Sky and Telescope - February 2016 - 20

Galactic Center Goings-On

Magnetar

INTO THE HEART This image from the
Chandra space telescope captures X-rays from
the busy galactic center in a view 50 light-years
across. Red, green, and blue indicate low,
medium, and high-energy X-rays, respectively.

STELLAR DISK + MAGNETAR Drawing from near-infrared data gathered by
the Keck telescopes, this frame shows stellar orbits in the galactic center. Stars
farther out tend to orbit clockwise in a disk, while the closer-in cluster contains
stars on more chaotic orbits. The X-ray-emitting magnetar is a projected 0.3
light-year from the center. G2's current orbit follows the red track.

Due to the different nature of their data, the two
teams might be looking at different sides of the same
coin: even if the black hole yanked some gas off G2, there
may yet be a star lurking within the gaseous envelope.
What of Sgr A* itself? Did the black hole notice all
this action in its domain? In the midst of the G2 observing frenzy, Sgr A* offered up some surprises of its own.

Inflows, Outflows, and Flares
X-ray observations confirm that Sgr A* feeds off stellar
winds. But most of that mass never makes it all the way
to the gaping maw. Daniel Wang (University of Massachusetts, Amherst) led a team that studied X-ray emission around Sgr A*. The researchers reported that less
than 1% of the material flowing toward the black hole
successfully makes the journey inside - the other 99%
blows back out into the surrounding environment.
In the process, our delicately snacking black hole
belches X-ray flares: mild bursts occur roughly once a
day, and bright spikes appear every 10 days or so. Thanks
to G2 monitoring in 2013 and 2014, one of us (Haggard)
led a team that discovered two of the most impressive
X-ray flares ever seen. Over the course of a couple of
hours, each released several hundred times the X-rays
normally seen from this region.
Simultaneous multiwavelength observations (when
they exist) generally show infrared and radio-wave spikes
accompanying X-ray flares. But the inverse isn't true - a
typical day sees four times as many infrared flares as
X-ray flares. So it's not clear how emissions at different
wavelengths relate to each other.
Because of these uncertainties, we still don't understand the flares' origin. A reordering of magnetic fields
20

Februar y 2016 sky & telescope

could create flares in a way similar to those seen from
our Sun. Or the black hole's extreme gravity might shred
the occasional asteroid that comes in a little too close.
The X-rays brighten and fade within a couple of
hours. If they arise in the accretion disk, then their
source must lie just outside the black hole's event horizon,
the point of no return for material and light entering the
black hole. Since that puts the escaping X-rays within an
a.u. from the black hole, in the thick of whatever gaseous
flow feeds Sgr A*, the flares are unlikely to be connected to
G2's closest approach at roughly 150 a.u.
Nevertheless, a recent study of 150 Chandra and
XMM-Newton observations spanning 15 years, led by
Gabriele Ponti (Max Planck Institute for Extraterrestrial
Physics, Germany), shows that bright flares became
more frequent about six months after G2 made its closest approach to the black hole. Coincidence? Maybe -
other black holes show similar flare clustering.
Continued multiwavelength monitoring of Sgr A* is
our best hope of pinpointing the source of the flares.

Discovery of an Exotic Pulsar
The same monitoring that followed G2's approach and
captured two brilliant flares led to another surprise on
April 24, 2013. As G2 approached Sgr A*, daily monitoring of the black hole with the Swift satellite jolted scientists with the announcement of a bright X-ray outburst.
Observers around the world rushed to their telescopes
to capture a more complete picture of the anticipated
disruption of the G2 cloud.
But the picture quickly became much more complex.
The X-ray outburst lasted for hours, then days - much
longer than the typical 1- or 2-hour duration of an X-ray



Sky and Telescope - February 2016

Table of Contents for the Digital Edition of Sky and Telescope - February 2016

Contents
Sky and Telescope - February 2016 - Cover1
Sky and Telescope - February 2016 - Cover2
Sky and Telescope - February 2016 - 1
Sky and Telescope - February 2016 - Contents
Sky and Telescope - February 2016 - 3
Sky and Telescope - February 2016 - A
Sky and Telescope - February 2016 - B
Sky and Telescope - February 2016 - 4
Sky and Telescope - February 2016 - 5
Sky and Telescope - February 2016 - 6
Sky and Telescope - February 2016 - 7
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