Correlative studies of astrophysical sources of very high and ultra high energy gamma-rays

During the period of this contract, June 1, 1991 to November 14, 1992, the major results of our research effort have come from the Whipple air shower experiment in Tucson, AZ. The most notable development has been the discovery of TeV photons from the BL Lac object, Markarian 421. This result depended critically on the identification of Mrk 421 by the EGRET team as a source of GeV gamma rays

An Estimation of the Gamma-Ray Burst Afterglow Apparent Optical Brightness Distribution Function

By using recent publicly available observational data obtained in conjunction with the NASA Swift gamma-ray burst mission and a novel data analysis technique, we have been able to make some rough estimates of the GRB afterglow apparent optical brightness distribution function. The results suggest that 71% of all burst afterglows have optical magnitudes with mR < 22.1 at 1000 seconds after the burst onset, the dimmest detected object in the data sample. There is a strong indication that the apparent optical magnitude distribution function peaks at mR ~ 19.5. Such estimates may prove useful in guiding future plans to improve GRB counterpart observation programs. The employed numerical techniques might find application in a variety of other data analysis problems in which the intrinsic distributions must be inferred from a heterogeneous sample.Comment: 15 pages including 2 tables and 7 figures, accepted for publication in Ap

Faint high-energy gamma-ray photon emission of GRB 081006A from Fermi observations

Since the launch of the Fermi gamma - ray Space Telescope on June 11, 2008, the LAT instrument has solidly detected more than 20 GRBs with high energy photon emission above 100 MeV. Using the matched filter technique, 3 more GRBs have also shown evidence of correlation with high energy photon emission as demonstrated by Akerlof et al. In this paper, we present another GRB unambiguously detected by the matched filter technique, GRB 081006A. This event is associated with more than 13 high energy photons above 100 MeV. The likelihood analysis code provided by the $Fermi$ Science Support Center (FSSC) generated an independent verification of this detection by comparison of the Test Statistics (TS) value with similar calculations for random LAT data fields. We have performed detailed temporal and spectral analysis of photons from 8 keV up to 0.8 GeV from the GBM and the LAT. The properties of GRB 081006A can be compared to the other two long duration GRBs detected at similar significance, GRB 080825C and GRB 090217A. We find that GRB 081006A is more similar to GRB 080825C with comparable appearances of late high energy photon emission. As demonstrated previously, there appears to be a surprising dearth of faint LAT GRBs, with only one additional GRB identified in a sample of 74 GRBs. In this unique period when both $Swift$ and $Fermi$ are operational, there is some urgency to explore this aspect of GRBs as fully as possible.Comment: ApJ, 745, 7

An inexpensive circuit for monitoring photomultiplier anode current

An 8-channel current-to-frequency converter module has been developed for measuring photomultiplier anode currents in the range of 1-100 [mu]A. Although this circuit was designed specifically for monitoring photomultiplier currents in a ground-based very high energy gamma-ray telescope, it would be useful in many other applications where cost is a consideration. An interesting consequence of this technique is that, by measuring the pulse-to-pulse time interval, it is possible to synthesize the kind of measurements usually performed with lock-in amplifiers. Tests show that with a few seconds integration time, a 1 kHz modulated signal can be detected at a level of 10-5 of the ambient current.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30337/1/0000739.pd

