Spitzer Space Telescope - General Observer Proposal #60108


First Multi-Orbit Time Series Observations of Jets in Ultracompact X-ray
Binaries

Principal Investigator: Stefanie Wachter
           Institution: California Institute of Technology

     Technical Contact: Donald Hoard, IPAC, California Institute of Technology

Co-Investigators: 
D. W. Hoard, Spitzer Science Center                                   
Steve B. Howell, NOAO                                                 

Science Category: compact objects 
 Observing Modes: IRAC Post-Cryo Mapping
  Hours Approved: 22.0 

Abstract:
Jets are a ubiquitous feature in many different astrophysical objects. They are
found in extragalactic sources such as quasars and active galactic nuclei, in
Galactic X-ray binary systems, and in newly forming young stellar objects.
While the occurrence of jets is clearly connected to accretion processes in all
of these objects, the exact mechanism for initiating the outflow and the roles
specific parameters play in these wildly different sources are poorly
understood. We propose to explore jet properties in the physically smallest
systems among the XRBs, the ultracompact XRBs. The known ultracompact XRBs
harbor a neutron star accretor and have orbital periods of less than an hour,
requiring non-main sequence donor stars (typically a C/O white dwarf). The
formation rate of ultracompact XRBs could be higher than that of hydrogen-rich
(i.e., main sequence donor) XRBs, making them a significant - if currently
poorly sampled - part of the Galactic population of interacting binary stars.
Ultracompact XRBs are important to jet studies, because they expand the
parameter space in physical size and chemical composition compared to XRBs with
main sequence donors. The main focus of our observing program is to, for the
first time, study the behavior of an XRB jet across several orbital cycles, by
obtaining mid-infrared light curves of the ultracompact XRBs 4U 0614+091 and 4U
1626-67. Our observations are designed to characterize both orbital variations
and long-term secular variations. No similar studies of jets in normal XRBs have
been carried out because of lack of persistent jets, need to observe in the
mid-IR, and prohibitively long orbital periods (several hours to tens of days).
Because of their short periods, ultracompact XRBs are the only XRBs that allow
continuous study over several orbital cycles. Currently, we only know what XRB
jets look like in instantaneous snapshots; we do not have any information about
their orbitally-resolved behavior. This Spitzer program will remedy that
unfortunate situation.