Point source

About: Point source is a research topic. Over the lifetime, 5077 publications have been published within this topic receiving 94091 citations.

Using the ROSAT catalogue to find counterparts for unidentified objects in the first Fermi/LAT catalogue

Abstract: There are a total of 1451 gamma-ray emitting objects in the Fermi Large Area Telescope First Source Catalogue. The point source location accuracy of typically a few arcmin has allowed the counterparts for many of these sources to be found at other wavelengths, but even so there are 630 which are described as having no plausible counterpart at 80 per cent confidence. In order to help identify the unknown objects, we have cross-correlated the positions of these sources with the ROSAT All Sky Survey Bright Source Catalogue. In this way, for Fermi sources which have a possible counterpart in soft X-rays, we can use the much smaller ROSAT error box to search for identifications. We find a strong correlation between the two samples and calculate that there are about 60 sources with a ROSAT counterpart. Using the ROSAT error boxes we provide tentative associations for half of them, demonstrate that the majority of these are either blazars or blazar candidates, and give evidence that most belong to the BL Lac class. Given that they are X-ray selected and most are high synchrotron peaked objects, which indicates the presence of high-energy electrons, these sources are also good candidates for TeV emission, and therefore good probes of the extragalactic background light.

White light optical processing and holography.

Abstract: A method is described wherein optical processing and holography using transparencies are performed using point source white light. The method is coherent, in that the optical systems are linear in amplitude rather than intensity. Perfect wavelength compensation is achieved, with no restriction on the source spectral width.

Anisotropic effects in surface acoustic wave propagation from a point source in a crystal

Abstract: Strong anisotropic effects in the propagation of surface acoustic waves (SAWs) from a point-like source are studied experimentally and theoretically. Nanosecond SAW pulses are generated by focused laser pulses and detected with a cw probe laser beam at a large distance from the source compared to the SAW wavelength, which allows us to resolve fine intricate features in SAW wavefronts. In our theoretical model, we represent the laser excitation by a localized impulsive force acting on the sample surface and calculate the far-field surface response of an elastically anisotropic solid to such a force. The model simulates the measured SAW waveforms very well and accounts for all experimentally observed features. Using the data obtained for the (111) and (001) surfaces of GaAs, we describe a variety of effects encountered in the SAW propagation from a point source in crystals. The most interesting phenomenon is the existence of cuspidal structures in SAW wavefronts resulting in multiple SAW arrivals for certain ranges of the observation angle. Cuspidal edges correspond to the “phonon focusing” directions yielding sharp peaks in the SAW amplitude. A finite SAW wavelength results in “internal diffraction” whereby the SAW wavefront spreads beyond the group velocity cusps. Degeneration of a SAW into a transverse bulk wave is another strong effect influencing the anisotropy of the SAW amplitude and making whole sections of the SAW wavefront including some phonon focusing directions unobservable in the experiment. The propagation of a leaky SAW mode (pseudo-SAW) is affected by a specific additional effect i.e. anisotropic attenuation. We also demonstrate that many of the discussed features are reproduced in “powder patterns”, a simple technique developed by us earlier for visualization of SAW amplitude anisotropy.

A persistent ~1 HZ quasi-periodic oscillation in the dipping low-mass x-ray binary 4U 1323-62

Abstract: We have discovered a ~1 Hz quasi-periodic oscillation (QPO) in the persistent emission, the dips, and the type I X-ray bursts of the low-mass X-ray binary 4U 1323-62. The rms amplitude of the QPO is approximately 9%, only weakly depending on photon energy. The amplitude is consistent with being constant throughout the persistent emission, the dips, and the bursts in all but one observation, where it is much weaker during one dip. These properties suggest that we have observed a new type of QPO, which is caused by quasi-periodic obscuration of the central X-ray source by a structure in the accretion disk. This can only occur when the binary inclination is high, consistent with the fact that 4U 1323-62 is a dipping source. The quasi-periodic obscuration could take place by partial covering of an extended central X-ray source by a near-opaque medium or by covering of a point source by a medium having suitable characteristics to produce the relatively energy-independent oscillations.

Power-Law Template for IR Point Source Clustering

Abstract: We perform a combined fit to angular power spectra of unresolved infrared (IR) point sources from the Planck satellite (at 217, 353, 545 and 857 GHz, over angular scales 100 < l < 2200), the Balloon-borne Large-Aperture Submillimeter Telescope (BLAST; 250, 350 and 500 um; 1000 < l < 9000), and from correlating BLAST and Atacama Cosmology Telescope (ACT; 148 and 218 GHz) maps. We find that the clustered power over the range of angular scales and frequencies considered is well fit by a simple power law of the form C_l \propto l^-n with n = 1.25 +/- 0.06. While the IR sources are understood to lie at a range of redshifts, with a variety of dust properties, we find that the frequency dependence of the clustering power can be described by the square of a modified blackbody, nu^beta B(nu,T_eff), with a single emissivity index beta = 2.20 +/- 0.07 and effective temperature T_eff = 9.7 K. Our predictions for the clustering amplitude are consistent with existing ACT and South Pole Telescope results at around 150 and 220 GHz, as is our prediction for the effective dust spectral index, which we find to be alpha_150-220 = 3.68 +/- 0.07 between 150 and 220 GHz. Our constraints on the clustering shape and frequency dependence can be used to model the IR clustering as a contaminant in Cosmic Microwave Background anisotropy measurements. The combined Planck and BLAST data also rule out a linear bias clustering model.