Astronomical Image Subtraction by Cross-Convolution

In recent years, there has been a proliferation of wide-field sky surveys to search for a variety of transient objects. Using relatively short focal lengths, the optics of these systems produce undersampled stellar images often marred by a variety of aberrations. As participants in such activities, we have developed a new algorithm for image subtraction that no longer requires high-quality reference images for comparison. The computational efficiency is comparable with similar procedures currently in use. The general technique is cross-convolution: two convolution kernels are generated to make a test image and a reference image separately transform to match as closely as possible. In analogy to the optimization technique for generating smoothing splines, the inclusion of an rms width penalty term constrains the diffusion of stellar images. In addition, by evaluating the convolution kernels on uniformly spaced subimages across the total area, these routines can accommodate point-spread functions that vary considerably across the focal plane.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/57484/26/xconvolve_cross_convolve.prohttp://deepblue.lib.umich.edu/bitstream/2027.42/57484/25/xconvolve_extend_grid.prohttp://deepblue.lib.umich.edu/bitstream/2027.42/57484/24/xconvolve_get_sky.prohttp://deepblue.lib.umich.edu/bitstream/2027.42/57484/23/xconvolve_make_kernels.prohttp://deepblue.lib.umich.edu/bitstream/2027.42/57484/22/xconvolve_make_mask.prohttp://deepblue.lib.umich.edu/bitstream/2027.42/57484/21/xconvolve_subtract_images.prohttp://deepblue.lib.umich.edu/bitstream/2027.42/57484/20/xconvolve_warp_image.prohttp://deepblue.lib.umich.edu/bitstream/2027.42/57484/19/xconvolve_regress_matrix.chttp://deepblue.lib.umich.edu/bitstream/2027.42/57484/18/binary_search.prohttp://deepblue.lib.umich.edu/bitstream/2027.42/57484/17/close_match_radec.prohttp://deepblue.lib.umich.edu/bitstream/2027.42/57484/16/iqd.prohttp://deepblue.lib.umich.edu/bitstream/2027.42/57484/15/xconvolve_splie2.prohttp://deepblue.lib.umich.edu/bitstream/2027.42/57484/14/xconvolve_splin2.prohttp://deepblue.lib.umich.edu/bitstream/2027.42/57484/13/xconvolve_make_sharelib.prohttp://deepblue.lib.umich.edu/bitstream/2027.42/57484/11/sample_subtract.prohttp://deepblue.lib.umich.edu/bitstream/2027.42/57484/10/070802_sks1650+2342-164942+235329_3b007_c.fithttp://deepblue.lib.umich.edu/bitstream/2027.42/57484/9/070802_sks1650+2342-164942+235329_3b007_cobj.fithttp://deepblue.lib.umich.edu/bitstream/2027.42/57484/8/070820_sks1650+2342-164942+235329_3b003_c.fithttp://deepblue.lib.umich.edu/bitstream/2027.42/57484/7/070820_sks1650+2342-164942+235329_3b003_cobj.fithttp://deepblue.lib.umich.edu/bitstream/2027.42/57484/6/sample_test.fithttp://deepblue.lib.umich.edu/bitstream/2027.42/57484/27/xconvolve_descrip.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/57484/28/ApJ_677_808.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/57484/31/apj_677_808-archive.zi

Limits to the fraction of high-energy photon emitting gamma-ray bursts

After almost 4 years of operation, the two instruments onboard the Fermi Gamma-ray Space Telescope have shown that the number of gamma-ray bursts with high energy photon emission above 100 MeV cannot exceed roughly 9% of the total number of all such events, at least at the present detection limits. In a recent paper (Zheng et al. 2012c), we found that GRBs with photons detected in the Large Area Telescope (LAT) have a surprisingly broad distribution with respect to the photon number above background. Extrapolation of our empirical fit to numbers of photons below our quoted detection limit suggests that the overall rate of such events could be determined by standard image co-adding techniques. In this case, we have taken advantage of the excellent angular resolution of the Swift mission to provide accurate reference points for 79 GRB events which have eluded any previous correlations with high energy photons. We find a small but significant signal. Guided by the power law fit obtained previously for the number distribution of GRBs, the data suggests that only a small fraction of GRBs are sources of high energy photons.Comment: 16 pages, 3 figures, 2 tables, accepted for publication in Ap

GRB 110709A, 111117A and 120107A: Faint high-energy gamma-ray photon emission from Fermi/LAT observations and demographic implications

Launched on June 11, 2008, the LAT instrument onboard the $Fermi$ Gamma-ray Space Telescope has provided a rare opportunity to study high energy photon emission from gamma-ray bursts. Although the majority of such events (27) have been iden tified by the Fermi LAT Collaboration, four were uncovered by using more sensiti ve statistical techniques (Akerlof et al 2010, Akerlof et al 2011, Zheng et al 2 012). In this paper, we continue our earlier work by finding three more GRBs ass ociated with high energy photon emission, GRB 110709A, 111117A and 120107A. To s ystematize our matched filter approach, a pipeline has been developed to identif y these objects in near real time. GRB 120107A is the first product of this anal ysis procedure. Despite the reduced threshold for identification, the number of GRB events has not increased significantly. This relative dearth of events with low photon number prompted a study of the apparent photon number distribution. W e find an extremely good fit to a simple power-law with an exponent of -1.8 $\pm$ 0.3 for the differential distribution. As might be expected, there is a substa ntial correlation between the number of lower energy photons detected by the GBM and the number observed by the LAT. Thus, high energy photon emission is associ ated with some but not all of the brighter GBM events. Deeper studies of the pro perties of the small population of high energy emitting bursts may eventually yi eld a better understanding of these entire phenomena.Comment: accepted to Ap