40 research outputs found
Unusual diffuse X-ray source in the Galactic center region
We report the ASCA and Chandra discovery of a diffuse X-ray source in the
Galactic center region. The X-ray spectrum is fitted with a non-equilibrium
ionization (NEI) plasma model of about 6-keV temperature. The model requires
higher than solar metal abundances, a young plasma age of 100 years
and a large N_H value of about 10^{23} cm^{-2}. The N_H value constrains the
source position to be in the Galactic center region at about 8.5 kpc distance.
The high resolution X-ray image with the Chandra ACIS shows a ring of 10''
radius which corresponds to 0.4 pc at the Galactic center, and a tail-like
structure. Although the morphology is peculiar, the other X-ray features are
likely to be a very young supernova remnant, possibly in a free expansion
phase.Comment: 11 pages, 4 figures, to be published in Ap
A flexible format for exchanging pulsar data
We describe a data format currently in use amongst European institutions for exchanging and archiving pulsar data. The format is designed to be as flexible as possible with regard to present and future compatibility with different operating systems. One application of the common format is simultaneous multi-frequency observations of single pulses. A data archive containing over 2500 pulse profiles stored in this format is now available via the Internet (see http://www.mpifr-bonn.mpg.de/pulsar/data), together with a small suite of computer programs that can read, write and display the data
Anomalous scattering of highly dispersed pulsars
We report multifrequency measurements of scatter broadening times for nine
highly dispersed pulsars over a wide frequency range (0.6 -- 4.9 GHz). We find
the scatter broadening times to be larger than expected and to scale with
frequency with an average power-law index of , i.e. significantly
less than that expected from standard theories. Such possible discrepancies
have been predicted very recently by Cordes & Lazio.Comment: 7 pages, 4 figures, accepted for publication in ApJ Letter
Structure and Dynamics of the 2012 November 13/14 Eclipse White-light Corona
Continuing our series of observations of coronal motion and dynamics over the solar-activity cycle, we observed from sites in Queensland, Australia, during the 2012 November 13 (UT)/14 (local time) total solar eclipse. The corona took the low-ellipticity shape typical of solar maximum (flattening index Δ = 0.01), a change from the composite coronal images we observed and analyzed in this journal and elsewhere for the 2006 and 2008-2010 eclipses. After crossing the northeast Australian coast, the path of totality was over the ocean, so further totality was seen only by shipborne observers. Our results include velocities of a coronal mass ejection (CME; during the 36 minutes of passage from the Queensland coast to a ship north of New Zealand, we measured 413 km s6(â1)) and we analyze its dynamics. We discuss the shapes and positions of several types of coronal features seen on our higher-resolution composite Queensland coronal images, including many helmet streamers, very faint bright and dark loops at the bases of helmet streamers, voids, and radially oriented thin streamers. We compare our eclipse observations with models of the magnetic field, confirming the validity of the predictions, and relate the eclipse phenomenology seen with the near-simultaneous images from NASA's Solar Dynamics Observatory (SDO/AIA), NASA's Extreme Ultraviolet Imager on Solar Terrestrial Relations Observatory, ESA/Royal Observatory of Belgium's Sun Watcher with Active Pixels and Image Processing (SWAP) on PROBA2, and Naval Research Laboratory's Large Angle and Spectrometric Coronagraph Experiment on ESA's Solar and Heliospheric Observatory. For example, the southeastern CME is related to the solar flare whose origin we trace with a SWAP series of images
Toward An Empirical Theory of Pulsar Emission. VII. On the Spectral Behavior of Conal Beam Radii and Emission Heights
In this paper we return to the old problem of conal component-pair widths and
profile dimensions. Observationally, we consider a set of 10 pulsars with
prominent conal component pairs, for which well measured profiles exist over
the largest frequency range now possible. Apart from some tendency to narrow at
high frequency, the conal components exhibit almost constant widths. We use all
three profile measures, the component separation as well as the outside
half-power and 10% widths, to determine conal beam radii, which are the focus
of our subsequent analysis. These radii at different frequencies are well
fitted by a relationship introduced by Thorsett (1991), but the resulting
parameters are highly correlated. Three different types of behavior are found:
one group of stars exhibits a continuous variation of beam radius which can be
extrapolated down to the stellar surface along the ``last open field lines''; a
