Saturday, January 15, 2011

GRS 1915+105: Taking the Pulse of a Black Hole System

GRS 1915+105
Credit X-ray (NASA/CXC/Harvard/J.Neilsen et al); Optical (Palomar DSS2)

This optical and infrared image from the Digitized Sky Survey shows the crowded field around the binary system GRS 1915+105 (GRS 1915 for short) located near the plane of our Galaxy. The top-left inset shows a close-up of the Chandra image of GRS 1915 , and the bottom-right inset shows the remarkable "heartbeats" seen in the X-ray light from this system. Using Chandra and the Rossi X-ray Timing Explorer (RXTE), astronomers have discovered what drives these heartbeats and given new insight into the ways that black holes can regulate their intake and severely curtail their growth.

GRS 1915 contains a black hole about 14 times the mass of the Sun that is feeding off material from a nearby companion star. As the material swirls toward the black hole, a disk forms. The black hole in GRS 1915 has been estimated to rotate at the maximum possible rate, allowing material in the inner disk to orbit very close to the black hole -- at a radius only 20% larger than the event horizon -- where the material travels at 50% the speed of light.

Researchers monitored this black hole system with Chandra and RXTE over a period of eight hours. As they watched, GRS 1915 gave off a short, bright pulse of X-ray light approximately every 50 seconds. This type of rhythmic cycle closely resembles an electrocardiogram of a human heart -- though at a slower pace. It was previously known that GRS 1915 can develop such heartbeats, but researchers gained new understanding into what drives the beats, and used the pulses to figure out what controls how much material the black hole consumes from the RXTE data.

The astronomers also used Chandra's high-resolution spectrograph to study the effects of this heartbeat variation on regions of the disk very far from the black hole, at distances of about 100,000 to a million times the radius of the event horizon. By analyzing the Chandra spectrum, they found a very strong wind being driven away from the outer parts of the disk. The rate of mass expelled in this wind is remarkably high, as much as 25 times the maximum rate at which matter falls onto the black hole. This massive wind drains material from the outer disk and eventually causes the heartbeat variation to shut down.

Fast Facts for GRS 1915+105:

Scale: 5 degrees across (58 light years)
Category: Black Holes, Neutron Stars/X-ray Binaries
Coordinates: (J2000) RA 19h 15m 11.60s | Dec +10° 56' 44.00''
Constellation: Aquila
Observation Date: May 23rd, 2001
Observation Time: 8 hours 20 min
Obs. ID: 1945
Color Code: X-ray (Violet); Optical (Red, Green, Blue)
Instrument: ACIS/HETG
Distance Estimate: 40,000 light years