1,119 research outputs found
Absolute Flux Calibration of the IRAC Instrument on the Spitzer Space Telescope using Hubble Space Telescope Flux Standards
The absolute flux calibration of the James Webb Space Telescope will be based
on a set of stars observed by the Hubble and Spitzer Space Telescopes. In order
to cross-calibrate the two facilities, several A, G, and white dwarf (WD) stars
are observed with both Spitzer and Hubble and are the prototypes for a set of
JWST calibration standards. The flux calibration constants for the four Spitzer
IRAC bands 1-4 are derived from these stars and are 2.3, 1.9, 2.0, and 0.5%
lower than the official cold-mission IRAC calibration of Reach et al. (2005),
i.e. in agreement within their estimated errors of ~2%. The causes of these
differences lie primarily in the IRAC data reduction and secondarily in the
SEDs of our standard stars. The independent IRAC 8 micron band-4 fluxes of
Rieke et al. (2008) are about 1.5 +/- 2% higher than those of Reach et al. and
are also in agreement with our 8 micron result.Comment: 16 pages, 6 figure
An analysis of the shapes of interstellar extinction curves. VII Milky Way spectrophotometric optical-through-ultraviolet extinction and its R-dependence
We produce a set of 72 NIR-through-UV extinction curves by combining new Hubble Space Telescope/STIS optical spectrophotometry with existing International Ultraviolet Explorer spectrophotometry (yielding gapless coverage from 1150 to 10000 ?) and NIR photometry. These curves are used to determine a new, internally consistent NIR-through-UV Milky Way mean curve and to characterize how the shapes of the extinction curves depend on R(V). We emphasize that while this dependence captures much of the curve variability, considerable variation remains that is independent of R(V). We use the optical spectrophotometry to verify the presence of structure at intermediate wavelength scales in the curves. The fact that the optical-through-UV portions of the curves are sampled at relatively high resolution makes them very useful for determining how extinction affects different broadband systems, and we provide several examples. Finally, we compare our results to previous investigations
New Ultraviolet Extinction Curves for Interstellar Dust in M31
New low-resolution UV spectra of a sample of reddened OB stars in M31 were
obtained with HST/STIS to study the wavelength dependence of interstellar
extinction and the nature of the underlying dust grain populations. Extinction
curves were constructed for four reddened sightlines in M31 paired with closely
matching stellar atmosphere models. The new curves have a much higher S/N than
previous studies. Direct measurements of N(H I) were made using the Ly
absorption lines enabling gas-to-dust ratios to be calculated. The sightlines
have a range in galactocentric distance of 5 to 14 kpc and represent dust from
regions of different metallicities and gas-to-dust ratios. The metallicities
sampled range from Solar to 1.5 Solar. The measured curves show similarity to
those seen in the Milky Way and the Large Magellanic Cloud. The Maximum Entropy
Method was used to investigate the dust composition and size distribution for
the sightlines observed in this program finding that the extinction curves can
be produced with the available carbon and silicon abundances if the metallicity
is super-Solar.Comment: ApJ, in press, 9 pages, 5 figure
Strange magnification pattern in the large separation lens SDSS J1004+4112 from optical to X-rays
We present simultaneous XMM-Newton UV and X-ray observations of the quadruply
lensed quasar SDSS J1004+4112 (RBS 825). Simultaneously with the XMM-Newton
observations we also performed integral field spectroscopy on the two closest
lens images A and B using the Calar Alto PMAS spectrograph. In X-rays the
widely spaced components C and D are clearly resolved, while the closer pair of
images A and B is marginally resolved in the XMM-EPIC images. The integrated
X-ray flux of the system has decreased by a factor of 6 since it was observed
in the ROSAT All Sky Survey in 1990, while the X-ray spectrum became much
harder with the power law index evolving from Gamma=-2.3 to -1.86. By
deblending the X-ray images of the lensed QSO we find that the X-ray flux
ratios between the lens images A and B are significantly different from the
simultaneously obtained UV ratios and previously measured optical flux ratios.
Our optical spectrum of lens image A shows an enhancement in the blue emission
line wings, which has been observed in previous epochs as a transient feature.
