What Are the Faint X-ray Transients Near the

Galactic Center?

Michael Muno (UCLA/Hubble Fellow)Fred Baganoff (MIT), Eric Pfahl (UVa),

Niel Brandt, Gordon Garmire (Penn State)Mark Morris, Andrea Ghez,

Jessica Lu, Seth Hornstein (UCLA)

Five Years of Chandra Observations of the Galactic

Center

Sgr A*

5 pcGalactic Plane

0.1% of the Galactic stellar mass!

X-ray Sources within 25 pc of Sgr A*

• Previous wide-field surveys were only sensitive to LX>1036 erg s-1.

• Most of the Chandra X-ray sources have LX=1031 to 1033 erg s-1. These are likely to be CVs.

• Seven transient X-ray sources have LX>1034 to 1036 erg s-1. These are a bit unexpected. . .

Sgr A*

5 pc

X-ray Sources within 25 pc of Sgr A*

• Previous wide-field surveys were only sensitive to LX>1036 erg s-1.

• Most of the Chandra X-ray sources have LX=1031 to 1033 erg s-1. These are likely to be CVs.

• Seven transient X-ray sources have LX>1034 to 1036 erg s-1. These are a bit unexpected. . .

Sgr A*

New Transient

5 pc

Your Standard Transient X-ray Source

• Most bright, transient X-ray sources are accreting black holes and neutron stars.• When accretion occurs at low rates, the disk tends to be unstable, producing outbursts with LX>1037 erg s-1.

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Location ofCompact Object

Thermonuclear Bursts from a Neutron Star Low-Mass X-ray

Binary

Sgr A*

New Transient

5 pc An X-ray burst lasting 100 swas produced by unstable Heburning on the neutron star.

Another LMXB (0.1 pc from Sgr A*) with Periodic Eclipses

The X-ray light curve displays partial eclipses at the 8 hour orbital period.

Infrared images reveal no infrared companion with K<15, ruling out a high-mass star.

Muno et al. (2005b)

To Sgr A*(0.1 pc)

Other Transients also have Unusual Properties

• Transient outbursts last anywhere from less than a month, to the full five years.

• The long, slow transient to the left has a soft spectrum reminiscent of transient magnetars.

Making Progress on Faint X-ray Transients

• We need better X-ray coverage, to identify new examples and to measure outburst time scales and spectral evolution.

• Radio observations can reveal jets, which are common from X-ray binaries.– Likewise, we need to determine whether the radio

transients seen near the Galactic center have X-ray counterparts.

• Infrared observations can detect accreting companions.

• TeV instruments will soon have the baseline to identify transients.