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Jets and outflows are observed around a wide variety of accreting objects and seem to be a near-ubiquitous feature of accretion disks. Large-scale magnetic fields are thought to be necessary for jet formation in many systems, but a longstanding puzzle is that the turbulence which is responsible for inward disk accretion should be even more efficient at causing a large-scale magnetic field to diffuse outward; it just doesn't seem possible to build up a strong field in the inner disk through advection of a weak one from outside. Here, we report theoretical work which challenges this conventional wisdom and shows that the surface layer of the disk (which is nonturbulent) can easily advect magnetic fields inward. This opens up the possibility for more diverse and robust jet formation than is seen in numerical simulations which assume that the large-scale field must be generated entirely via a dynamo within the inner disk. In addition, strong disk turbulence (as inferred from observations) can naturally be explained by the magnetorotational instability feeding off a large-scale magnetic field anchored in the surface layer., Comment: 5 pages, 1 figure; to be published in proceedings of "A Population Explosion: The Nature and Evolution of X-ray Binaries in Diverse Environments," 28 Oct. - 2 Nov. 2007, St. Petersburg Beach, FL (R. M. Bandyopadhyay, S. Wachter, D. Gelino, & C. R. Gelino, eds.)

We show that a large-scale, weak magnetic field threading a turbulent accretion disk tends to be advected inward, contrary to previous suggestions that it will be stopped by outward diffusion. The efficient inward transport is a consequence of the diffuse, magnetically-dominated surface layers of the disk, where the turbulence is suppressed and the conductivity is very high. This structure arises naturally in three-dimensional simulations of magnetorotationally unstable disks, and we demonstrate here that it can easily support inward advection and compression of a weak field. The advected field is anchored in the surface layer but penetrates the main body of the disk, where it can generate strong turbulence and produce values of alpha (i.e., the turbulent stress) large enough to match observational constraints; typical values of the vertical magnetic field merely need to reach a few percent of equipartition for this to occur. Overall, these results have important implications for models of jet formation which require strong, large-scale magnetic fields to exist over a region of the inner accretion disk., Comment: 13 pages, 4 figures; accepted for publication in The Astrophysical Journal (v2: added references to papers by McKinney et al.)

We report infrared observations of the microquasar GRS 1915+105 using the NICMOS instrument of the Hubble Space Telescope during 9 visits in April-June 2003. During epochs of high X-ray/radio activity near the beginning and end of this period, we find that the $1.87 $\um infrared flux is generally low ($\sim 2$ mJy) and relatively steady. However, during the X-ray/radio ``plateau'' state between these epochs, we find that the infrared flux is significantly higher ($\sim 4-6$ mJy), and strongly variable. In particular, we find events with amplitudes $\sim 20-30$% occurring on timescales of $\sim 10-20$s (e-folding timescales of $\sim 30$s). These flickering timescales are several times faster than any previously-observed infrared variability in GRS 1915+105 and the IR variations exceed corresponding X-ray variations at the same ($\sim 8s$) timescale. These results suggest an entirely new type of infrared variability from this object. Based on the properties of this flickering, we conclude that it arises in the plateau-state jet outflow itself, at a distance $<2.5$ AU from the accretion disk. We discuss the implications of this work and the potential of further flickering observations for understanding jet formation around black holes., Comment: 19 pages, incl. 4 figures; accepted for publication in ApJ

