34 research outputs found
Interpreting Flux from Broadband Photometry
We discuss the transformation of observed photometry into flux for the
creation of spectral energy distributions and the computation of bolometric
luminosities. We do this in the context of supernova studies, particularly as
observed with the Swift spacecraft, but the concepts and techniques should be
applicable to many other types of sources and wavelength regimes. Traditional
methods of converting observed magnitudes to flux densities are not very
accurate when applied to UV photometry. Common methods for extinction and the
integration of pseudo-bolometric fluxes can also lead to inaccurate results.
The sources of inaccuracy, though, also apply to other wavelengths. Because of
the complicated nature of translating broad-band photometry into monochromatic
flux densities, comparison between observed photometry and a spectroscopic
model is best done by comparing in the natural units of the observations. We
recommend that integrated flux measurements be made using a spectrum or
spectral energy distribution which is consistent with the multi-band photometry
rather than converting individual photometric measurements to flux densities,
linearly interpolating between the points, and integrating. We also highlight
some specific areas where the UV flux can be mischaracterized.Comment: Accepted for publication in the Astronomical Journal. 16 pages, 9
figures. A PDF file with wide-screen friendly figures is linked from this
blog post
http://ultravioletsupernova.blogspot.com/2016/08/interpreting-flux-from-broadband.htm
SOUSA: the Swift Optical/Ultraviolet Supernova Archive
The Ultra-Violet Optical Telescope on the Swift spacecraft has observed
hundreds of supernovae, covering all major types and most subtypes. Here we
introduce the Swift Optical/Ultraviolet Supernova Archive (SOUSA), which will
contain all of the supernova images and photometry. We describe the observation
and reduction procedures and how they impact the final data. We show photometry
from well-observed examples of most supernova classes, whose absolute
magnitudes and colors may be used to infer supernova types in the absence of a
spectrum. A full understanding of the variety within classes and a robust
photometric separation of the groups requires a larger sample, which will be
provided by the final archive. The data from the existing Swift supernovae are
also useful for planning future observations with Swift as well as future UV
observatories.Comment: Accepted for publication in the UV issue of Astrophysics and Space
Science 10 pages, 6 figures SOUSA is an archive in progress with data being
posted to the Swift SN website:
http://swift.gsfc.nasa.gov/docs/swift/sne/swift_sn.htm
Swift/UVOT Photometry of the Planetary Nebula WeBo 1: Unmasking A Faint Hot Companion Star
We present an analysis of over 150 ks of data on the planetary nebula WeBo 1
(PN G135.6+01.0) obtained with the Swift Ultraviolet Optical Telescope (UVOT).
The central object of this nebula has previously been described as a late-type
K giant barium star with a possible hot companion, most likely a young
pre-white dwarf. UVOT photometry shows that while the optical photometry is
consistent with a large cool object, the near-ultraviolet (UV) photometry shows
far more UV flux than could be produced by any late-type object. Using model
stellar atmospheres and a comparison to UVOT photometry for the pre-white dwarf
PG 1159-035, we find that the companion has a temperature of at least 40,000 K
and a radius of, at most, 0.056 R_sun. While the temperature and radius are
consistent with a hot compact stellar remnant, they are lower and larger,
respectively, than expected for a typical young pre-white dwarf. This likely
indicates a deficiency in the assumed UV extinction curve. We find that higher
temperatures more consistent with expectations for a pre-white dwarf can be
derived if the foreground dust has a strong "blue bump" at 2175 AA and a lower
R_V. Our results demonstrate the ability of Swift to both uncover and
characterize hot hidden companion stars and to constrain the UV extinction
properties of foreground dust based solely on UVOT photometry.Comment: 26 pages, 9 figure, accepted to Astronomical Journa
Faint NUV/FUV Standards from Swift/UVOT, GALEX and SDSS Photometry
At present, the precision of deep ultraviolet photometry is somewhat limited
by the dearth of faint ultraviolet standard stars. In an effort to improve this
situation, we present a uniform catalog of eleven new faint (u sim17)
ultraviolet standard stars. High-precision photometry of these stars has been
taken from the Sloan Digital Sky Survey and Galaxy Evolution Explorer and
combined with new data from the Swift Ultraviolet Optical Telescope to provide
precise photometric measures extending from the Near Infrared to the Far
Ultraviolet. These stars were chosen because they are known to be hot (20,000 <
T_eff < 50,000 K) DA white dwarfs with published Sloan spectra that should be
photometrically stable. This careful selection allows us to compare the
combined photometry and Sloan spectroscopy to models of pure hydrogen
atmospheres to both constrain the underlying properties of the white dwarfs and
test the ability of white dwarf models to predict the photometric measures. We
find that the photometry provides good constraint on white dwarf temperatures,
which demonstrates the ability of Swift/UVOT to investigate the properties of
hot luminous stars. We further find that the models reproduce the photometric
measures in all eleven passbands to within their systematic uncertainties.
