A study led by an IRAP astronomer (Université Paul Sabatier / CNRS / CNES) led to the discovery of a pulsar beaming with the energy of about 10 million suns. It is so powerful that it radiates as much energy as the disk surrounding a black hole, although it does not have the same mass. This is the brightest pulsar – a dense stellar remnant left over from a supernova explosion – ever observed in the Universe. Its periodic X-rays emission relates to the frequency of rotation of the star itself. The discovery was made with NuSTAR (Nuclear Spectroscopic Telescope NASA’s Array). It will be published in Nature on October 9, 2014.
“You might think of this pulsar as the ‘Mighty Mouse’ of stellar
remnants,” said Fiona Harrison, the NuSTAR principal investigator at the
California Institute of Technology in Pasadena, Calif. “It has all the
power of a black hole but with much less mass.”
The surprising discovery is helping astronomers better understand mysterious sources of fierce X-rays, called ultraluminous X-ray sources, or ULXs. Before now, all ULXs were thought to be black holes. New data from NuSTAR show that at least one ULX, about 12 million light-years away in the galaxy Messier 82 (M82), is actually a pulsar.
“The pulsar appears to be eating the equivalent of a black hole diet,” said Harrison. The discovery appears in a new report in the October 9 issue of Nature. It will help astronomers understand how black holes gorge, and grow, so quickly – an important event in the formation of galaxies and structures in the universe.
ULXs are generally thought to be black holes feeding, or “accreting,” off companion stars. They are suspected to be the long-sought “medium-size” black holes – missing links between smaller, stellar-size black holes and the gargantuan ones that dominate the hearts of all galaxies. But research into the true nature of ULXs is ongoing.
NuSTAR didn’t initially set out to study the two ULXs in M82.Astronomers had been observing a recent supernova in M82, when they serendipitously noticed bright X-rays coming from a point nearby – what turned out to be the ULX called M82 X-2. The NuSTAR team was shocked to find that this source had a pulse. Black holes don’t pulse, but pulsars do.
Pulsars belong to a class of stars called neutron stars. Like black holes, neutron stars are the burnt-out cores of exploded stars, only puny in mass by comparison. Pulsars are neutron stars that send out beams of light. As the star spins, these beams intercept Earth like lighthouse beacons, producing a pulsed signal.
“We took it for granted that the powerful ULXs must be massive black holes,” said Matteo Bachetti, lead author of the paper from the University of Toulouse, France. “When we first saw the pulsations in the data, we thought they must be from another source.”
NASA’s Chandra X-ray Observatory and Swift satellite had also been
monitoring M82 to study the same supernova, and confirm that the intense
X-rays of M82 X-2 were coming from a pulsar.
“Having a diverse array of telescopes in space means that they can help each other out,” said Paul Hertz, director of NASA’s astrophysics division in Washington. “When one telescope makes a discovery, others can be called in for backup.”
The key to NuSTAR’s discovery was in its sensitivity to X-rays in the highest-energy ranges as well as its ability to precisely measure the timing of the signals. This timing capability allowed the astronomers to measure a pulse rate from M82 X-2 of 1.37 seconds. They also measured its energy output at the equivalent of 10 million suns, or 10 times more than what was measured before – a big punch for something about the mass of the sun and the size of Pasadena.
How is this puny dead star radiating so fervently? Astronomers aren’t sure, but they say it is likely due to a lavish feast of the cosmic kind. As is the case with black holes, the gravity of a neutron star can pull matter off companion stars. As the matter is dragged into the neutron star, it heats up and glows with X-rays. If the pulsar is indeed feeding off surrounding matter, it is doing so at such an extreme rate to have theorists scratching their heads.
Astronomers are planning follow-up observations with NuSTAR, Swift and Chandra to help explain the bizarre behavior. The NuSTAR team will also look at more ULXs, and it’s possible they could turn up more pulsars. At this point, it is not clear if the M82 X-2 is an oddball or if more ULXs beat with the pulse of dead stars. NuSTAR, a relatively small telescope, has thrown a big loop into the mystery of black holes.
“In the news recently, we have seen that another source of unusually bright X-rays in the M82 galaxy seems to be a medium-sized black hole,” said Jeanette Gladstone of the University of Alberta, Canada, who is not affiliated with the study. “Now, we find that the second source of bright X-rays in M82 isn’t a black hole at all. This is going to challenge theorists and pave the way for a new understanding of the diversity of these fascinating objects.”
- An ultraluminous X-ray source powered by an accreting neutron star, M. Bachetti et Al., Nature, doi:10.1038/nature13791, octobre 2014
- NASA Press Release
IRAP Contacts :
- Matteo Bachetti (INAF-OAC, anciennement IRAP-CNRS) l T +39 070 711 80 271 l firstname.lastname@example.org
- Didier Barret (IRAP – CNRS) l T 05 61 55 85 61 l email@example.com
- Natalie WEBB (IRAP – Univ. Paul Sabatier) l T 05 61 55 75 70 l firstname.lastname@example.org