Time Evolution and the Nature of the Near-Infrared Jets in GRS1915+105

We observed the galactic microquasar GRS1915+105 in the K ($2.2 \mu$m) band on October 16 and 17, 1995 UTC using the COB infrared (IR) imager on the Kitt Peak National Observatory 2.1-m telescope with a 0.2-arcsec/pixel plate scale and under good ($\sim 0.7$-arcsec) seeing conditions. Using a neighboring star in the image frames to determine the point spread function (PSF), we PSF-subtract the images of GRS1915+105. We find no evidence of extended emission such as the apparent near-IR jets seen by Sams et al. (1996) in July, 1995. Simple modelling of the star + jet structure allows us to place an upper limit on any similar emission at that position of $K>16.4$ at the 95% confidence level, as compared to $K=13.9$ as seen by Sams et al. (1996). This lack of extended IR flux during continued hard X-ray flaring activity confirms the hypothesis that the extended IR emission arises from the superluminal radio-emitting jets rather than reprocessing of the X-ray emission on other structures around the compact central object. Given the large apparent velocity of the radio-emitting jets, by the time of our observations the Sams et al. feature would have moved $>1$ arcsec from GRS1915+105, and we can place a limit of $K>17.7$ (95% confidence level) on any infrared emission in this region. We can thus place an upper limit on the radiative timescale of the feature of $\tau < 25$ days, which is consistent with synchrotron jet emission.Comment: 10 pages, 3 figures; submitted to ApJ Letter

A Comparison Between Lucky Imaging and Speckle Stabilization for Astronomical Imaging

The new technique of Speckle Stabilization has great potential to provide optical imaging data at the highest angular resolutions from the ground. While Speckle Stabilization was initially conceived for integral field spectroscopic analyses, the technique shares many similarities with speckle imaging (specifically shift-and-add and Lucky Imaging). Therefore, it is worth comparing the two for imaging applications. We have modeled observations on a 2.5-meter class telescope to assess the strengths and weaknesses of the two techniques. While the differences are relatively minor, we find that Speckle Stabilization is a viable competitor to current Lucky Imaging systems. Specifically, we find that Speckle Stabilization is 3.35 times more efficient (where efficiency is defined as signal-to-noise per observing interval) than shift-and-add and able to detect targets 1.42 magnitudes fainter when using a standard system. If we employ a high-speed shutter to compare to Lucky Imaging at 1% image selection, Speckle Stabilization is 1.28 times more efficient and 0.31 magnitudes more sensitive. However, when we incorporate potential modifications to Lucky Imaging systems we find the advantages are significantly mitigated and even reversed in the 1% frame selection cases. In particular, we find that in the limiting case of Optimal Lucky Imaging, that is zero read noise {\it and} photon counting, we find Lucky Imaging is 1.80 times more efficient and 0.96 magnitudes more sensitive than Speckle Stabilization. For the cases in between, we find there is a gradation in advantages to the different techniques depending on target magnitude, fraction of frames used and system modifications.Comment: 21 page, 6 figures. Accepted for publication in PAS

A Next Generation High-speed Data Acquisition System for Multi-channel Infrared and Optical Photometry

We report the design, operation, and performance of a next generation high-speed data acquisition system for multi-channel infrared and optical photometry based on the modern technologies of Field Programmable Gate Arrays, the Peripheral Component Interconnect bus, and the Global Positioning System. This system allows either direct recording of photon arrival times or binned photon counting with time resolution up to 1-$\mu$s precision in Universal Time, as well as real-time data monitoring and analysis. The system also allows simultaneous recording of multi-channel observations with very flexible, reconfigurable observational modes. We present successful 20-$\mu$s resolution simultaneous observations of the Crab Nebula Pulsar in the infrared (H-band) and optical (V-band) wavebands obtained with this system and 100-$\mu$s resolution V-band observations of the dwarf nova IY Uma with the 5-m Hale telescope at the Palomar Observatory.Comment: 11 pages, including 4 figures, to appear in PAS

QPO Frequency - Color Radius Connection in GRS 1915+105: a Possible Turnover supporting AEI predictions

It is widely believed that the low frequency quasi-periodic X-ray oscillations observed in microquasars are correlated to, but do not originate at, the physical radius of the inner edge of the accretion disk. Models relating the QPO frequency and color radius are hindered by observations showing contradicting trend correlations between the microquasars GRO 1655-40, XTE J1550-564 and GRS 1915+105. The first shows a negative correlation and the latter two a positive one. By taking into account relativistic rotation in the accretion disk, the Accretion-Ejection Instability (AEI) model predicts a turnover in the frequency-radius relationship, and has been successfully compared with observations of GRO J1655-40 and GRS 1915+105. We present further evidence supporting the AEI model prediction by using observations of the microquasar GRS 1915+105. By combining a data set including $\theta$-, $\beta$- and $\alpha$-class X-ray light curves, we observe positive, negative and null correlations in the frequency-radius relationship. This is the first time a single source has shown a possible inversion in the QPO frequency-color radius curve predicted by the AEI model