After an X-ray binary experiences a transient jet ejection, it undergoes a phase in which its X-ray light curve is dominated, for some time, by thermal emission from an accretion disk surrounding the black hole. The accretion physics in the thermal-dominant state is understood better than in any other, and it is therefore the best state for comparing observations to theoretical models. Here, I present simulations that study the way a thermally-emitting disk might be expected to behave immediately after a large-scale, steady jet has been removed from the system in the form of a sudden ejection. I simulate the ejection's effect on the disk by allowing the strength of turbulence (modeled by the alpha parameter of Shakura and Sunyaev) to increase rapidly in time, and I show how this change can lead to an outburst in an otherwise-steady disk. The motivation for treating the jet removal in this way is the fact that many models for jets involve large-scale magnetic fields that should inhibit the magnetorotational instability believed to drive turbulence; this should naturally lead to a rapid increase in turbulence when the magnetic field is ejected from the system or otherwise destroyed during the ejection event. I show how the timescale and luminosity of the outburst can be controlled by the manner in which alpha is allowed to change, and I briefly discuss ways in which these simulations can be compared to observations of X-ray binaries, in particular GRS 1915+105, which shows the most complex and variable behavior of any black hole system in outburst., Comment: 10 pages, 6 figures; accepted for publication in the proceedings of VI Microquasar Workshop: Microquasars and Beyond, Sept 18-22 2006, Como, Italy, ed: T. Belloni (2006), PoS(MQW6)037

We present evidence that ~ 30 minute episodes of jet formation in GRS 1915+105 may sometimes be a superposition of smaller, faster phenomena. Based on observations in 2002 July using the Rossi X-ray Timing Explorer and the Palomar 5 meter telescope, we show that GRS 1915+105 sometimes produces ~ 30 minute infrared flares that can entirely be explained as a combination of small (~ 150 second) flares, one for each oscillation in the accompanying X-ray light curve. We discuss the differences between these observations and similar ones in 1997 August and conclude that an X-ray "trigger spike" seen during each cycle in 1997 is a key ingredient for large ejections to occur on ~ 30 minute timescales in this source., Comment: 6 pages, 3 figures; published in Proceedings of the 5th Microquasar Workshop (Beijing, June 2004); this paper is a shorter version of astro-ph/0501624

We present evidence that ~ 30 minute episodes of jet formation in the Galactic microquasar GRS 1915+105 may sometimes entirely be a superposition of smaller, faster phenomena. We base this conclusion on simultaneous X-ray and infrared observations in July 2002, using the Rossi X-ray Timing Explorer and the Palomar 5 meter telescope. On two nights, we observed quasi-periodic infrared flares from GRS 1915+105, each accompanied by a set of fast oscillations in the X-ray light curve (indicating an interaction between the jet and accretion disk). In contrast to similar observations in 1997, we find that the duration of each X-ray cycle matches the duration of its accompanying infrared flare, and we observed one instance in which an isolated X-ray oscillation occurred at the same time as a faint infrared "subflare" (of duration ~ 150 seconds) superimposed on one of the main flares. From these data, we are able to conclude that each X-ray oscillation had an associated faint infrared flare and that these flares blend together to form, and entirely comprise, the ~ 30 minute events we observed. Part of the infrared emission in 1997 also appears to be due to superimposed small flares, but it was overshadowed by infrared-bright ejections associated with the appearance of a sharp "trigger" spike in each X-ray cycle that were not present in 2002. We also study the evolution of the X-ray spectrum and find significant differences in the high energy power law component, which was strongly variable in 1997 but not in 2002. Taken together, these observations reveal the diversity of ways in which the accretion disk and jet in black hole systems are capable of interacting and solidify the importance of the trigger spike for large ejections to occur on ~ 30 minute timescales in GRS 1915+105., Comment: 17 pages, 9 figures; accepted for publication in The Astrophysical Journal

We report the discovery of fast (~ 100 sec) 2.2 micron flares from the microquasar GRS 1915+105, which are superimposed on longer (~ 30 min), brighter flares corresponding to episodes of jet formation. The number and strength of the sub-flares in each bright flare varies, and does not seem to correlate in any obvious way with the underlying light curve or with changes in the X-ray emission. However, the fact that these sub-flares only occur in tandem with the larger flares indicates that they might be related to the jet ejection process., Comment: 4 pages, 2 figures; to appear in Proceedings of the 4th Microquasar Workshop, eds. Ph Durouchoux, Y. Fuchs and J. Rodriguez, published by the Center for Space Physics: Kolkata