Vampire Pulsar Sucks Companion Star Down to Planet-Size* - April 2002
by Christopher Wanjek
Scientists have discovered a new millisecond "accreting" pulsar in a binary star system, only the second such object known. This star orbits its companion every 42 minutes, one of the tightest orbits known. The entire system -- the neutron star, the companion, and the orbit -- could fit inside our own sun.
Also, the companion star is now only 15 Jupiter masses, whittled down from a star originally up to half the mass of the sun. This a strong indication that neutron stars slowly dissolve their companions, increase their spin rate, and ultimately evolve into isolated millisecond pulsars.
The source was discovered in early April 2002 in twice-weekly monitoring scans of the galactic bulge using the NASA Rossi X-ray Timing Explorer's Proportional Counter Array, specifically designed to catch fast-evolving and low luminosity transient outbursts of X-ray sources.
RXTE PCA team members Craig Markwardt and Jean Swank discuss the observation today in a press conference at the joint meeting of the American Physical Society and the High Energy Astrophysics Division of the American Astronomical Society in Albuquerque, New Mexico.
The first, and until now the only, accreting millisecond pulsar is SAX J1808.4-3658, which has a spin frequency of 401.5 Hz. This souce is thought to be a link between low-mass X-ray binaries, which are systems where a neutron star accretes matter and is presumably spun up by accretion torque, and isolated millisecond pulsars, which are rapidly spinning but have no companions. SAX J1808.4-3658 was the first source where a definitively rapidly spinning neutron star was discovered in an accreting system.
The outburst of SAX J1808.4-3658 was both low luminosity and rapidly decaying, making it difficult to detect and make follow-up observations. The RXTE PCA bulge monitoring program was an effort to detect sources like SAX J1808.4-3658 quickly before they had a chance to fade. The effort has paid off with the discovery of this new millisecond pulsar system, designated XTE J1751-305.
XTE J1751-305 has a faster pulsar spin frequency than SAX J1808.4-3658. In further analysis of RXTE data, it was determined that the observed spin frequency was modulated at a 42-minute period, also much shorter than the 2 hour period of SAX J1808.4-3658. Markwardt and Swank interpret the modulation to be the Doppler effect due to motion in a binary orbit. The orbital period of 42 minutes is one of the shortest for known neutron star systems.
The modulations also constrain the nature of the companion star. The projected binary orbital motion of the neutron star is only about 3000 km. At such a small separation, the companion must be very low mass, about 0.014 solar masses, or approximately 15 Jupiter masses. This is a lower mass companion than that of SAX J1808.4-3658.
The companion is likely to be a helium-rich star, probably a semi-degenerate white dwarf. In some binary evolutionary scenarios, such systems might be formed during a period of mass transfer driven by gravitational wave dissipation. It is not clear whether the neutron star will eventually completely disrupt its companion, converting it to an isolated millisecond pulsar.
One mystery is why these sources are such a rarity. The discovery of a second accreting millisecond pulsar confirms that SAX J1808.4-3658 was not a fluke, and that the theory of accretion torque is a viable one.
For images, refer to http://universe.nasa.gov/press/images/neutron/
*Note: This notice was not an official NASA news release, but rather accompanied NASA press release #02-56 "Scientists Catch Speeding Neutron Star With "Radar Gun" Technique, Confirm Theory," the feature of a APS/AAS-HEAD press conference on April 22, 2002.