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Astronomy Colloquium: "Stars Wake up the Giants: Flares and Transient Jets from Quiescent Galactic Centers"

Event Type
Department of Astronomy
Astronomy 134
Dec 3, 2013   4:00 - 5:00 pm  
Dimitrios Giannios, Assistant Professor, Purdue University
Originating Calendar
Astronomy Colloquium Speaker Calendar

The majority of the supemassive black holes in galactic centers,

including that in our own Galaxy, are dormant. Our knowledge of the

properties of quiescent accretion and the environments of quiescent

galactic centers is very limited. In this talk, I will discuss 2 examples

in which stars can be used as unique probes of quiescent galactic centers.

The tidal disruption of a star by a supermassive black hole provides

us with a rare glimpse of these otherwise dormant beasts.

It has long been predicted that the disruption will be accompanied

by a thermal `flare', powered by the accretion of bound stellar debris.

Recently, we explored the observational consequences if a fraction of

the accretion power is channeled into an ultra-relativistic outflow.

The high-energy transient Sw 1644+57 provides strong support to the

presence of powerful relativistic jets during tidal

disruption events. I will discuss the rich behavior of Sw 1644+57 in

the radio and X-rays, focusing on the information we gain

on the circum-nuclear medium and the jet physics by modeling this event.

Our Galactic center contains a cluster of stars in the inner 0.1 pc.

This, so called, S cluster is thought to coexist with the quiescent

accretion disk. Because of its early stellar type, the S2 star of the cluster

is expected to posses a fairly powerful wind. We show here that the

ram pressure of the accretion

disk shocks the stellar wind fairly close to the star. The shocked

fluid reaches a temperature of ˜1 keV and cools efficiently through

optically thin, thermal bremsstrahlung emission. The radiation from

the shocked wind peaks around the epoch of the pericentre passage

of the star at a luminosity potentially comparable to the quiescent

emission detected from Sgr A*. Detection of shocked wind radiation

can constrain the density of the accretion disc at a distance of

several thousands of gravitational radii from the black hole.

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