Spitzer Space Telescope - General Observer Proposal #60086


Characterizing the Periodic Infrared Brightening Events of GX17+2: A Precessing
Synchrotron Jet?

Principal Investigator: Thomas Harrison
           Institution: New Mexico State University

     Technical Contact: Thomas Harrison, New Mexico State University

Co-Investigators: 
Jillian Bornak, New Mexico State University                           
Bernie McNamara, New Mexico State University                          
Dawn Gelino, NExScI                                                   
Michael Rupen, NRAO                                                   

Science Category: compact objects 
 Observing Modes: IRAC Post-Cryo Mapping
  Hours Approved: 3.6 

Abstract:
GX17+2 is a low mass X-ray binary, and one of the brightest X-ray sources in the
sky. It is also a "Z Source" so named by the path this small family of objects
traces out in the X-ray color-color diagram. This Z-pattern is believed to occur
due to the changing rate of accretion, which is normally quite close to the
Eddington limit for a neutron star. GX17+2 seems to be a normal member of this
class of object, with stochastic X-ray and radio variations. The one exception
to this is that the infrared counterpart shows four magnitude variations in the
K-band. Even more peculiar is that these "outbursts" are periodic and recur
every 3.01 days! Unfortunately, GX17+2 is highly reddened (Av > 19) and is not
detectable at other ground-based wavelengths. Thus, we cannot conclusively
identify the nature of these outbursts, but our radio observations show that it
is radio loud during these infrared events. This suggests that we are seeing a
precessing synchrotron jet. While there is evidence for precessing jets in other
X-ray binaries, such a short precession period is unprecedented. We propose to
obtain simultaneous K-band and IRAC 3.6 and 4.5 micron observations of GX17+2
during two IR bright phases to allow us to deconvolve the reddening and spectrum
of the source to allow us to confirm whether these outbursts are from optically
thin synchrotron emission. Such jets are expected to be highly variable, and the
short exposure times offered by Spitzer will allow us to characterize this
variability. The proposed program requires 3.6 hr.