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    GX 17+2: jet-powered Z source
    An x-ray binary is a system of two stars orbiting about their common center of mass, so named for their large x-ray luminosities. The primary (the heavier star) is more evolved and has exhausted its nuclear fuel, existing as a stellar cinder like a neutron star or a black hole. The secondary (the lighter star) has not yet used up its fuel and exists as a main sequence, giant, or supergiant star. In a High Mass X-ray Binary (HMXB) the primary accretes matter from the secondary's stellar wind. The secondary is usually an early type (young and massive) star with a strong stellar wind. In a Low Mass X-ray Binary (LMXB) the secondary is larger than its Roche Lobe, the surface of gravitational equipotential inside of which material is bound to the star. Material overflowing the Roche Lobe is attracted by the primary, again a compact object.

    In both types of x-ray binaries, material attracted to the primary forms (by conservation of angular momentum) an accretion disk around it. Friction in the disk serves to heat the material and redistribute angular momentum such that material migrates to the inner part of the disk. Material loses gravitational potential energy as it falls toward the primary, half of which is released as light. This radiation is very high energy and is the main source of the binary's eponymous x-rays.


    The 5' x 5' field of GX 17+2, shown subtracted from another image to remove the sky background.
    X-ray binaries have multiple sources of light at various wavelengths. The accretion disk itself usually overwhelms all other sources from visible to x-rays. The disk has a corona (somewhat similar to how the Sun has its corona) of tenuous hot gas which can reprocess (absorb and reemit) the accretion disk's x-rays as IR radiation. In LMXBs the secondary star would only contribute significantly to the IR, but in HMXBs it would be much more hot and luminous and contribute to visible and UV light.

    X-ray binaries are thought to have synchrotron jets (usually) perpendicular to the accretion disk, which are streams of relativistic plasma radiating across the spectrum from x-rays to radio. Due to the other competing bright sources of light, these jets are usually only observed in radio and hard x-rays and only in black hole systems. Synchrotron jets have been observed in X-ray binaries such as GRS 1915+105 (Fender et al. 1997), GX 339-4 (Corbel & Fender 2002), and XRB 4U 0614+091 (Migliari et al. 2006). This project is an attempt to observe a synchrotron jet in a neutron star system, proving that such jets are a process of accretion rather than a property of black holes, as was previously thought.

    Why is the IR important? As has been stated, the IR is where synchrotron will be brightest compared to other radiation sources. It is suspected that all x-ray binaries can host such jets. If so, this changes how we interpret other binary systems. IR observations of quiescent x-ray novae are used to constrain system parameters like the mass ratio, the component masses, and the orbital plane inclination to the line of sight, all under the assumption that light from the secondary star dominates IR emission. The presence of an IR bright synchrotron jet would mean all previous estimates for these systems could be invalid (Callanan et al. 2002).


    X-ray color-color diagram for GX 17+2, showing the classic three branches of the Z: the Flaring Branch (FB), the Normal Branch (NB), and the Horizontal Branch (HB).
    GX 17+2 is a LMXB Z source in the constellation Serpens (near the galactic center), whose Type I x-ray bursts are indicative of a neutron star primary. These bursts are occasional large increases in x-ray brightness caused by sudden nuclear ignition of accreted material built up on the surface of the neutron star. This system is also a Z source, meaning that the neutron star is accreting at near Eddington rate (accreting almost to the point where the energy released blows away the accreting material). Z sources are so named for the pattern they trace out on a x-ray color-color diagram. It is thought that this 'z' pattern relates to the ratio of the mass accretion rate and the average mass accretion rate, which increases from Horizontal Branch (HB) to Normal Branch (NB) to Flaring Branch (FB) (Hasinger & van der Klis 1989). GX 17+2 is one of the most active of the 8 known Z sources. Its x-ray and radio activity seems correlated with radio brightness increasing from the NB to the HB (Penninx 1988, Migliari & Fender 2006), and it evidences the largest amplitude of rapid radio variations (Penninx 1989). Migliari & Fender (2006) found a correlation between radio and X-ray activity in Z sources in the hard X-ray state, where rapid state changes are associated with bright, optically thick, transient radio events (flares).


    GX 17+2 K band lightcurve for ARC 2006 IR brightening event.

    GX 17+2 K band lightcurve for second KPNO 2007 IR brightening event.
    This system demonstrates IR variations of an amazing 3 magnitudes in the K band (2.2 microns). Incidentally, this allows for correct identification of the IR counterpart distinct from the field star NP Ser which is almost superimposed on GX 17+2 (Callanan et al. 2002). These IR events are not associated with x-ray activity, implying that they are not caused by accretion disk events or corona x-ray reprocessing. Instead, they are interpreted as synchrotron jet emission. Current theoretical models predict such IR synchrotron radiation but also predict it will only be brighter than the reprocessed accretion disk corona emission for short-period systems (Migilari, et al. 2006). GX 17+2's orbital parameters are unknown, however.

    In 2006 we observed this system with the ARC 3.5m at APO, the RXTE PCA, and the VLA; in 2007 we observed with the KPNO 2.1m using SQIID and with the CTIO 1.3m with the SMARTS ANDICAM; and in 2008 we have been granted time on the KPNO 2.1m. These KPNO data, combined with ARC 3.5m, suggest a periodicity in the IR events of about 3 days. This period is a curiosity, for the Migliari compact jet model implies a stochastic jet. It is possible to explain these observations as a precessing synchrotron jet, as seen in BL Lacs (supermassive black holes at the centers of some galaxies). It is also possible that GX 17+2 is an eccentric binary with the periodic activity caused by Roche Lobe overflow at periastron passage, as seen in Cir X-1.


    RXTE ASM soft X-ray lightcurve with the KPNO 2007 IR brightening events indicated.
    RXTE PCA data were contemporaneous with IR observations. Three hours after the ARC 2006 event GX 17+2 was found to be in the middle of the NB. RXTE ASM data show that three out of four IR brightening events are preceded roughly 1.5 days by an increase in soft X-ray brightness and they all seem to occur during low soft X-ray emission; but not all X-ray increases+lows lead to an IR event. Small numbers statistics prevents us from drawing any conclusions. X-ray results provided by the ASM/RXTE teams at MIT and at the RXTE SOF and GOF at NASA's GSFC.

    GX 17+2's IR variations are not due to eclipses, for the system brightens during the course of the event, which is fairly short in relation to the period. They cannot be due to mass ejection events (flares), for flares are stochastic in nature and our observed behavior is periodic. We suspect GX 17+2 posses a precessing IR synchrotron jet. Evidence exists for precessing jets in AGN, such as BL Lac S5 0716+714 (Nesci et al. 2005). We presume that we observe a chord of the conical jet as it precesses across our line of sight. This explains both the periodic and variable aspect of the observed IR brightening events. To prove the synchrotron nature of this radiation, we must determine its spectral index. This can only be done with NASA's Spitzer telescope. A proposal has been submitted for the upcoming cycle.

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