25 research outputs found
A Method of Correcting Near-Infrared Spectra for Telluric Absorption
We present a method for correcting near-infrared medium-resolution spectra
for telluric absorption. The method makes use of a spectrum of an A0V star,
observed near in time and close in airmass to the target object, and a
high-resolution model of Vega, to construct a telluric correction spectrum that
is free of stellar absorption features. The technique was designed specifically
to perform telluric corrections on spectra obtained with SpeX, a 0.8-5.5
micron, medium-resolution cross-dispersed spectrograph at the NASA Infrared
Telescope Facility, and uses the fact that for medium resolutions there exist
spectral regions uncontaminated by atmospheric absorption lines. However, it is
also applicable (in a somewhat modified form) to spectra obtained with other
near-infrared spectrographs. An IDL-based code that carries out the procedures
is available for downloading via the World Wide Web from the IRTF website.Comment: 39 pages, 10 figures, To appear in the Feb 2003 issue of PASP; IDL
source code, as well as full resolution versions of the figures, are
available at http://irtfweb.ifa.hawaii.edu/Facility/spex
Near-Infrared Light Curves of the Black Hole Binary A0620-00
We measured the near-infrared orbital light curve of the black hole binary
A0620-00 in 1995 and 1996. The light curves show an asymmetric, double-humped
modulation with extra emission in the peak at orbital phase 0.75. There were no
significant changes in the shape of the light curve over the one-year
observation period. There were no sharp dips in the light curves nor reversals
of the asymmetry between the two peaks as seen in earlier observations. The
light curves are well fit by models incorporating ellipsoidal variations from
the mass-losing K-type star plus a beamed bright spot on the accretion disk
around the compact star. The long-term stability of the light curve shape rules
out superhumps and star spots as sources of asymmetry when we observed
A0620-00. The ellipsoidal variations yield a lower limit i >= 38 deg on the
orbital inclination. The light curves show no eclipse features, which places an
upper limit i <= 75 deg. This range of inclinations constrains the mass of the
compact object to 3.3 < M_1 < 13.6 Msun. The light curves do not further
constrain the orbital inclination because the contribution of the accretion
disk to the observed flux is unknown. We argue that a previous attempt to
measure the near-infrared flux from the accretion disk using the dilution of
the 12CO(2,0) bandhead in the spectrum of the K star is not reliable because
the band strength depends strongly on surface gravity.Comment: Accepted for publication in the Astronomical Journal. 17 pages, 4
figures. Prepared using AASTEX V. 5.
RJK Observations of the Optical Afterglow of GRB 991216
We present near-infrared and optical observations of the afterglow to the
Gamma-Ray Burst (GRB) 991216 obtained with the F. L. Whipple Observatory 1.2-m
telescope and the University of Hawaii 2.2-m telescope. The observations range
from 15 hours to 3.8 days after the burst. The temporal behavior of the data is
well described by a single power-law decay with index -1.36 +/-0.04,
independent of wavelength. The optical spectral energy distribution, corrected
for significant Galactic reddening of E(B-V)=0.626, is well fitted by a single
power-law with index -0.58 +/- 0.08. Combining the IR/optical observations with
a Chandra X-ray measurement gives a spectral index of -0.8 +/- 0.1 in the
synchrotron cooling regime. A comparison between the spectral and temporal
power-law indices suggest that a jet is a better match to the observations than
a simple spherical shock.Comment: Accepted to the Astrophysical Journal, 12 pages, 4 postscript figure
Toward 1% Photometry: End-to-end Calibration of Astronomical Telescopes and Detectors
We review the systematic uncertainties that have plagued attempts to obtain
high precision and high accuracy from ground-based photometric measurements
using CCDs. We identify two main challenges in breaking through the 1%
precision barrier: 1) fully characterizing atmospheric transmission, along the
instrument's line of sight, and 2) properly identifying, measuring and removing
instrumental artifacts. We discuss approximations and limitations inherent in
the present methodology, and we estimate their contributions to systematic
photometric uncertainties. We propose an alternative conceptual scheme for the
relative calibration of astronomical apparatus: the availability of calibrated
detectors whose relative spectral sensitivity is known to better than one part
in opens up the possibility of in situ relative throughput measurements,
normalized to a precision calibrated detector, using a stable but uncalibrated
narrowband light source. An implementation scheme is outlined, which exploits
the availability of tunable lasers to map out the relative wavelength response
of an imaging system, using a flatfield screen and a calibrated reference
photodiode. The merits and limitations of this scheme are discussed. In tandem
with careful measurements of atmospheric transmission, this approach could
potentially lead to reliable ground-based photometry with fractional
uncertainties below the percent level.Comment: 25 pages, no figures. To be published in Ap
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
A Study of the B-V Colour Temperature Relation
We attempt to construct a B-V colour temperature relation for stars in the
least model dependent way employing the best modern data. The fit we obtained
with the form Teff = Teff((B-V)0,[Fe/H],log g) is well constrained and a number
of tests show the consistency of the procedures for the fit. Our relation
covers from F0 to K5 stars with metallicity [Fe/H] = -1.5 to +0.3 for both
dwarfs and giants. The residual of the fit is 66 K, which is consistent with
what are expected from the quality of the present data. Metallicity and surface
gravity effects are well separated from the colour dependence. Dwarfs and
giants match well in a single family of fit, differing only in log g. The fit
also detects the Galactic extinction correction for nearby stars with the
amount E(B-V) = 0.26 +/-0.03 mag/kpc. Taking the newly obtained relation as a
reference we examine a number of B-V colour temperature relations and
atmosphere models available in the literature. We show the presence of a
systematic error in the colour temperature relation from synthetic calculations
of model atmospheres; the systematic error across K0 to K5 dwarfs is 0.04-0.05
mag in B-V, which means 0.25-0.3 mag in Mv for the K star range. We also argue
for the error in the temperature scale used in currently popular stellar
population synthesis models; synthetic colours from these models are somewhat
too blue for aged elliptical galaxies. We derive the colour index of the sun
(B-V)sun = 0.627 +/-0.018, and discuss that redder colours (e.g., 0.66-0.67)
often quoted in the literature are incompatible with the colour-temperature
relation.Comment: AASLaTeX (aaspp4.sty),36 pages (13 figures included), submitted to
Astronomical Journal, replaced (typo in author name
Gaia broad band photometry
The scientific community needs to be prepared to analyse the data from Gaia,
one of the most ambitious ESA space missions, to be launched in 2012. The
purpose of this paper is to provide data and tools in order to predict in
advance how Gaia photometry is expected to be. To do so, we provide
relationships among colours involving Gaia magnitudes and colours from other
commonly used photometric systems (Johnson-Cousins, SDSS, Hipparcos and Tycho).
