Astro News
- Recent Updates of Astro News
- Active Mercury(07/09)
- Hubble Status Report: Directly Observes a Planet Orbiting Another Star(01/09)
- A Non-trivial Answer to a Trivial Astronomical Question-The Origin Of Absolute Magnitude(07/08)
- Assault by a Black Hole(04/08)
- New Lakes Discovered on Titan(01/08)
- ˇ§Deviant Behaviourˇ¨ in the Solar System(10/07)
- Cosmic Ripples - Cosmic Microwave Background - CMB(07/07)
- Interplanetary Superhighway(04/07)
- Is Pluto a Planet?(01/07)
- Breathing Moonrocks(10/06)
- My Thoughts on the Theory of Relativity, Quantum Mechanics, Superstring Theory and Dark Matter(07/06)
- Space-time Vortex(04/06)
- Radio Astronomy(01/06)
- Neutrino Astronomy(10/05)
- The Active Earth(07/05)
- What is Dark Energy?(04/05)
- The Mysterious Black Holes(01/05)
- Intermediate-Mass Black Holes And Quasisoft X-Ray Sources(10/04)
- Time Travel: From a Scientific Approach(07/04)
- What is Astrobiology?(04/04)
- Black Hole: From Fantasy To Reality (II)(01/04)
- Black Hole: From Fantasy To Reality (I)(10/03)
- From The Oldest Light In The Universe To The Fate Of The Universe(7/03)
- The Cosmic HERO(4/03)
- Quaoar - the Tenth Member of the Solar System?(1/03)
- The First Chinese Telescope in Space(10/02)
- Diamonds and Other Stardust(7/02)
- Supermassive Black Hole in Andromeda Galaxy(4/02)
- Detection of Solar Neutrinos(1/02)
- Simultaneous Multiple Wavwlength Observation(10/01)
- Celestial Distance(7/01)
- Solar-Terrestrial Relations(7/00)
- Fundamental Particles in Astronomy(4/00)
- The Solar Maximum in 2000(1/00)
- Hubble Constant(10/99)
- New Findings on Cosmology(7/99)
- Strange Stars(4/99)
- How Strong Stellar Magnetic Field Can Be?(1/99)

Important notices

Black hole is a strong gravitational pull object in which even light cannot escape from it. Black holes are invisible (by definition), yet the gas and dust falling into a black hole are heated to high temperatures and glow furiously. If there is a companion star orbiting around a black hole, astronomers can investigate the central black hole by examining the accreting matters from the nearby companion star. Most astronomers are convinced of the existence of the so-called stellar black holes, which collapse out of single star and have masses up to 10 times that of the sun. A famous example of this kind of black holes is Cygnus X-1. Likewise, supermassive black holes, which can have the mass of billions of stars and sit in the centre of galaxies, are also believed to exist. For example, the centre of our Milky Way contains of a 3 million solar mass black hole, known to astronomers as Sgr A*.

Recently, astronomers suspect that a class of black holes, namely intermediate-mass black hole, should exist for which the mass of the black hole is between hundreds and tens of thousands of suns. The existence of this kind of objects would defy conventional black hole formation theory. However, there is no solid evidence that intermediate-mass black holes exist because of the lack of observations and there are still a lot of discussions about it. Scientists believe that many nearby galaxies harbour intermediate-mass black holes as very luminous X-ray sources, also known as ultraluminous X-ray sources. Unlike supermassive black hole in the centre of a galaxy, these ultraluminous X-ray sources usually locate at the galactic disks. Although many ultraluminous X-ray sources in nearby galaxies were discovered by the Einstein X-ray Observatory and ROSAT, the poor sensitivity and spatial resolution provide little information about their nature. In particular, due to source confusion, it is very difficult for astronomers to search for signatures at other wavelengths in order to study the physical nature of ultraluminous X-ray sources.

With the advent of the NASA's Chandra X-ray Observatory and the European Space Agency's XMM-Newton satellite, the search of intermediate-mass black holes in nearby galaxies becomes possible and promising. The sub-arcsecond spatial resolution of Chandra, and XMM-Newton's unrivaled sensitivity open a new area for the study of nearby galaxies; many X-ray point sources are resolved without confusion and astronomers can study their X-ray properties such as timing behaviors and spectra in details. There are more than 100 ultraluminous X-ray sources in the local universe and their X-ray properties provide important clues to their physical nature. More recently, Dr. Rosanne Di Stefano (also from the Harvard-Smithsonian Centre for Astrophysics) and I used the Chandra X-ray Observatory to detect objects dubbed "quasisoft" X-ray sources in nearby galaxies because of their low temperatures, which are at most four million degrees Celsius. That is well below the temperatures expected for "hard" X-ray sources such as neutron stars or stellar mass black holes - which run between 10 million and 100 million degrees Celsius - and still hotter than "soft" X-ray sources like white dwarfs. But despite their low temperatures, these quasisoft objects emitted X-ray signals that are more powerful than expected from simple neutron stars or stellar black holes. Instead, the natural model for quasisoft X-ray sources is the one involving an intermediate-mass black hole. Another intriguing feature of quasisoft X-ray sources is that they locate at different type of galaxies like spirals and ellipticals. Quasisoft X-ray sources are also found in the halos of galaxies and even in globular clusters. This discovery suggests the existence of a new class of X-ray sources. It appears that quasisoft X-ray sources are quite common in the local universe, and if we can establish a strong link between quasisoft X-ray sources and intermediate-mass black holes, searching quasisoft X-ray sources will become a novel approach to hunt for intermediate-mass black holes.

Artists' illustrations of the Chandra X-Ray Observatory ( Credit : CXC / NGST )


Artists' illustrations of the XMM-Newton Satellite ( Credit : ESA )

Nevertheless, there is no direct evidence for intermediate-mass black holes and it will be very difficult to establish them in the same way that we have for supermassive black holes, at the centre of galaxies, or stellar black holes. The temperature range of quasisoft X-ray sources falls just right for an accreting black hole that is more massive than a stellar-mass black hole. However, we should be cautioned that in principle, the low temperatures of many quasisoft X-ray sources can be explained by stellar-mass black holes, neutron stars, and even white dwarfs under very specific physical conditions. To rule out other models, we will need more X-ray observations as well as observations at other wavelengths. Therefore, the accurate positions provided by Chandra will be crucial for further studies. The discovery of quasisoft X-ray sources, along with other ultraluminous X-ray sources, should give astronomers valuable clues to the processes responsible to intermediate-mass black holes and their formation.

Chandra observations of the spiral galaxy M83 and other nearby galaxies have revealed a possible new class of X-ray sources. These mysterious X-ray sources, marked with green diamonds in the image, are called "quasisoft" sources because they have a temperature in the range of one to four million degrees Celsius.
(Credit : NASA / CXC / SAO / R.DiStefano et al. )