We propose a scenario where intrinsic UV and X-ray variability gives rise to
line variations which are selectively magnified in image A by microlensing. The
extended emission of the lensing cluster of galaxies is clearly detected in the
EPIC images, we measure a 0.5-2.0 keV luminosity of 1.4 E44 erg/s. Based on the
cluster X-ray properties, we estimate a mass of 2-6 E14 solar masses.Comment: 11 pages, 10 figures, accepted for publication in Astronomy &
Astrophysic
Measuring the properties of extragalactic dust and implications for the Hubble diagram
Scattering and absorption of light by a homogeneous distribution of
intergalactic large dust grains has been proposed as an alternative,
non-cosmological explanation for the faintness of Type Ia supernovae at z\s im
0.5. We investigate the differential extinction for high-redshift sources
caused by extragalactic dust along the line of sight.
Future observations of Type Ia supernovae up to , e.g. by the
proposed SNAP satellite, will allow the measurement of the properties of dust
over cosmological distances. We show that 1% {\em relative} spectrophotometric
accuracy (or broadband photometry) in the wavelength interval 0.7--1.5 m
is required to measure the extinction caused by ``grey'' dust down to magnitudes.
We also argue that the presence of grey dust is not necessarily inconsistent
with the recent measurement of the brightness of a supernova at (SN
1997ff), in the absence of accurate spectrophotometric information of the
supernova.Comment: Accepted by A&
The Responses of the University of Iowa Charged Particle Detectors on Explorer 12 Graphic Summary
Graphs of charged particle detectors responses on Explorer XII satellit
On the photon polarization in radiative B -> phi K gamma decay
The photon polarization in radiative decays B -> Y gamma is known to be a
subtle probe of the effective Lagrangian structure and possible New Physics
effects. We discuss exclusive decay mode B -> phi K gamma where the
experimentally distinct final state makes analysis especially promising. The
possibility to extract information on the photon polarization out of the data
entirely depends on the partial waves interference pattern in the phi K system.Comment: RevTeX, 6 pages, 1 figure; the journal versio
Optical spectroscopy of the microquasar GRS 1758-258: a possible intermediate mass system?
Context. GRS 1758-258 is one of two prototypical microquasars towards the
Galactic Center direction discovered almost a quarter of a century ago. The
system remains poorly studied in the optical domain due to its counterpart
being a very faint and absorbed target in a crowded region of the sky. Aims.
Our aim is to investigate GRS 1758-258 in order to shed light on the nature of
the stellar binary components. In particular, the main physical parameters of
the donor star, such as the mass or the spectral type, are not yet well
constrained. Methods. GRS 1758-258 has remained so far elusive to optical
spectroscopy owing to its observational difficulties. Here, we use this
traditional tool of stellar astronomy at low spectral resolution with a 10 m
class telescope and a long slit spectrograph. Results. An improved spectrum is
obtained as compared to previous work. The quality of the data does not allow
the detection of emission or absorption features but, nevertheless, we manage
to partially achieve our aims comparing the de-reddened continuum with the
spectral energy distribution expected from an irradiated disc model and
different donor star templates. Conclusions. We tentatively propose that GRS
1758-258 does not host a giant star companion. Instead, a main sequence star
with mid-A spectral type appears to better agree with our data. The main
impacts of this finding are the possibility that we are dealing with an
intermediate mass system and, in this case, the prediction of an orbital period
significantly shorter than previously proposed.Comment: 5 pages, 6 figures, accepted for publication in A&
Discovery of a massive variable star with Z=Zo/36 in the galaxy DDO 68
The Local Volume dwarf galaxy DDO 68, from the spectroscopy of its two
brightest HII regions (Knots 1 and 2) was designated as the second most
metal-poor star-forming galaxy [12+log(O/H)=7.14]. In the repeated spectral
observations in 2008 January with the 6-m telescope (BTA) of the HII region
Knot 3 [having 12+log(O/H)=7.10+-0.06], we find a strong evidence of a
transient event related to a massive star evolution. From the follow-up
observation with the higher spectral resolution in 2008 February, we confirm
this phenomenon, and give parameters of its emission-line spectrum comprising
of Balmer HI and HeI lines. The luminosities of the strongest transient lines
(Ha, Hb) are of a few 10^36 erg s^-1. We also detected an additional continuum
component in the new spectrum of Knot 3, which displays the spectral energy
distribution raising to ultraviolet. The estimate of the flux of this continuum
leads us to its absolute V-band magnitude of ~-7.1. Based on the spectral
properties of this transient component, we suggest that it is related to an
evolved massive star of luminous blue variable type with Z=Zo/36. We briefly
discuss observational constraints on parameters of this unique (in the aspect
of the record low metallicity of the progenitor massive star) event and propose
several lines of its study.Comment: 6 pages, 5 Postscript figures, to appear in MNRAS Letters in June
2008 issu
- …