25-Year Mystery of X-ray Emissions Solved

25 years ago, astronomers discovered diffuse X-ray emissions coming from the plane of the Milky Way, but were puzzled by the source of those emissions. The mystery has now been solved by an international team of astronomers using the Chandra X-ray Observatory. These diffuse emissions do not originate from one single source but from white dwarf stars and stars with active outer gas layers.

Energetic X-ray emissions usually originate from very hot gases in a temperature range between 10 and 100 million degrees Celsius. And this so called “Galactic Ridge X-ray Emission” (GRXE) can also be found in very hot, optically thin plasma.

However, a gas with these thermal properties would immediately dissipate. Cosmic particles colliding with the interstellar medium could also be ruled out as an explanation for the GRXE.

Recently observations from two different satellites, the RXTE and Integral satellites, have shown that the X-ray emissions of the Milky Way exhibit the same distribution pattern as the stars. Since then, it has been assumed that a large portion of the GRXE originates from individual stars. These findings motivated the international team to carry out more precise measurements with the Chandra X-ray telescope.

The test area chosen was a small celestial region near the center of the Milky Way, and was about one and a half time the size of a full moon. Chandra identified 473 point sources of X-rays in a sector of the search field covering only 2.6 arcminutes. In a further step, the group used measurements from the Spitzer space telescope to prove that the results of the sector observed could be applied to the whole galaxy.

Most of the 473 X-ray sources are likely white dwarfs, which accrete matter from their surroundings. The sources could also be stars that have high activity in their outermost gas layer, the corona. White dwarfs are the remnants of extinct, low-mass suns. These cooling dead stars frequently orbit a partner, and in such a binary star system the white dwarf extracts matter from its larger partner until it becomes a Type Ia supernova.

The resolution of the diffuse X-ray emissions in our galaxy into discrete sources has far-reaching consequences for our understanding of a number of astrophysical phenomena. Astronomers can use the GRXE as a calibration for the spatial distribution of star populations within the Milky Way, for example. The results are also relevant for research into other galaxies, to determine if diffuse X-ray radiation from these objects also originates from white dwarfs and active stars.

The work was done by Mikhail Revnivtsev from the Excellence Cluster Universe at the TU Munich and his colleagues at the Max Planck Institute for Astrophysics in Garching, the Space Research Institute in Moscow and the Harvard-Smithsonian Center for Astrophysics in Cambridge, and was published in the April 30, 2009 edition of Nature.

Source: Max Planck Institute

9 Replies to “25-Year Mystery of X-ray Emissions Solved”

  1. Question. I know that white dwarfs radiate alot of heat still, so if they were to float into a nebula on their journeys, they would not accrete(sp?) matter at some additional rate due to its solar winds, correct?

    Followup question is this: say two rather comparable in size galaxies (like the antenna galaxies) collide. There is a chance that some white/brown dwarf would pass through some heavy gas area of the other galaxy. Could it accrete mater enough to light up as another, much larger star? Granted, its life would be shortened, but could it happen?

  2. prajna: The gas in a nebula is extremely diffuse. Even “dense” nebulae are still practically hard vacuums by practical definitions. A dense object such as a white dwarf just wouldn’t accrete enough material to change much from its initial state without passing very close to a regular star and getting “captured” in is gravity to become a binary system, and that is very tough to do.

    Also, in the rare event a white dwarf was to accumulate enough material from its surroundings, it wouldn’t become a star, but a Type 1a Supernova. This link has more information about that type of event.

  3. Dave Finton, prajna was not necessarily speculating about white dwarf stars, but also about the possibility of a brown dwarf, aka a “failed star” due to not having enough mass, gathering more material and becoming a “real star”.
    I haven’t read about such a scenario, I can’t imagine a mechanism which makes it possible … but I’d like to hear educated opinions about it.

    and prajna, a white dwarf, I expect, wont “light up” again. I’d rather expect it to surround itself with a colourful disc or halo of all sorts of debris it finds on its travels.

  4. Freenix: prajna did frame his/her question in the contexts of both white and brown dwarfs. I was just responding to the white dwarf context. Due to my lack of coffee this morning I missed the brown dwarf part of his question.

    If a brown dwarf did somehow manage to accrete enough material to form a star, it would become a red dwarf star. Those stars would be extremely long-lived (they would live up to several trillion years, so I’ve heard). Once again Wikipedia to the rescue on that subject. But I agree with you Freenix, I can’t imagine a scenario where that would be possible except for under highly exceptional circumstances. But hey, I guess anything is possible. =)

  5. Lovely how the ‘simplest’ explanation works without any need for inventing new and exotic mechanisms. Sometimes it’s nice to know that the universe can be ‘dull’.

    (No, I’m not a basher. I lurrrve Dark Matter and Energy.)

  6. There is a chance that some white/brown dwarf would pass through some heavy gas area of the other galaxy. Could it accrete mater enough to light up as another, much larger star?

    Well. I guess that there is just too few material for both kinds of dwarfs to change its view. When astronomers talk about a “dense cloud”, they are talking about probably 1 million particles per ccm. That sounds much, but compared to the 10^23 (there is no name for that number!) particles per ccm in the atmosphere, it is even more than a hard vaccum. So just going through the cloud won’t give them considerably more mass (also take in mind that both objects are of planetary size, “brownies” a little bigger and “whities” a little smaller).

    I think it’s unlikely that a brown dwarf becomes a “normal” star or a white dwarf goes SN Typa 1a in that way.

  7. “but compared to the 10^23 (there is no name for that number!)”

    … I smell a mole.

  8. 10^23 = 100 sextillion (American terminology)
    10^23 = 100 trilliards (European terminology)
    10^23 =~ 100 zettabytes (computer terminology)

    So there are lots of names for 10^23 =)

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