99 research outputs found
A search of the SAS-2 data for pulsed gamma-ray emission from radio pulsars
Data from the SAS-2 high energy gamma ray experiment were examined for pulsed emission from each of 75 radio pulsars which were viewed by the instrument and which have sufficiently well defined period and period derivative information from radio observations to allow for gamma ray periodicity searches. When gamma ray arrival times were converted to pulsar phase using the radio reference timing information, two pulsars, PSR 1747-46 and PSR 1818-04, showed positive effects, each with a probability less than 0.0001 of being a random fluctuation in the data for that pulsar. These are in addition to PSR 0531+21 and PSR 0833-45, previously reported. The results of this study suggest that gamma-ray astronomy has reached the detection threshold for gamma ray pulsars and that work in the near future should give important information on the nature of pulsars
Variability of the Vela Pulsar-wind Nebula Observed with Chandra
The observations of the pulsar-wind nebula (PWN) around the Vela pulsar with
the Advanced CCD Imaging Spectrometer aboard the Chandra X-ray Observatory,
taken on 2000 April 30 and November 30, reveal its complex morphology
reminiscent of that of the Crab PWN. Comparison of the two observations shows
changes up to 30% in the surface brightness of the PWN features. Some of the
PWN elements show appreciable shifts, up to a few arcseconds (about 10^{16}
cm), and/or spectral changes. To elucidate the nature of the observed
variations, further monitoring of the Vela PWN is needed.Comment: 7 pages (incl. 3 embedded PS figures), AASTEX, uses emulateapj5.sty.
Submitted to ApJ Lett. For a high-resolution color PS image of Figure 3 (6.3
Mby), see http://www.astro.psu.edu/users/divas/velaneb_fig3.p
Properties of the Chandra Sources in M81
The Chandra X-ray Observatory obtained a 50-ks observation of the central
region of M81 using the ACIS-S in imaging mode. The global properties of the 97
x-ray sources detected in the inner 8.3x8.3 arcmin field of M81 are examined.
Roughly half the sources are concentrated within the central bulge. The
remainder are distributed throughout the disk with the brightest disk sources
lying preferentially along spiral arms. The average hardness ratios of both
bulge and disk sources are consistent with power law spectra of index Gamma~1.6
indicative of a population of x-ray binaries. A group of much softer sources
are also present. The background source-subtracted logN-logS distribution of
the disk follows a power law of index ~ -0.5 with no change in slope over three
decades in flux. The logN-logS distribution of the bulge follows a similar
shape but with a steeper slope above ~4.0e+37 ergs/s. There is unresolved x-ray
flux from the bulge with a radial profile similar to that of the bulge sources.
This unresolved flux is softer than the average of the bulge sources and
extrapolating the bulge logN-logS distribution towards weaker sources can only
account for 20% of the unresolved flux. No strong time variability was observed
for any source with the exception of one bright, soft source.Comment: 5 pages, 3 color PS figures, to appear in ApJ
Changes in the long term intensity variations in Cyg X-2 and LMC X-3
We report the detection of changes in the long-term intensity variations in
two X-ray binaries, Cyg X-2 and LMC X-3. In this work, we have used the
long-term light curves obtained with the All-Sky Monitors (ASMs) of the Rossi
X-Ray Timing Explorer (RXTE), Ginga, Ariel 5, and Vela 5B and the scanning
modulation collimator of HEAO 1. It is found that in the light curves of both
the sources, obtained with these instruments at various times over the last 30
years, more than one periodic or quasi-periodic component is always present.
The multiple prominent peaks in the periodograms have frequencies unrelated to
each other. In Cyg X-2, RXTE-ASM data show strong peaks at 40.4 and 68.8 days,
and Ginga-ASM data show strong peaks at 53.7 and 61.3 days. Multiple peaks are
also observed in LMC X-3. The various strong peaks in the periodograms of LMC
X-3 appear at 104, 169, and 216 days (observed with RXTE-ASM) and 105, 214, and
328 days (observed with Ginga-ASM). The present results, when compared with the
earlier observations of periodicities in these two systems, demonstrate the
absence of any stable long period. The 78 day periodicity detected earlier in
Cyg X-2 was probably due to the short time base in the RXTE data that were
used, and the periodicity of 198 days in LMC X-3 was due to a relatively short
duration of observation with HEAO 1.Comment: 11 pages, 7 postscript figures include
Probing the Neutron Star Interior with Glitches
With the aim of constraining the structural properties of neutron stars and
the equation of state of dense matter, we study sudden spin-ups, glitches,
occurring in the Vela pulsar and in six other pulsars. We present evidence that
glitches represent a self-regulating instability for which the star prepares
over a waiting time. The angular momentum requirements of glitches in Vela
indicate that at least 1.4% of the star's moment of inertia drives these
events. If glitches originate in the liquid of the inner crust, Vela's
`radiation radius' must exceed ~12 km for a mass of 1.4 solar
masses. The isolated neutron star RX J18563-3754 is a promising candidate for a
definitive radius measurement, and offers to further our understanding of dense
matter and the origin of glitches.Comment: Invited talk at the Pacific Rim Conference on Stellar Astrophysics,
