoveries is perhaps the weirdest object of all, an astrophysical one-hit wonder sighted just two weeks ago by scientists at the Parkes Observatory in Australia. The object resembles the now controversial “Lorimer burst,” a superbright, superfast extragalactic radio outburst that made headlines in 2007 when it leapt out of a pulsar survey.
Since then, though, events in Earth’s atmosphere have mimicked the signal from the original Lorimer burst, causing some scientists to doubt its astrophysical nature, said Michael Keith, a postdoctoral fellow at Australia’s Commonwealth Scientific and Industrial Research Organisation, who briefly discussed the object in his presentation.
But the new burst looks better than the original. It’s a good, clean signal, and coming from much farther away. “The model of the galaxy would have to be really, really wrong for this thing to be inside,” Keith said.
Nobody knows what causes these outbursts, though some have suggested the bursts could result from collapsing black holes or cosmic collisions. “It could be some completely new phenomenon,” Ransom says. He notes that if this second observation is real, it means these mysterious bursts could be quite common and waiting to be discovered with the right survey. “Some amazing event that is causing these bursts could be happening all the time.”
S. Thorsett et al. The triple pulsar system PSR B1620-26 in M4. The Astrophysical Journal. Volume 253, October 1, 1999, p. 763. [Go to]
M. Bailes et al. Transformation of a star into a planet in a millisecond binary. Science. Vol. 333. Published online August 25, 2011. [Go to]
D. Lorimer et al. A bright millisecond radio burst of extragalactic origin. Science. Vol. 318. Published online September 27, 2007. [Go to]