Three orbiting X-ray space telescopes have detected an
increased rate of X-ray flares from the usually quiet giant black hole
at the center of our Milky Way galaxy after new long-term monitoring.
Scientists are trying to learn whether this is normal behavior that was
unnoticed due to limited monitoring, or these flares are triggered by
the recent close passage of a mysterious, dusty object.
By combining information from long monitoring campaigns by NASA’s
Chandra X-ray Observatory and ESA’s XMM-Newton, with observations by the
Swift satellite, astronomers were able to carefully trace the activity
of the Milky Way’s supermassive black hole over the last 15 years. The
supermassive black hole, a.k.a. Sagittarius A*, weighs in at slightly
more than 4 million times the mass of the Sun. X-rays are produced by
hot gas flowing toward the black hole.
The new study reveals that Sagittarius A* (Sgr A* for short) has been
producing one bright X-ray flare about every ten days. However, within
the past year, there has been a ten-fold increase in the rate of bright
flares from Sgr A*, at about one every day. This increase happened soon
after the close approach to Sgr A* by a mysterious object called G2.
“For several years, we’ve been tracking the X-ray emission from Sgr
A*. This includes also the close passage of this dusty object” said
Gabriele Ponti of the Max Planck Institute for Extraterrestrial Physics
in Germany. “A year or so ago, we thought it had absolutely no effect on
Sgr A*, but our new data raise the possibility that that might not be
the case."
Originally, astronomers thought G2 was an extended cloud of gas and
dust. However, after passing close to Sgr A* in late 2013, its
appearance did not change much, apart from being slightly stretched by
the gravity of the black hole. This led to new theories that G2 was
not simply a gas cloud, but instead a star swathed in an extended dusty
cocoon.
“There isn’t universal agreement on what G2 is,” said Mark Morris of
the University of California at Los Angeles. “However, the fact that Sgr
A* became more active not long after G2 passed by suggests that the
matter coming off of G2 might have caused an increase in the black
hole’s feeding rate.”
While the timing of G2’s passage with the surge in X-rays from Sgr A*
is intriguing astronomers see other black holes that seem to behave
like Sgr A*. Therefore, it’s possible this increased chatter from Sgr A*
may be a common trait among black holes and unrelated to G2. For
example, the increased X-ray activity could be due to a change in the
strength of winds from nearby massive stars that are feeding material to
the black hole.
“It’s too soon to say for sure, but we will be keeping X-ray eyes on
Sgr A* in the coming months,” said co-author Barbara De Marco, also of
Max Planck. “Hopefully, new observations will tell us whether G2 is
responsible for the changed behavior or if the new flaring is just part
of how the black hole behaves.”
The analysis included 150 Chandra and XMM-Newton observations pointed
at the center of the Milky Way over the last 15 years, extending from
September 1999 to November 2014. An increase in the rate and brightness
of bright flares from Sgr A* occurred after mid-2014, several months
after the closest approach of G2 to the huge black hole.
If the G2 explanation is correct, the spike in bright X-ray flares
would be the first sign of excess material falling onto the black hole
because of the cloud’s close passage. Some gas would likely have been
stripped off the cloud, and captured by the gravity of Sgr A*. It then
could have started interacting with hot material flowing towards the
black hole, funneling more gas toward the black hole that could later be
consumed by Sgr A*.
A paper on these findings has been accepted by the Monthly Notices of the Royal Astronomical Society. A
preprint
is available online. NASA's Marshall Space Flight Center in Huntsville,
Alabama, manages the Chandra program for NASA's Science Mission
Directorate in Washington. The Smithsonian Astrophysical Observatory in
Cambridge, Massachusetts, controls Chandra's science and flight
operations.
Image credit: NASA/CXC/MPE/G. Ponti et al.; Illustration: NASA/CXC/M. Weiss
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Last Updated: Sep. 23, 2015
Editor: Lee Mohon
KARACHI:
Rangers personnel conducted a raid in Karachi Surjani town area late
Friday night and arrested an alleged target-killer accused of being
involved in the murders of policemen and several political activists in
the city.