Hong Kong, Aug. 1999. 9 pages, 5 figure
Constraints on a Putative Pulsar in SN 1987A
We assume that all the energy loss of the putative pulsar in SN 1987A would
contribute to the luminosity of the remnant, which acts like a bolometer. The
bolometric luminosity of SN 1987A provides an upper bound on the pulsar's rate
of energy loss. An isolated pulsar spinning down by magnetic dipole radiation
alone, with initial rotation periods of 10-30 ms, as extrapolated for galactic
young pulsars, can have a luminosity below the bolometric bound if either the
magnetic field is weak, B 10 G, or if it is so strong that
the pulsar luminosity decays rapidly, B 10 G.Comment: Accepted for publication in ApJ Letter
Tabulated data from the SAS-2 high energy gamma ray telescope
The second small astronomy satellite (SAS-2) carried a high energy gamma ray telescope into an equitorial orbit with a 2 D inclination, an apogee of 610 km, and a perigee of 440 km. The energy threshold of the instrument was about 30 MeV, the energy of the gamma rays could be measured up to about 200 MeV, and the integral intensity above 200 MeV could also be measured. Summary tables of the gamma ray data are presented in two energy bands, 35-100 MeV and 100 MeV. The sky was divided into 144 solid angle elements, and, in each solid angle element for which data exist, the number of gamma rays observed is given and also the exposure factor. Information is provided to permit conversion of these data into approximate intensities
Tracking and imaging gamma ray experiment (TIGRE) for 1 to 100 MEV gamma ray astronomy
A large international collaboration from the high energy astrophysics community has proposed the Tracking and Imaging Gamma Ray Experiment (TIGRE) for future space observations. TIGRE will image and perform energy spectroscopy measurements on celestial sources of gamma rays in the energy range from 1 to 100 MeV. This has been a difficult energy range experimentally for gamma ray astronomy but is vital for the future considering the recent exciting measurements below 1 and above 100 MeV. TIGRE is both a double scatter Compton and gamma ray pair telescope with direct imaging of individual gamma ray events.
Multi‐layers of Si strip detectors are used as Compton and pair converters CsI(Tl) scintillation detectors are used as a position sensitive calorimeter. Alternatively, thick GE strip detectors may be used for the calorimeter. The Si detectors are able to track electrons and positrons through successive Si layers and measure their directions and energy losses. Compton and pair events are completely reconstructed allowing each event to be imaged on the sky. TIGRE will provide an order‐of‐magnitude improvement in discrete source sensitivity in the 1 to 100 MeV energy range and determine spectra with excellent energy and excellent angular resolutions. It’s wide field‐of‐view of π sr permits observations of the entire sky for extended periods of time over the life of the mission
On the Population of Wind-Accreting Neutron Stars in the Galaxy
We explore the possibility that neutron stars accreting from the winds of
main-sequence stellar companions account for a significant fraction of
low-luminosity, hard X-ray sources (L_X <~ 10^35 ergs/s; 1-10 keV) in the
Galaxy. This work was motivated by recent Chandra observations of the Galactic
center by Wang et al. (2002). Our calculations indicate that many of the
discrete X-ray sources detected in this survey may be wind-accreting neutron
stars, and that many more may be discovered with deeper X-ray observations. We
propose that an infrared observing campaign be undertaken to search for the
stellar counterparts of these X-ray sources.Comment: submitted to ApJ Letter
A Dual Frequency, Multi-Year Monitoring Program of Compact Radio Sources
We present light curves for 149 sources monitored with the Green Bank
Interferometer. The light curves are at two radio frequencies (approximately
2.5 and 8.2 GHz) and range from 3 to 15 yrs in length, covering the interval
1979--1996, and have a typical sampling of one flux density measurement every 2
days. We have used these light curves to conduct various variability analysis
(rms flux density variations and autoregressive, integrated, moving average
modeling) of these sources. We find suggestive, though not unambiguous
evidence, that these sources have a common, broadband mechanism for intrinsic
variations, in agreement with previous studies of a subset of these source. We
also find that the sources generally display a short-term variability (~ 10 d)
that arises from radio-wave scattering in an extended medium. These conclusions
extend those of Fiedler et al. (1987) who used a sub-sample of these data. The
primary motivation for this monitoring program was the identification of
extreme scattering events. In an effort to identify ESEs in a systematic
manner, we have taken the wavelet transform of the light curves. We find 15
events in the light curves of 12 sources that we classify as probable ESEs.
However, we also find that five ESEs previously identified from these data do
not survive our wavelet selection criteria. Future identification of ESEs will
probably continue to rely on both visual and systematic methods. Instructions
for obtaining the data are also presented.Comment: 34 pages, LaTeX with AASTeX-5, 7 figures in 20 PostScript files;
because of their length, Table 1 and Figures 8 and 9 can be obtained from
http://ese.nrl.navy.mil/GBI/GBI.htm
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