455 research outputs found
The NANOGrav 11 yr Data Set: Limits on Gravitational Wave Memory
The mergers of supermassive black hole binaries (SMBHBs) promise to be incredible sources of gravitational waves (GWs). While the oscillatory part of the merger gravitational waveform will be outside the frequency sensitivity range of pulsar timing arrays, the nonoscillatory GW memory effect is detectable. Further, any burst of GWs will produce GW memory, making memory a useful probe of unmodeled exotic sources and new physics. We searched the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) 11 yr data set for GW memory. This data set is sensitive to very low-frequency GWs of ~3 to 400 nHz (periods of ~11 yr–1 month). Finding no evidence for GWs, we placed limits on the strain amplitude of GW memory events during the observation period. We then used the strain upper limits to place limits on the rate of GW memory causing events. At a strain of 2.5 × 10⁻¹⁴, corresponding to the median upper limit as a function of source sky position, we set a limit on the rate of GW memory events at <0.4 yr⁻¹. That strain corresponds to an SMBHB merger with reduced mass of ηM ~ 2 × 10¹⁰ M_⊙ and inclination of ι = π/3 at a distance of 1 Gpc. As a test of our analysis, we analyzed the NANOGrav 9 yr data set as well. This analysis found an anomolous signal, which does not appear in the 11 yr data set. This signal is not a GW, and its origin remains unknown
A Search for Supernova-Remnant Masers Toward Unidentified EGRET Sources
Supernova remnants expanding into adjacent molecular clouds are believed to
be sites of cosmic ray acceleration and sources of energetic gamma-rays. Under
certain environmental conditions, such interactions also give rise to unusual
OH masers in which the 1720 MHz satellite line dominates over the more common
1665/7 MHz emission. Motivated by the apparent coincidence of a handful of
EGRET sources with OH(1720 MHz) maser-producing supernova remnants, we have
carried out a search using the Very Large Array for new OH(1720 MHz) masers
within the error regions of 11 unidentified EGRET sources at low Galactic
latitude. While a previously known maser associated with an HII region was
serendipitously detected, initial results indicate that no new masers were
found down to a limiting flux of, typically, 50 mJy. We discuss the
implications of this result on the nature of the unidentified Galactic EGRET
sources.Comment: 5 pages, 1 figure. To appear in Proceedings, GAMMA2001 (Baltimore,
MD, April 4-6, 2001), eds. N. Gehrels, C. Shrader, and S. Rit
Lensing of Fast Radio Bursts by Plasma Structures in Host Galaxies
Plasma lenses in the host galaxies of fast radio bursts (FRBs) can strongly
modulate FRB amplitudes for a wide range of distances, including the
Gpc distance of the repeater FRB121102. To produce caustics, the lens'
dispersion-measure depth (), scale size (), and distance
from the source () must satisfy . Caustics produce strong
magnifications () on short time scales ( hours to days and
perhaps shorter) along with narrow, epoch dependent spectral peaks (0.1 to
1~GHz). However, strong suppression also occurs in long-duration (
months) troughs. For geometries that produce multiple images, the resulting
burst components will arrive differentially by s to tens of ms and
they will show different apparent dispersion measures, pc cm. Arrival time perturbations may mask any
underlying periodicity with period s. When arrival times differ by
less than the burst width, interference effects in dynamic spectra are
expected. Strong lensing requires source sizes smaller than , which can be satisfied by compact objects such as
neutron star magnetospheres but not by AGNs. Much of the phenomenology of the
repeating fast radio burst source FRB121102 is similar to lensing effects. The
overall picture can be tested by obtaining wideband spectra of bursts (from
to 10 GHz and possibly higher), which can also be used to characterize the
plasma environment near FRB sources. A rich variety of phenomena is expected
from an ensemble of lenses near the FRB source. We discuss constraints on
densities, magnetic fields, and locations of plasma lenses related to
requirements for lensing to occur.Comment: 11 pages, 7 figures, submitted to the Astrophysical Journa
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