Astrophysicists find evidence of black holes' destruction of stars

Oct 11, 2011
This artist's concept shows a galaxy with a supermassive black hole at its core. The black hole is shooting out jets of radio waves. Image credit: NASA/JPL-Caltech

Astrophysicists have found evidence of black holes destroying stars, a long-sought phenomenon that provides a new window into general relativity. The research, reported in the latest issue of the Astrophysical Journal, also opens up a method to search for the possible existence of a large population of presently undetectable "intermediate mass" black holes which are hypothesized to be precursors to the super-massive black holes at the centers of most large galaxies.

The study was carried out primarily by Glennys Farrar and Sjoert van Velzen at New York University's Center for and , and also included the following researchers: Suvi Gezari of Johns Hopkins University's Department of Physics and ; Linda Ostman of Spain's Universitat Autònoma de Barcelona; Nidia Morrell of the Las Campanas Observatory in Chile; Dennis Zaritsky of the University of Arizona; Matthew Smith of South Africa's University of Cape Town; Joseph Gelfand of NYU-Abu Dhabi; and Andrew Drake of Caltech. Van Velzen is currently a doctoral candidate at Radboud University in the Netherlands.

Cosmologists have calculated that, on occasion, a star's orbit will be disturbed in such a way that it passes very near the super-massive black hole at the center of its galaxy—but not so close that it is captured whole. Such a star will be torn apart by the extreme tidal forces it experiences: the force of gravity on the near side of the star is so much stronger than that on the far side that the gravitational force holding the star together is overwhelmed, causing the star to simply come apart. While some of the star's matter falls into the black hole, much of it continues in chaotic orbits, crashing into itself and producing intense radiation lasting days to months. These phenomena are called stellar tidal disruption flares, or TDFs.

Although discovering evidence of TDFs has been a high priority of astrophysicists for many years, and several possible examples have been found using X-ray and UV satellites, discovering TDFs in a large-scale, systematic survey using ground-based optical telescopes as has now been achieved, is critical to controlling bias and avoiding misidentifications.

The difficulty in detecting TDFs is largely due to the challenge of distinguishing them from more common types of flares such as supernovae. (For every TDF there are about 1000 supernovae.) In addition, some super-massive have an "accretion disk" surrounding them—gas and dust, often left from an earlier merger with another galaxy—which is continuously feeding the hole. Such accreting black holes are usually evident from the bright emission they produce and are known as quasars or Active Galactic Nuclei (AGN). However, a hiccup in the accretion of an undetected active black hole could produce a flare that might be mistakenly identified as a TDF.

The researchers on the study uncovered sound evidence for the presence of two TDFs through a rigorous analysis of archival data from the Sloan Digital Sky Survey (SDSS).

To do so, they sifted through voluminous SDSS data, in which more than 2 million were repeatedly observed over 10 years. By very carefully registering the images and looking at differences between consecutive images, they obtained a sample of 342 intense and well-measured flares.

Of these, almost all could be classified into supernovae and AGN flares. However, two cases were left that did not fit either classification. By relying on multi-year observations, the researchers could see that the two flares' host galaxies showed no other flaring activity, as would be the case if the flares came from a hidden variable AGN. This means the possibility these two flares were produced by undetected AGNs is extremely small.

In addition, the researchers located these flares at the nucleus of their galaxy with high precision, which reduces the likelihood that they are supernovae to less than 1 percent since supernovae are randomly distributed through galaxies.

Finally, the properties of these flares are very different from flares of AGNs and supernovae—and their spectra are unlike any supernovae observed to date. Supernovae flares are characteristically very blue at first but become red as they cool and rapidly decay, whereas the TDF flares are very blue throughout—slowly decaying without changing color. This behavior is consistent with expectations for a TDF—the debris from the star should rapidly form an accretion disk and look like a short-lived AGN.

Sjoert van Velzen, the study's lead author, was a Dutch first-year graduate student who came to NYU to work under the direction of Glennys Farrar, a Professor of Physics at NYU and senior scientist of the project. Van Velzen is now completing his Ph. D. in Holland.

About his first encounter with real scientific work, van Velzen says, "Searching through 2.6 million galaxies was actually a lot of fun—there is so much to discover! Based on our search criteria and observing two TDFs that met those criteria, the rate of TDFs is about once per 100,000 years, per galaxy. It's quite thrilling to have been able to make such a measurement."

"The next step is to develop models to explain in detail the flares' properties and duration, and address the question of whether TDFs could be responsible for producing Ultrahigh Energy Cosmic Rays, whose sources have been elusive up to now," says Farrar. "It is very exciting that we are on the verge of obtaining a large and better-observed sample of TDFs to study—though a more sensitive search of SDSS archival data and the new generation of transient surveys which will observe more flares in real-time and with multi-wavelength follow-up. A large sample will be invaluable to understanding many outstanding questions in astrophysics."

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User comments : 14

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Robert_Wells
2.3 / 5 (6) Oct 11, 2011
wait for it....

neutron stars disguised as black holes to confuse humans are tearing up the other stars because they're repulsed.
CHollman82
1.9 / 5 (12) Oct 11, 2011
Okay so now we have micro black holes, normal black holes, intermediate black holes, and super massive black holes...

