Black Hole Mass Boundary Drops Lower With Two New Lightweights

3 June 2002, Albuquerque, NM

Aided by the distorted shape of the stars in normally faint binary systems called Soft X-ray Transients, an astronomer at the University of California-San Diego has determined that two nearby black holes each have a mass no larger than a handful of stars like our Sun.

"Astronomical theory says that these extremely low-mass black holes should exist, and now we've finally found at least two," says Dawn Gelino, a postdoctoral fellow at the UCSD Center for Astrophysics and Space Sciences. "These two finds take us near the uncertain boundary between a neutron star and a black hole."

Today in Albuquerque, NM, at the 200th meeting of the American Astronomical Society (AAS), Gelino presents results on a candidate black hole system with a tentative mass of just 5.25 solar masses, using observational data from the 4-meter Blanco telescope at Cerro Tololo Inter-American Observatory in Chile.

Known as N Vel 93 or GRS 1009-45, this binary system is located about 13,000 light-years from Earth, half of the distance toward the center of the Milky Way. The two elements of the binary system are separated by just 3.3 solar radii and they complete an orbit about their common center of mass every seven hours.

These types of X-ray binary systems are believed to consist of a neutron star or a black hole, and a late-type dwarf star slightly smaller than our Sun that is feeding stellar material into its partner. Due to the dwarf star's proximity to its compact partner, as the star evolves it fills its so-called Roche lobe, creating a teardrop-shaped volume of space full of stellar material that remains gravitationally bound to the star. Any overflow is pulled onto the neutron star or the black hole, through its accretion disk. When the overflow matter begins to fall onto the compact object, these tiny systems suddenly erupt to emit copious quantities of X-rays, thereby signaling their presence.

"We study these objects in the infrared rather than the optical because it helps separate the light of the cooler star from the hotter material in the accretion disk. Since the dwarf star is teardrop-shaped, we see differing amounts of surface area as it orbits the compact object. When the star is 'beside' it, we see more surface area, and therefore more light coming from the system than when the star is 'in front' or 'behind' the compact object," Gelino explains. "If the light we see is purely from the star, then we observe what are called 'ellipsoidal variations.' However, if light from the accretion disk is present, the light curve becomes more tricky to interpret."

By fitting the variations in the data to a computer model, the orbital inclination of the system can be determined. The combination of this inclination and careful measurements of the radial velocity change and rotation of the companion object can be used to infer the mass of the primary object.

The nature of the compact object is based upon its mass. The generally accepted maximum mass of a neutron star is between 3-3.2 solar masses. Therefore, if an object is found with a mass larger than this and it is very compact, it is designated as a black hole.

Last month, Gelino submitted results to the Astrophysical Journal Letters that identify a 4.25-solar mass black hole in the system known as J0422+32, using data from the 3.5-meter telescope at Apache Point Observatory in New Mexico. This system has a separation of just 2.5 solar radii, and is most likely the lowest mass black hole currently known in the Universe.

Black holes are believed to be the end result of a very massive star's life. Their progenitor stars are thought to have masses greater than about 25 times that of the Sun.

Soft X-Ray Transients allow astronomers to measure the properties of black holes and observe the process of the accretion of matter into them. Knowing the masses of these objects will help astronomers better understand how a massive star behaves during its life and its death. Astronomers can also learn more about what is going on in Active Galactic Nuclei (AGNs), and in other galaxies like our own, which are believed to harbor supermassive black holes at their centers.

Only 16 of these Soft X-ray Transient systems have been studied in detail, and none of them have previously been confirmed to have a central black hole with a mass less than five solar masses.

The new results regarding N Vel 93 and J0422+32, done in collaboration with Thomas E. Harrison of New Mexico State University, are available for detailed review at display poster 8.11 in the Southwest Exhibit Hall of the AAS meeting.

Notes

The data on N Vel 93 from the Blanco telescope was obtained with the OSIRIS infrared imager/spectrometer, which is a collaborative project between The Ohio State University and Cerro Tololo Inter-American Observatory (CTIO). Based in La Serena, Chile, CTIO is part of the National Optical Astronomy Observatory (NOAO), which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under a cooperative agreement with the National Science Foundation.

Contacts

Douglas Isbell
Public Information Officer National Optical Astronomy Observatory
Tel: 520/318-8214
Email: disbell@noao.edu

Kim McDonald
Director of Science Communications University of California-San Diego
San Diego
Tel: 858/534-7572
Email: kimmcdonald@ucsd.edu