38,451 research outputs found
The Thousand Star Magnitudes in the Catalogues of Ptolemy, Al Sufi, and Tycho Are All Corrected For Atmospheric Extinction
Three pre-telescopic star catalogues contain about a thousand star magnitudes
each (with magnitudes 1, 2, 3, 4, 5, and 6), with these reported brightnesses
as the original basis for what has become the modern magnitude scale. These
catalogues are those of Ptolemy (c. 137, from Alexandria at a latitude of
31.2), Al Sufi (c. 960, from Isfahan at a latitude of 32.6), and Tycho Brahe
(c. 1590, from the island of Hven at a latitude of 55.9). Previously, extensive
work has been made on the positions of the catalogued stars, but only scant
attention has been paid to the magnitudes as reported. These magnitudes will be
affected by a variety of processes, including the dimming of the light by our
Earth's atmosphere (atmospheric extinction), the quantization of the
brightnesses into magnitude bins, and copying or influence from prior
catalogues. This paper provides a detailed examination of these effects.
Indeed, I find all three catalogues to report magnitudes that have near-zero
extinction effects, so the old observers in some way extinction corrected their
observations.Comment: Four appendices appear only in the on-line edition available to
subscribers (see http://www.shpltd.co.uk/jha.html), or in the last half of
this arXiv PDF fil
Explaining the Gamma-Ray Burst E_peak Distribution
The characteristic photon energy for Gamma Ray Bursts, E_peak, has a
remarkably narrow distribution for bursts of similar peak flux, with values
between 150 and 600 keV for most faint bursts. This result is surprising within
the framework of internal shock models, since spectral shifts associated with
the jet's blue shift (by a Lorentz factor of Gamma) and the cosmological red
shift (by a factor of 1+z) should cause substantial smearing in the
distribution of the spectral peak in the jet's co-moving frame, E_rest. For the
general case where the luminosity (L) varies as Gamma^N and E_rest varies as
Gamma^M, then the observed E_peak will vary as L^{(M+1)/N}(1+z)^{-1}. For two
independent set of 20 and 84 bursts, E_peak(1+z) varies as a power law of the
luminosity with an index of (M+1)/N=0.36+-0.03. With this measured value, the
above functional dependence of E_peak on L and z results in E_peak being
roughly constant for bursts of similar peak flux, P_256. Thus, the kinematic
smearing will be small, hence allowing the E_peak distribution to be narrow.
This model also predicts that bright bursts will have high E_peak values
because they all have some combination of high luminosity (and hence a large
blue shift Gamma) and a nearby distance (and hence a small cosmological red
shift). Quantitatively, E_peak should vary roughly as P_256^0.36, and this
model prediction is strikingly confirmed with BATSE data by Mallozzi et al. A
prediction of this model is that GRBs at very high red shift z~10 should all
appear with E_peak at ~200 keV. A further prediction of this model is that
normal bursts with P_256 below the BATSE trigger threshold will appear as x-ray
flashes with E_peak~70 keV; just as is reported by Kippen et al. and Heise et
al.Comment: ApJ Letters in press, 16 pages, 3 figure
Severe Limits on Variations of the Speed of Light with Frequency
Explosive astrophysical events at high red shift can be used to place severe
limits on the fractional variation in the speed of light (), the
photon mass (), and the energy scale of quantum gravity ().
I find based on the simultaneous arrival of
a flare in GRB 930229 with a rise time of for photons of 30
keV and 200 keV. The limit on is for GRB
980703 from radio to gamma ray observations. The limit on is GeV for GRB 930131 from 30 keV to 80 MeV photons.Comment: 7 pages, Submitted to PR
All Known Hot RCB Stars Are Fading Fast Over the Last Century
The R Coronae Borealis (RCB) stars are cool supergiants that display
irregular and deep dips in their light curves, caused by dust formation. There
are four known hot RCB stars (DY Cen, MV Sgr, V348 Sgr, and HV 2671), with
surface temperatures of 15,000--25,000 K, and prior work has suggested that
three of these have secular fading in brightness. I have tested this result by
measuring century-long light curves in the Johnson B-band with modern
comparison star magnitudes, and I have extended this by measuring many
magnitudes over a wide time range as well as for the fourth hot RCB star. In
all four cases, the B-band magnitude of the maximum light is now fast fading.
The fading rates (in units of magnitudes per century) are 2.5 for DY Cen after
1960, 1.3 for MV Sgr, 1.3 for V348 Sgr, and 0.7 for HV 2671. This secular
fading is caused by the expected evolution of the star across the top of the HR
diagram at constant luminosity, as the temperature rises and the bolometric
correction changes. For DY Cen, the brightness at maximum light is rising from
1906 to 1932, and this is caused by the temperature increase from near 5,800 to
7,500 K. Before 1934 DY Cen had frequent dust dips, while after 1934 there are
zero dust dips, so there is some apparent connection between the rising
temperature and the formation of the dust. Thus, we have watched DY Cen evolve
from an ordinary RCB star up to a hot RCB star and now appearing as an extreme
helium star, all in under one century.Comment: Just published in Monthly Notices of the Royal Astronomical Society.
11 pages, 3 figure
An Analytical Study for Subsonic Oblique Wing Transport Concept
For abstract, see N77-10045
Investigation of the Progenitors of the Type Ia Supernovae Associated With the LMC Supernova Remnants 0505-67.9 and 0509-68.7
Although Type Ia supernovae have been heavily scrutinized due to their use in
making cosmological distance estimates, we are still unable to definitively
identify the progenitors for the entire population. While answers have been
presented for certain specific systems, a complete solution remains elusive. We
present observations of two supernova remnants (SNRs) in the Large Magellanic
Cloud, SNR 0505-67.9 and SNR 0509-68.7, for which we have identified the center
of the remnant and the 99.73% containment central region in which any companion
star left over after the supernova must be located. Both remnants have a number
of potential ex-companion stars near their centers; all possible single and
double degenerate progenitor models remain viable for these two supernovae.
Future observations may be able to identify the true ex-companions for both
remnants.Comment: 8 pages, 4 figures, 4 tables, ApJ In Press; Table 2 truncated, full
version available in published paper or directly from author
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