Summer 2001 -- Vol. 18, No. 2

XMM-Newton Satellite Captures X-rays from Most-Distant Quasar

The XMM-Newton space observatory has set a new record for the most distant X-rays ever found by detecting X-rays from the farthest known quasar. With a redshift of 5.8, the quasar provides a glimpse of the universe when it was less than one billion years old, or about 6 percent of its current age. The most distant object previously detected with X-rays had a redshift of 4.7.

The feat was achieved by a team of U.S. and European astronomers, led by Niel Brandt of Penn State, as part of the group’s project to determine the properties of the most ancient of the most luminous known objects in the universe, using XMM-Newton and other new-generation X-ray observatories.

A scientific paper reporting these results, titled “An XMM-Newton Detection of the z=5.80 X-ray Weak Quasar SDSSp J1044-0125,” was published in the February 2001 issue of The Astronomical Journal.

“We are pushing our X-ray observations into the very early epoch shortly after the Big Bang to understand whether quasars back then were different from those in our local universe,” says Brandt. “We also are trying to answer other fundamental questions about how the first quasars and the first generation of galaxies were formed.”

By looking so far away with X-ray observations, the astronomers are able to look back in time at some of the hottest objects in the early universe.

There is evidence that quasars—short for “quasi stellar” objects because of their star-like appearance—are fuelled by supermassive black holes feeding on their host galaxies. Quasars can emit 1,000 times the energy of our entire galaxy even though they only occupy a volume similar to that of our solar system.

The team mobilized the great X-ray-collecting power of the European Space Agency’s XMM-Newton observatory to target the most remote of all known quasars.

Known as SDSS 1044-0125, the object was discovered in April 2000 by a Sloan Digital Sky Survey (SDSS) team lead by Xiaohui Fan, who was a graduate student at Princeton University at the time. “Although this quasar is at an enormous distance, it is very luminous, so there was a good chance that its X-ray emission could be detected by XMM-Newton,” Fan says.

Astronomers gauge an object’s distance from Earth by measuring its redshift—the amount by which the wavelength of its light is lengthened, or moved toward the red part of the spectrum, as the object moves away from Earth in the expanding universe. The higher the redshift, the more distant and younger the object.

More than ten thousand quasars have been found since the first quasar redshift was measured by Maarten Schmidt in 1963.

The first quasar redshift of 0.16, measured for a highly luminous object known as 3C 273, is considered relatively nearby by today’s standards.

“We are seeing this redshift 5.80 quasar as it was only a billion years after the Big Bang, which is quite early in the history of the universe, since current estimates put the age of the universe at between 12 and 15 billion years,” Fan comments.

Brandt’s team includes postdoctoral fellow Shai Kaspi and professor Donald Schneider of Penn State; Fan, a member of the Institute for Advanced Study; and associate professor Michael Strauss and professor James Gunn of Princeton University. The team worked closely with XMM-Newton scientists Matteo Guainazzi and Jean Clavel, of the European Space Agency’s XMM-Newton Science Operations Center in Madrid, Spain, to interpret the X-ray data.

During an 8-hour-long observation at the end of May 2000, the team used the XMM-Newton observatory’s EPIC-pn camera to register this quasar’s X-ray emission.

“Surprisingly, only about 30 X-ray photons were collected when we expected about ten times as much radiation,” says Guainazzi. “Such a low number of photons does not allow for detailed spectral analysis, but we were able to calculate the X-ray flux, and hence the high-energy luminosity, of this quasar.”

The astronomers suggest two possible explanations for the quasar’s surprisingly low X-ray emission, explaining that the intergalactic medium, even out to a redshift of 5.80, is insufficiently dense to block the “missing” X-rays from reaching Earth.

“On the one hand, it could be absorbed by gas in the galaxy surrounding the quasar,” Brandt explains.

Such absorption is well known in objects such as “broad-absorption-line” quasars.

The object might, on the other hand, be a “precursor quasar.” “Its enormous black hole—about 3 billion times the mass of our Sun—would be undergoing an exceptionally strong accretion process,” explains Brandt.

“The accretion would be so rapid that even the X-rays being generated by the immense temperatures are dragged back into the hole.

This so-called ‘Super-Eddington’ accretion might explain the formation of such a massive black hole less than a billion years after the Big Bang.”

For the moment, the astronomers say, other extremely-high-redshift quasars need to be observed to draw general conclusions because this X-ray study concerns only one object.

“If we find objects with stronger X-ray emissions, we would expect to gain more insight into the hot early universe,” Kaspi says. “But if flux levels remain low, real answers may have to wait until the arrival of even more powerful X-ray observatories such as the XEUS space-based observatory being planned by the European Space Agency.”

This research received financial support from the U.S. National Science Foundation, the Alfred P. Sloan Foundation, NASA, Research Corporation, and a Porter O. Jacobus Fellowship.

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