second group exhibits beam radii which asymptotically approach a minimum high
frequency value that is 3--5 times larger; and a third set shows almost no
spectral change in beam radius at all. The first two behaviors are associated
with outer-cone component pairs; whereas the constant separation appears to
reflect inner-cone emission.Comment: 21 pages, 11 figures, accepted for publication in Astrophysical
Journal, uses aaste
Finding Radio Pulsars in and Beyond the Galactic Center
Radio-wave scattering is enhanced dramatically for Galactic center sources in
a region with radius >~ 15 arc min. Using scattering from Sgr A* and other
sources, we show that pulse broadening for pulsars in the Galactic center is
{\em at least} 6.3 \nu^{-4} seconds (\nu = radio frequency in GHz) and is most
likely 50--200 times larger because the relevant scattering screen appears to
be within the Galactic center region itself. Pulsars beyond---but viewed
through---the Galactic center suffer even greater pulse broadening and are
angularly broadened by <~ 2 {\em arc min}. Periodicity searches at radio
frequencies are likely to find only long period pulsars and, then, only if
optimized by using frequencies >~ 7 GHz and by testing for small numbers of
harmonics in the power spectrum. The optimal frequency is where \Delta_{0.1} is the distance of the
scattering region from Sgr A* in units of 0.1 kpc, P is the period (seconds),
and \alpha is the spectral index. A search for compact sources using aperture
synthesis may be far more successful than searches for periodicities because
the angular broadening is not so large as to desensitize the survey. We
estimate that the number of {\em detectable} pulsars in the Galactic center may
range from <= 1 to 100, with the larger values resulting from recent, vigorous
starbursts. Such pulsars provide unique opportunities for probing the ionized
gas, gravitational potential, and stellar population near Sgr A*.Comment: 13 pages, 4 PS figures, LaTeX and requires AASTeX macro aas2pp4,
accepted by ApJ, also available as
http://astrosun.tn.cornell.edu/SPIGOT/papers/pulsar/gc_psr.web
The characteristics of millisecond pulsar emission: I. Spectra, pulse shapes and the beaming fraction
We have monitored a large sample of millisecond pulsars using the 100-m
Effelsberg radio telescope in order to compare their radio emission properties
to the slowly rotating population. With some notable exceptions, our findings
suggest that the two groups of objects share many common properties. A
comparison of the spectral indices between samples of normal and millisecond
pulsars demonstrates that millisecond pulsar spectra are not significantly
different from those of normal pulsars. There is evidence, however, that
millisecond pulsars are slightly less luminous and less efficient radio
emitters compared to normal pulsars. We confirm recent suggestions that a
diversity exists among the luminosities of millisecond pulsars with the
isolated millisecond pulsars being less luminous than the binary millisecond
pulsars. There are indications that old millisecond pulsars exhibit somewhat
flatter spectra than the presumably younger ones. We present evidence that
millisecond pulsar profiles are only marginally more complex than those found
among the normal pulsar population. Moreover, the development of the profiles
with frequency is rather slow, suggesting very compact magnetospheres. The
profile development seems to anti-correlate with the companion mass and the
spin period, again suggesting that the amount of mass transfer in a binary
system might directly influence the emission properties. The angular radius of
radio beams of millisecond pulsars does not follow the scaling predicted from a
canonical pulsar model which is applicable for normal pulsars. Instead they are
systematically smaller. The smaller inferred luminosity and narrower emission
beams will need to be considered in future calculations of the birth-rate of
the Galactic population.Comment: 40 pages, 14 figures, accepted for publication in Ap
A 20cm Survey of the Galactic Center Region I: Detection of Numerous Linear Filaments
We present a sensitive 20cm VLA continuum survey of the Galactic
center region using new and archival data based on multi-configuration
observations taken with relatively uniform {\it uv} coverage. The high dynamic
range images cover the regions within and with a spatial resolution of and 10. The wide field
imaging technique is used to construct a low-resolution mosaic of 40
overlapping pointings. The mosaic image includes the Effelsburg observations
filling the low spatial frequency {\it uv} data. We also present high
resolution images of twenty three overlapping fields using DnC and CnB array
configurations. The survey has resulted in a catalog of 345 discrete sources as
well as 140 images revealing structural details of HII regions, SNRs, pulsar