They just keep adding more missing links!

There used to only be normal black holes and super massive black holes and there was only one missing link, then they added micro black holes and then there were two missing links, now with this new "intermediate" black hole we have a third missing link!

When will scientists realize that the more they discover the more missing links they add?
Isaacsname
not rated yet Oct 11, 2011
If a star passes by a black hole and loses a little bit of it's mass, does the black hole shrink after the star is beyond reach ?
antialias_physorg
4 / 5 (4) Oct 11, 2011
does the black hole shrink after the star is beyond reach ?

Why should it? The black hole will increase in size due to the captured mass. At least the event horizon will. The definition of the 'size' of the black hole entity itself is a bit iffy.

Once the star leaves the vicinity and doesn't shed any more mass then only gravitational interactions continue to take place.
TopherTO
5 / 5 (5) Oct 11, 2011
While they had their moments, I think the mock neutron repulsion comments are getting quite old.
omatranter
3.4 / 5 (5) Oct 11, 2011
Flares... Flares, I have never stopped wearing flares, people give me the strangest looks, I put that down to my Fashion Hipness of never letting go of anything including, Flares.

What has this to do with this article, as always nothing.

Just kidding,
Flares and Repellent Neutronium Rock On!

We seek it here, we seek it there,
Those Repulsive Neutrons we seek everywhere.
Is it in heaven?Is it in hell?
That demmed, elusive Manuel.
dtyarbrough
1 / 5 (7) Oct 11, 2011
Astrophysicists find evidence of black holes where their brains should be.
CHollman82
1.4 / 5 (5) Oct 11, 2011
Oh come on the neutron repulsion joke got a higher rating than my missing link joke?
A2G
1 / 5 (7) Oct 11, 2011
What is really sad is that there is more truth in the spam ad for T-shirts and other crap than in the article.

Just show me a star disappear into a black hole or behind one.
Isaacsname
2 / 5 (1) Oct 11, 2011
does the black hole shrink after the star is beyond reach ?

Why should it? The black hole will increase in size due to the captured mass. At least the event horizon will. The definition of the 'size' of the black hole entity itself is a bit iffy.

Once the star leaves the vicinity and doesn't shed any more mass then only gravitational interactions continue to take place.

Interesting, maybe somebody should tell Penrose

http://en.wikiped..._process
jsdarkdestruction
not rated yet Oct 12, 2011
does the black hole shrink after the star is beyond reach ?

Why should it? The black hole will increase in size due to the captured mass. At least the event horizon will. The definition of the 'size' of the black hole entity itself is a bit iffy.

Once the star leaves the vicinity and doesn't shed any more mass then only gravitational interactions continue to take place.

well, technically if it was not absorbing any other matter and hawking radiation is real then i guess you could say its very slowly shrinking.
Pirouette
3 / 5 (2) Oct 12, 2011
Okay so now we have micro black holes, normal black holes, intermediate black holes, and super massive black holes...

From what I gather, and I could be wrong, the article is telling us that an intermediate sized black hole has the potential of eventually growing into a super massive black hole due to pulling in matter/energy into itself from passing stars and other matter/energy. And that the matter/energy need not necessarily be touching the event horizon before the nearest side of the matter/energy from the star, for example, starts to pull apart from the main body and gets sucked into the black hole. Essentially, that should mean that the black hole does grow in size as long as it continues to feed and that there is a magnetic/gravitational force field that is just beyond its event horizon, where the force field is not strong enough to draw in the whole star, but just a piece of it.

Pirouette
1 / 5 (1) Oct 12, 2011
I have felt for a long time that the largest black holes have a way to reach out further in order to acquire ever more matter/energy apart from what falls into it inadvertently from passing comets, stars and the usual detritus. I felt that there had to be something further and beyond the event horizon to draw in bigger game, so to speak, otherwise the black hole's mass could not eventually grow larger into a super massive hole. Certainly, a black hole's mass only equals the matter/energy that it swallows. But I think that it would need to have a way to reach out beyond its own gravitational well. Otherwise, the black hole's condition would be fairly static and never change very much almost throughout eternity. It would be tantamount to a beggar on a street corner with a cup in his hand, and an occasional coin dropped into his cup.
antialias_physorg
5 / 5 (3) Oct 12, 2011
well, technically if it was not absorbing any other matter and hawking radiation is real then i guess you could say its very slowly shrinking.

Hawking radiation only makes a black hole shrink once that radiation is of higher temperature than the comsic microwave background. Currently that is FAR from the case (i.e. in order for a black hole to shrink the universe must first cool down/expand a LOT more than it has. We're talking many orders of magnitude longer than the current age of the universe)

Interesting, maybe somebody should tell Penrose

Penrose processes are only relevant in the ergosphere (which is not much larger than the event horizon and identical to the event horizon for non-rotating black holes).

Our hypothetical 'escapee' star would not have escaped (intact) if it had gotten close to that.