Point source

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

Surface motion due to a point source in a semi-infinite elastic medium

Abstract: The theory of the motion of the surface of a semi-infinite elastic solid due to an impulsive internal point source is developed, both for P-wave and S-wave point sources. The resulting motions have been computed numerically for the case λ = μ and are presented both in tabular form and graphically.

The nature of the compact X-ray source in the supernova remnant G27.4+0.0

Abstract: High-resolution X-ray imaging data obtained with ROSAT is used to constrain the nature of the central compact source in the supernova remnant G27.4+0.0. Diffuse emission is seen from throughout the approximately 4 min diameter radio shell, while the central source remains unresolved at approximately 3 sec. We combine archival data from the Einstein HRI, IPC, and MPC with the ROSAT HRI data to define the X-ray spectra of the diffuse and point-like emission. The bulk of the shell radiation is consistent with that of a approximately 10(exp 7) K plasma, although a higher temperature component is also suggested by the data; coupled with the remnant's size and distance, we derive an age of between 500 and 2500 yr. The point source has a substantially harder spectrum, with a power-law photon index less than or approximately equal to 1. A search for periodic modulation from the point source yields upper limits ranging from 10%-35% for periods between 0.025 and 1000 s, depending on the assumed pulse shape. No aperiodic variability on timescales of from 10(exp 3) to 10(exp 8) s is required, although a factor of approximately 2 change between the Einstein and ROSAT eras is possible. We show that the point source cannot represent thermal emission from the surface of a young neutron star and is unlikely to be explained as nonthermal, Crab-like X-ray pulses or a small synchrotron nebula. The most likely models involve accretion-powered systems -- either a wind-fed neutron star with a massive companion or a low-mass X-ray binary. In all probability, this is the youngest X-ray binary in the Galaxy.

The Diffuse Emission and a Variable Ultraluminous X-Ray Point Source in the Elliptical Galaxy NGC 3379

Abstract: A Chandra observation of the intermediate-luminosity (MB = -20) elliptical galaxy NGC 3379 resolves 75% of the X-ray emission within the central 5 kpc into point sources. Spectral analysis of the remaining unresolved emission within the central 770 pc indicates that 90% of the emission probably arises from undetected point sources, while 10% arises from thermal emission from kT = 0.6 keV gas. Assuming a uniform density distribution in the central region of the galaxy gives a gas mass of 5 × 105 M☉. Such a small amount of gas can be supplied by stellar mass loss in only 107 yr. Thus, the gas must be accreting into the central supermassive black hole at a very low radiative efficiency as in the ADAF or RIAF models, or it is being expelled in a galactic wind driven by the same AGN feedback mechanism as that observed in cluster cooling flows. If the gas is being expelled in an AGN-driven wind, then the ratio of mechanical to radio power of the AGN must be 104, which is comparable to that measured in cluster cooling flows that have recently been perturbed by radio outbursts. Only 8% of the detected point sources are coincident with globular cluster positions, which is significantly less than that found among other elliptical galaxies observed by Chandra. The low specific frequency of globular clusters and the small fraction of X-ray point sources associated with globular clusters in NGC 3379 is more similar to the properties of lenticular galaxies rather than elliptical galaxies. The brightest point source in NGC 3379 is located 360 pc from the central AGN with a peak luminosity of 3.5 × 1039 ergs s-1, which places it in the class of ultraluminous X-ray point sources (ULXs). Analysis of an archival ROSAT HRI observation of NGC 3379 shows that this source was at a comparable luminosity 5 yr prior to the Chandra observation. The spectrum of the ULX is well described by a power-law model with Γ = 1.6 ± 0.1 and galactic absorption, similar to other ULXs observed by Chandra and XMM-Newton and to the low-hard state observed in Galactic black hole binaries. During the Chandra observation, the source intensity smoothly varies by a factor of 2 with the suggestion of an 8-10 hr period. No changes in hardness ratio are detected as the intensity of the source varies. While periodic behavior has recently been detected in several ULXs, all of these reside within spiral galaxies. The ULX in NGC 3379 is the only known ULX in an elliptical galaxy with a smoothly varying light curve suggestive of an eclipsing binary system.

Higher-degree moment tensors – the importance of source finiteness and rupture propagation on seismograms

Abstract: Summary. The moment tensor representation of seismic sources is used to study source finiteness. This extension of previous work makes use of force moments of degree one to three. The importance of the higher degree moments in modelling finite propagating faults is investigated. The conditions under which only the first-degree moment tensor (point source approximation) is adequate are studied. Convergence of the moment series is found to be frequency-dependent, a function of station azimuth, and dependent upon fault model. The feasibility of using the higher-degree moments in an inversion scheme is dependent upon making simplifying assumptions about the fault model. If determined, these higher-degree moments yield information which can solve such problems as the fault plane ambiguity and the trade-off between source rupture time and rise time.