wind nebulae and more than 80 linear filaments distributed toward the complex
region of the Galactic center. These observations show the evidence for an
order of magnitude increase in the number of faint linear filaments with
typical lengths of few arcminutes. Many of the filaments show morphological
characteristics similar to the Galactic center nonthermal radio filaments
(NRFs). The linear filaments are not isolated but are generally clustered in
star forming regions where prominent NRFs had been detected previously. The
extensions of many of these linear filaments appear to terminate at either a
compact source or a resolved shell-like thermal source. A relationship between
the filaments, the compact and extended thermal sources as well as a lack of
preferred orientation for many RFs should constrain models that are proposed to
explain the origin of nonthermal radio filaments in the Galactic center.Comment: 48 pages. The complete version of the paper with 140 figures and four
tables can be found in the ApJ preprint website:
http://www.journals.uchicago.edu/ApJ/future.htm
A Sino-German 6\ cm polarization survey of the Galactic plane. V. Large supernova remnants
Observations of large supernova remnants (SNRs) at high frequencies are rare,
but provide valuable information about their physical properties. The total
intensity and polarization properties of 16 large SNRs in the Galactic plane
were investigated based on observations of the Urumqi 6\ cm
polarization survey of the Galactic plane with an angular resolution of
9\farcm5. We extracted total intensity and linear polarization maps of large
SNRs from the Urumqi 6\ cm survey, obtained their integrated flux
densities, and derived the radio spectra in context with previously published
flux densities at various frequencies. In particular, Effelsberg 11\
cm and 21\ cm survey data were used for calculating integrated flux
densities. The 6\ cm polarization data also delineate the magnetic
field structures of the SNRs. We present the first total intensity maps at
6\ cm for SNRs G106.3+2.7, G114.3+0.3, G116.5+1.1, G166.0+4.3 (VRO
42.05.01), G205.5+0.5 (Monoceros Nebula) and G206.9+2.3 (PKS 0646+06) and the
first polarization measurements at 6\ cm for SNRs G82.2+5.3 (W63),
G106.3+2.7, G114.3+0.3, G116.5+1.1, G166.0+4.3 (VRO 42.05.01), G205.5+0.5
(Monoceros Nebula) and G206.9+2.3 (PKS 0646+06). Most of the newly derived
integrated radio spectra are consistent with previous results. The new flux
densities obtained from the Urumqi 6\ cm, Effelsberg 11\ cm
and 21\ cm surveys are crucial to determine the spectra of SNR
G65.1+0.6, G69.0+2.7 (CTB 80), G93.7-0.2 and G114.3+0.3. We find that
G192.81.1 (PKS 0607+17) consists of background sources, \ion{H}{II} regions
and the extended diffuse emission of thermal nature, and conclude that
G192.81.1 is not a SNR.Comment: 15 pages, 10 figures, accepted by A&\amp;A. Language improved. For
the version with high resolution figures, please go to:
http://zmtt.bao.ac.cn/6cm/paper/gxy_largeSNR.pd
Arecibo 430 MHz Pulsar Polarimetry: Faraday Rotation Measures and Morphological Classifications
We have measured Faraday Rotation Measures (RMs) at Arecibo Observatory for
36 pulsars, 17 of them new. We combine these and earlier measurements to study
the galactic magnetic field and its possible temporal variations. Many RM
values have changed significantly on several-year timescales, but these
variations probably do not reflect interstellar magnetic field changes. By
studying the distribution of pulsar RMs near the plane in conjunction with the
new NE2001 electron density model, we note the following structures in the
first galactic longitude quadrant: (1) The local field reversal can be traced
as a null in RM in a 0.5-kpc wide strip interior to the Solar Circle, extending
\~7 kpc around the Galaxy. (2) Steadily increasing RMs in a 1-kpc wide strip
interior to the local field reversal, and also in the wedge bounded by 42<l<52
deg, indicate that the large-scale field is approximately steady from the local
reversal in to the Sagittarius arm. (3) The RMs in the 1-kpc wide strip
interior to the Sagittarius arm indicate another field reversal in this strip.
(4) The RMs in a final 1-kpc wide interior strip, straddling the Scutum arm,
also support a second field reversal interior to the Sun,between the
Sagittarius and Scutum arms. (5) Exterior to the nearby reversal, RMs from
60<l<78 deg show evidence for two reversals, on the near and far side of the
Perseus arm. (6) In general, the maxima in the large-scale fields tend to lie
along the spiral arms, while the field minima tend to be found between them. We
have also determined polarized profiles of 48 pulsars at 430 MHz. We present
morphological pulse profile classifications of the pulsars, based on our new
measurements and previously published data.Comment: Accepted by Astrophysical Journal Supplement, 56 pages, 22 fig