3,594 research outputs found
Variations of the Selective Extinction Across the Galactic Bulge - Implications for the Galactic Bar
We propose a new method to investigate the coefficient of the selective
extinction, based on two band photometry. This method uses red clump stars as a
means to construct the reddening curve. We apply this method to the OGLE
color-magnitude diagrams to investigate the variations of the selective
extinction towards various parts of the Galactic bulge. We find that
coefficient is within the errors the same for
OGLE fields. Therefore, the difference of in the extinction
adjusted apparent magnitude of the red clump stars in these fields (Stanek et
al.~1994, 1995) cannot be assigned to a large-scale gradient of the selective
extinction coefficient. This strengthens the implication of this difference as
indicator of the presence of the bar in our Galaxy. However using present data
we cannot entirely exclude the possibility of variations of
the selective extinction coefficient on the large scales across the bulge.Comment: submitted to ApJ Letters, 10 pages, gziped PostScript with figures
included; also available through WWW at
http://www.astro.princeton.edu/~library/prep.htm
An Unexpectedly Swift Rise in the Gamma-ray Burst Rate
The association of long gamma-ray bursts with supernovae naturally suggests
that the cosmic GRB rate should trace the star formation history. Finding
otherwise would provide important clues concerning these rare, curious
phenomena. Using a new estimate of Swift GRB energetics to construct a sample
of 36 luminous GRBs with redshifts in the range z=0-4, we find evidence of
enhanced evolution in the GRB rate, with ~4 times as many GRBs observed at z~4
than expected from star formation measurements. This direct and empirical
demonstration of needed additional evolution is a new result. It is consistent
with theoretical expectations from metallicity effects, but other causes remain
possible, and we consider them systematically.Comment: 4 pages, 4 figures; minor changes to agree with published versio
BVRI Observations of the Optical Afterglow of GRB 990510
We present observations of the optical counterpart to the Gamma-Ray
Burst (GRB) 990510 obtained with the Las Campanas 1.0-m telescope between 15
and 48 hours after the burst. The temporal analysis of the data indicates
steepening decay, independent of wavelength, approaching asymptotically
at early times ( and at
late times, with the break time at . GRB 990510 is the
most rapidly fading of the well-documented GRB afterglows. It is also the first
observed example of broad-band break for a GRB optical counterpart. The optical
spectral energy distribution, corrected for significant Galactic reddening, is
well fitted by a single power-law with . However, when the
-band point is dropped from the fit, the power-law becomes , indicating a possible deviation from the power-law in the spectrum,
either intrinsic or due to additional extinction near the source or from an
intervening galaxy at . Broad-band break behavior broadly similar to
that observed in GRB 990510 has been predicted in some jet models of GRB
afterglows, thus supporting the idea that the GRB energy is beamed, at least in
some cases.Comment: submitted to the ApJ Letters, 13 pages, 5 tables, 3 figures;
additional data available at ftp://cfa-ftp.harvard.edu/pub/kstanek/GRB990510/
and through WWW at http://cfa-www.harvard.edu/cfa/oir/Research/GRB
On the Redshift Distribution of Gamma Ray Bursts in the Swift Era
A simple physical model for long-duration gamma ray bursts (GRBs) is used to
fit the redshift (z) and the jet opening-angle distributions measured with
earlier GRB missions and with Swift. The effect of different sensitivities for
GRB triggering is sufficient to explain the difference in the z distributions
of the pre-Swift and Swift samples, with mean redshifts of ~1.5 and
~2.7, respectively. Assuming that the emission properties of GRBs do not
change with time, we find that the data can only be fitted if the comoving
rate-density of GRB sources exhibits positive evolution to z >~ 3-5. The mean
intrinsic beaming factor of GRBs is found to range from ~34-42, with the Swift
average opening half-angle ~10 degree, compared to the pre-Swift
average of ~7 degree. Within the uniform jet model, the GRB
luminosity function is proportional to L^{-3.25}_*, as inferred from our best
fit to the opening angle distribution. Because of the unlikely detection of
several GRBs with z <~ 0.25, our analysis indicates that low redshift GRBs
represent a different population of GRBs than those detected at higher
redshifts. Neglecting possible metallicity effects on GRB host galaxies, we
find that ~1 GRB occurs every 600,000 yrs in a local L_* spiral galaxy like the
Milky Way. The fraction of high-redshift GRBs is estimated at 8-12% and 2.5-6%
at z >= 5 and z >= 7, respectively, assuming continued positive evolution of
the GRB rate density to high redshifts.Comment: Accepted for publication in ApJ. The paper contains 29 pages and 24
figure
Brownian movement of inorganic nanoparticles in sediments
The 100 nm particles of and metallic Fe sedimented jointly with powder from their suspension in oleic acid exhibit distinguished mobility which depends on the concentration of aluminum oxide. This observation is interpreted as the result of the interparticle Fe-Fe magnetic interactions which lead to the formation of the rigid network of magnetic metallic iron nanoparticles
Late-Time Optical Afterglow Observations with LBT and MDM
Using the 2.4m MDM and 8.4m Large Binocular Telescope, we observed nine GRB
afterglows to systematically probe the late time behaviors of afterglows
including jet breaks, flares, and supernova bumps. In particular, the LBT
observations have typical flux limits of 25-26 mag in the Sloan r' band, which
allows us to extend the temporal baseline for measuring jet breaks by another
decade in time scale. We detected four jet breaks (including a "textbook" jet
break in GRB070125) and a fifth candidate, all of which are not detectable
without deep, late time optical observations. In the other four cases, we do
not detect the jet breaks either because of contamination from the host galaxy
light, the presence of a supernova bump, or the intrinsic faintness of the
optical afterglow. This suggests that the basic picture that GRBs are
collimated is still valid and that the apparent lack of Swift jet breaks is due
to poorly sampled afterglow light curves, particularly at late times. Besides
the jet breaks, we also detected late time flares, which could attribute to
late central engine activities, and two supernova bumps.Comment: 5 pages, 5 figures, 2008 NANJING GAMMA-RAY BURST CONFERENCE. AIP
Conference Proceedings, Volume 1065, pp. 93-97 (2008), Eds. Y.F. Huang, Z.G.
Dai, B. Zhan
The Optical Gravitational Lensing Experiment. Catalog of stellar proper motions in the OGLE-II Galactic bulge fields
We present a proper motion (\mu) catalogue of 5,080,236 stars in 49 Optical
Gravitational Lensing Experiment II (OGLE-II) Galactic bulge (GB) fields,
covering a range of -11 deg. <l< 11 deg. and -6 deg. <b<3 deg., the total area
close to 11 square degrees. The proper motion measurements are based on 138 -
555 I-band images taken during four observing seasons: 1997-2000. The catalogue
stars are in the magnitude range 11 < I < 18 mag. In particular, the catalogue
includes Red Clump Giants (RCGs) and Red Giants in the GB, and main sequence
stars in the Galactic disc. The proper motions up to \mu = 500 mas/yr were
measured with the mean accuracy of 0.8-3.5 mas/yr, depending on the brightness
of a star. This catalogue may be useful for studying the kinematic of stars in
the GB and the Galactic disk.Comment: 13 pages, 16 figures, MNRAS in pres
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