The most up-to-date information from industrial partners has been used to
define the nominal passbands and based on the BaSeL3.1 stellar spectral energy
distribution library, relationships were obtained for stars with different
reddening values, ranges of temperatures, surface gravities and metallicities.
The transformations involving Gaia and Johnson-Cousins V-I_C and Sloan DSS g-z
colours have the lowest residuals. A polynomial expression for the relation
between the effective temperature and the colour G_BP-G_RP was derived for
stars with T > 4500 K. Transformations involving two Johnson or two Sloan DSS
colours yield lower residuals than using only one colour. We also computed
several ratios of total-to-selective absorption including absorption A_G in the
G band and colour excess E(G_BP-G_RP) for our sample stars. A relationship,
involving A_G/A_V and the intrinsic (V-I_C) colour, is provided. The derived
Gaia passbands have been used to compute tracks and isochrones using the Padova
and BASTI models. Finally, the performances of the predicted Gaia magnitudes
have been estimated according to the magnitude and the celestial coordinates of
the star. The provided dependencies among colours can be used for planning
scientific exploitation of Gaia data, performing simulations of the Gaia-like
sky, planning ground-based complementary observations and for building
catalogues with auxiliary data for the Gaia data processing and validation.Comment: 15 pages and 19 figure (accepted in A&A
Photometry and Spectroscopy of GRB 030329 and Its Associated Supernova 2003dh: The First Two Months
We present extensive optical and infrared photometry of the afterglow of
gamma-ray burst (GRB) 030329 and its associated supernova (SN) 2003dh over the
first two months after detection (2003 March 30-May 29 UT). Optical
spectroscopy from a variety of telescopes is shown and, when combined with the
photometry, allows an unambiguous separation between the afterglow and
supernova contributions. The optical afterglow of the GRB is initially a
power-law continuum but shows significant color variations during the first
week that are unrelated to the presence of a supernova. The early afterglow
light curve also shows deviations from the typical power-law decay. A supernova
spectrum is first detectable ~7 days after the burst and dominates the light
after ~11 days. The spectral evolution and the light curve are shown to closely
resemble those of SN 1998bw, a peculiar Type Ic SN associated with GRB 980425,
and the time of the supernova explosion is close to the observed time of the
GRB. It is now clear that at least some GRBs arise from core-collapse SNe.Comment: 57 pages, 13 figures, accepted by ApJ, revised per referee's
comments, includes full photometry table. Data available at
ftp://cfa-ftp.harvard.edu/pub/kstanek/GRB030329 or through WWW at
http://cfa-www.harvard.edu/cfa/oir/Research/GRB
Discovery of the Low-Redshift Optical Afterglow of GRB 011121 and Its Progenitor Supernova 2001ke
We present the discovery and follow-up observations of the afterglow of the
GRB 011121 and its associated supernova SN 2001ke. Images were obtained with
the OGLE 1.3m telescope in BVRI passbands, starting 10.3 hours after the burst.
The temporal analysis of our early data indicates a steep decay, independent of
wavelength with power-law index alpha=-1.72+/-0.05. There is no evidence for a
break in the light curve earlier than 2.5 days after the burst. The spectral
energy distribution determined from the early broad-band photometry is a
power-law with index beta=-0.66+/-0.13 after correcting for a large reddening.
Spectra, obtained with the Magellan 6.5m Baade telescope, reveal narrow
emission lines from the host galaxy which provide a redshift of z=0.362+/-0.001
to the GRB. We also present late R and J-band observations of the afterglow
between 7-17 days after the burst. The late-time photometry shows a large
deviation from the initial decline and our data combined with Hubble Space
Telescope photometry provide strong evidence for a supernova peaking about 12
rest-frame days after the GRB. The first spectrum ever obtained of a GRB
supernova at cosmological distance revealed a blue continuum. SN 2001ke was
more blue near maximum than SN 1998bw and faded more quickly which demonstrates
that a range of properties are possible in supernovae which generate GRB. The
blue color is consistent with a supernova interacting with circumstellar gas
and this progenitor wind is also evident in the optical afterglow. This is the
best evidence to date that classical, long gamma-ray bursts are generated by
core-collapse supernovae.Comment: 22 pages, 9 postscript figures. Submitted to Ap