Search results for "galaxy"

Pulsed high-energy γ-rays from the radio pulsar PSRI706–44

1992

Gamma radiation above 100 MeV in energy has been detected from the radio pulsar PSR1706-44. The gamma emission forms a single broad peak within the pulsar period of 102 ms, in contrast to the two narrow peaks seen in the other three known high-energy gamma-ray pulsars. The emission mechanism in all cases is probably the same, the differences arising from the geometry of the magnetic and rotation axes and the line of sight. Gamma-ray emission accounts for as much as 1 percent of the total neutron star spindown energy in these pulsars, much more than emerges at optical or radio frequencies. Thus, study of this emission is important in understanding pulsar emission and evolution.

Expansion of SN 1993J

1995

A sequence of images from very long baseline interferometry shows that the young radio supernova SN 1993J is expanding with circular symmetry. However, the circularly symmetric images show emission asymmetries. A scenario in which freely expanding supernova ejecta shock mostly isotropic circumstellar material is strongly favored. The sequence of images constitutes the first “movie” of a radio supernova.

2017

Accretion dynamics in the formation of young stars is still a matter of debate because of limitations in observations and modeling. Through scaled laboratory experiments of collimated plasma accretion onto a solid in the presence of a magnetic field, we open a first window on this phenomenon by tracking, with spatial and temporal resolution, the dynamics of the system and simultaneously measuring multiband emissions. We observe in these experiments that matter, upon impact, is ejected laterally from the solid surface and then refocused by the magnetic field toward the incoming stream. This ejected matter forms a plasma shell that envelops the shocked core, reducing escaped x-ray emission. T…

The unusual γ-ray burst GRB 101225A from a helium star/neutron star merger at redshift 0.33

2011

Long Gamma-Ray Bursts (GRBs) are the most dramatic examples of massive stellar deaths, usually associated with supernovae. They release ultra-relativistic jets producing non-thermal emission through synchrotron radiation as they interact with the surrounding medium. Here we report observations of the peculiar GRB 101225A (the "Christmas burst"). Its gamma-ray emission was exceptionally long and followed by a bright X-ray transient with a hot thermal component and an unusual optical couuterpart. During the first 10 days, the optical emission evolved as an expanding, cooling blackbody after which an additional component, consistent with a faint supernova, emerged. We determine its distance to…

Bright hot impacts by erupted fragments falling back on the Sun: a template for stellar accretion.

2013

Impacts of falling fragments observed after the eruption of a filament in a solar flare on 7 June 2011 are similar to those inferred for accretion flows on young stellar objects. As imaged in the ultraviolet (UV)-extreme UV range by the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory, many impacts of dark, dense matter display uncommonly intense, compact brightenings. High-resolution hydrodynamic simulations show that such bright spots, with plasma temperatures increasing from ~10(4) to ~10(6) kelvin, occur when high-density plasma (>>10(10) particles per cubic centimeter) hits the solar surface at several hundred kilometers per second, producing high-energy emission as …

Multiscaling Properties of Large-Scale Structure in the Universe

1995

The large-scale distribution of galaxies and galaxy clusters in the universe can be described in the mathematical language of multifractal sets. A particularly significant aspect of this description is that it furnishes a natural explanation for the observed differences in clustering properties of objects of different density in terms of multiscaling, the generic consequence of the application of a local density threshold to a multifractal set. The multiscaling hypothesis suggests ways of improving upon the traditional statistical measures of clustering pattern (correlation functions) and exploring further the connection between clustering pattern and dynamics.

Exploring Mergers of Galaxy Clusters in a Cosmological Context

2010

We present results of an Eulerian Adaptive Mesh Refinement (AMR) hydrodynamical and N-body simulation in a ΛCDM cosmology. The simulation incorporates common cooling and heating processes, a phenomenological description of the star formation and supernovae feedback. A specific halo finder has been designed and applied in order to extract a sample of galaxy clusters directly obtained from the simulation without considering any resimulating scheme. We have studied the evolutionary history of the cluster halos, and classified them in three categories depending on the merger events they have undergone. We pay special attention to discuss the role of merger events as a source of feedback and reh…

CCD uvby-beta photometry of young open clusters. I. The double cluster h and chi Persei

2002

We present CCD uvby-beta photometry for stars in the nuclei of the young double cluster h and chi Persei. We find that the reddening is highly variable through the h Persei nucleus, increasing from west to east, with values ranging from E(b-y)=0.328+-0.022 in the western part to E(b-y)=0.465+-0.025 in the south-east. Towards chi Persei the reddening is fairly constant, with E(b-y)=0.398+-0.025. Both clusters share a common distance modulus of 11.7+-0.1 mag., and an age of log t=7.10+-0.05 years.

A new sample of large angular size radio galaxies II. The optical data

2001

We constructed and presented in the first paper of this series a new sample of 84 large angular size radio galaxies by selecting from the NRAO VLA Sky Survey objects with angular size > 4', declination above +60 degrees and total flux density at 1.4 GHz > 100 mJy. In this paper we present optical spectra and images of those galaxies associated with the radio emission for which no redshift was known prior to our observations. Optical counterparts have been identified for all (but one) members of the sample. After our observations, a reliable spectroscopic redshift is available for 67 objects (80%) from the sample. This paper, second of a series of three, contributes to increase the num…

PSR 1758 – 24 and G5.4 – 1.2, a remarkable pulsar–supernova remnant association

1991

Τiming observations made over an 18-month period using the Parkes radiotelescope show that the pulsar PSR 1758-24 is young and is located at the extreme western edge of the compact nebula G5.27-0.90 which is associated with supernova remnant G5.4-1.2. These results confirm the association of this pulsar with the supernova remnant. They also imply that the pulsar was born with such a high velocity that is has overtaken the supernova remnant shell and now lies outside the shell