X-ray and Observational Astronomy

News May 2004: Discovery of a dipping probable black hole binary in NGC 55

The irregular galaxy NGC 55 is shown in this red-band optical image. The positions of bright X-ray sources are overlaid. The black hole candidate is the brightest X-ray source in the galaxy, and is highlighted by the arrow.	The soft, medium and hard X-ray lightcurve of the black hole candidate in NGC 55 over 16 hours, as seen by XMM-Newton. Prominent dips lasting a few minutes up to an hour can clearly be seen.

Leicester PhD student Ann-Marie Stobbart, together with Dr Tim Roberts and Prof Bob Warwick, recently published a paper announcing the discovery of dips in the X-ray light curve of the brightest X-ray source in the galaxy NCG 55. This X-ray source appears to be a black hole accreting gas from a close binary companion star.

At a distance of almost 6 million light-years (1.78 Mpc), this galaxy is relatively nearby. Even so, the X-ray source is extremely bright. It is almost a million times brighter in X-rays that our Sun is in visible light. The high X-ray luminosity of 1.6x10^39 erg/s itself is strongly suggestive of the presence of a black hole, as this is around 10 times the maximum brightness that a neutron star can sustain. The dips seen in the X-ray intensity (top right) are characteristic of a close binary system in which gas pulled off a normal-type star forms a disk around the other massive collapsed object, the dips are then due to the raised edge of this 'accretion disk' occasionally obstructing the view to the X-ray emission site around the accreting collapsed star.

A massive collapsed star, such as a black hole, neutron star or white dwarf, when close to a normal star has such a strong gravitational pull that gas from the companion star can fall off it onto the collapsed star. In this type of binary star the whole star does not fall onto the collapsed star because the stars are in orbit around each other, this rotation of the system holds the normal star up, away from the collapsed star, like water held in a bucket swung rapidly over your head. Such binary systems are typically very small (not much larger than the Sun) and orbit around each other very rapidly (about a day or less).

Gas falling onto a collapsed star orbits more rapidly as it falls inwards. The very high speed of the gas close to the collapsed star means that large amounts of energy are available in a small volume, and it is this which gives rise to the X-ray emission. Actually, the detailed means by which the accretion energy is transformed to X-rays is not well understood, the hot accretion disk likely emits X-rays from gas increasingly hot towards the centre, and this is likely to be overlaid by much hotter lower density gas emitting both higher and lower energy X-rays.

The brightness of the X-ray source in NGC 55 implies a mass 11 times that of the Sun, well above the maximum possible neutron star mass of 2-3 solar masses but within the range of other measured stellar black hole masses. A black hole can emit radiation (outside of its event horizon) at a greater rate than a neutron star because each accreting atom can fall further and be accelerated to higher speeds when falling onto a black hole.

There is a second line of evidence which points to a black hole in this system. The temperature and brightness of the inner accretion disk region depends on its distance from the accreting star, these are both measurable from the X-ray observations. This inner disk radius is in turn dependent on the mass of the accreting star. A stellar mass of ~20 times that of the Sun, or even greater, is implied following this line of reasoning. This mass again implies the presence of a black hole.

The X-ray source in NGC 55 is the first dipping black hole binary system found outside our own Galaxy. The dips are themselves rather surprising, normally lower energy X-rays are more absorbed than higher energy X-rays. So we would expect dips due to absorption in the accretion disk rim to be more pronounced in the soft X-ray light curve in the top panel, whereas we see that the dips are deepest in the hard (higher energy) X-rays shown in the bottom panel. A probable explanation is that the absorption is due to large clouds causing either total absorption or none at all, with the harder X-rays coming from the small inner disk region and the softer X-rays coming from a more extended region, it is easy to see that the soft X-rays may not be so substantially eclipsed.

It has to be noted that in this observation of this object the identification of the soft X-rays with the extended gas over the disk and the harder X-rays with the hot disk itself is the opposite to what is normally seen. This may be due in part to the relatively small amount of interstellar gas between us and this object, unusually allowing the softest X-rays to reach us. Although accreting black holes have been studied for over 30 years much remains to be understood.

The paper "A dipping black hole X-ray binary candidate in NGC 55" by Stobbart, Roberts and Warwick has been accepted for publication in Monthly Notices of the Royal Astronomical Society. It is available free from the arXiv service.

Other news items are available here.

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Last updated: 2004 May 11 by Julian Osborne