Within the limits of our photometry, we find the standard stars to be
photometrically stable. This success indicates that the models can be used to
calibrate additional filters to our standard system, permitting easier
comparison of photometry from heterogeneous sources. The largest source of
uncertainty in the model fitting is the uncertainty in the foreground reddening
curve, a problem that is especially acute in the UV.Comment: Accepted for publication in Astrophysical Journal. 31 pages, 13
figures, electronic tables available from ApJ or on reques
A Large Catalog of Homogeneous Ultra-Violet/Optical GRB Afterglows: Temporal and Spectral Evolution
We present the second Swift Ultra-Violet/Optical Telescope (UVOT) gamma-ray
burst (GRB) afterglow catalog, greatly expanding on the first Swift UVOT GRB
afterglow catalog. The second catalog is constructed from a database containing
over 120,000 independent UVOT observations of 538 GRBs first detected by Swift,
the High Energy Transient Explorer 2 (HETE2), the INTErnational Gamma-Ray
Astrophysics Laboratory (INTEGRAL), the Interplanetary Network (IPN), Fermi,
and Astro-rivelatore Gamma a Immagini Leggero (AGILE). The catalog covers GRBs
discovered from 2005 Jan 17 to 2010 Dec 25. Using photometric information in
three UV bands, three optical bands, and a `white' or open filter, the data are
optimally co-added to maximize the number of detections and normalized to one
band to provide a detailed light curve. The catalog provides positional,
temporal, and photometric information for each burst, as well as Swift Burst
Alert Telescope (BAT) and X-Ray Telescope (XRT) GRB parameters. Temporal slopes
are provided for each UVOT filter. The temporal slope per filter of almost half
the GRBs are fit with a single power-law, but one to three breaks are required
in the remaining bursts. Morphological comparisons with the X-ray reveal that
approximately 75% of the UVOT light curves are similar to one of the four
morphologies identified by Evans et al. (2009). The remaining approximately 25%
have a newly identified morphology. For many bursts, redshift and extinction
corrected UV/optical spectral slopes are also provided at 2000, 20,000, and
200,000 seconds.Comment: 44 pages, 14 figures, to be published in Astrophysical Journal
Supplementa
Early Ultraviolet, Optical and X-Ray Observations of the Type IIP SN 2005cs in M51 with Swift
We report early photospheric-phase observations of the Type IIP Supernova
(SN) 2005cs obtained by Swift's Ultraviolet-Optical and X-Ray Telescopes.
Observations started within two days of discovery and continued regularly for
three weeks. During this time the V-band magnitude remained essentially
constant, while the UV was initially bright but steadily faded until below the
brightness of an underlying UV-bright HII region. This UV decay is similar to
SNe II observed by the International Ultraviolet Explorer. UV grism spectra
show the P-Cygni absorption of MgII 2798A, indicating a photospheric origin of
the UV flux. Based on non-LTE model atmosphere calculations with the CMFGEN
code, we associate the rapid evolution of the UV flux with the cooling of the
ejecta, the peak of the spectral energy distribution (SED) shifting from ~700A
on June 30th to ~1200A on July 5th. Furthermore, the corresponding
recombination of the ejecta, e.g., the transition from FeIII to FeII, induces
considerable strengthening of metal line-blanketing at and above the
photosphere, blocking more effectively this fading UV flux. SN2005cs was not
detected in X-rays, and the upper limit to the X-ray luminosity yields a limit
to the mass loss rate of the progenitor of about 10^-5 solar masses per year.
Overall, Swift presents a unique opportunity to capture the early and fast
evolution of Type II SNe in the UV, providing additional constraints on the
reddening, the SED shortward of 4000A, and the ionization state and temperature
of the photon-decoupling regions.Comment: 15 pages, 6 figures. Accepted for publication by Astrophysical
Journa
THE SWIFT UVOT STARS SURVEY. I. METHODS AND TEST CLUSTERS
We describe the motivations and background of a large survey of nearby stel-
lar populations using the Ultraviolet Optical Telescope (UVOT) aboard the Swift
Gamma-Ray Burst Mission. UVOT, with its wide field, NUV sensitivity, and 2.3
spatial resolution, is uniquely suited to studying nearby stellar populations
and providing insight into the NUV properties of hot stars and the contribution
of those stars to the integrated light of more distant stellar populations. We
review the state of UV stellar photometry, outline the survey, and address
problems spe- cific to wide- and crowded-field UVOT photometry. We present
color-magnitude diagrams of the nearby open clusters M 67, NGC 188, and NGC
2539, and the globular cluster M 79. We demonstrate that UVOT can easily
discern the young- and intermediate-age main sequences, blue stragglers, and
hot white dwarfs, pro- ducing results consistent with previous studies. We also
find that it characterizes the blue horizontal branch of M 79 and easily
identifies a known post-asymptotic giant branch star.Comment: 35 pages, 8 figures, accepted for publication in Astronomical Journa
GRB 130831a: Rise and demise of a magnetar at z = 0.5
Open Access.--14th Marcel Grossman Meeting On Recent Developments in Theoretical and Experimental General Relativity, Astrophysics and Relativistic Field Theories; University of Rome "La Sapienza"Rome; Italy; 12 July 2015 through 18 July 2015; Code 142474.-- http://www.icra.it/mg/mg14/Gamma-ray bursts (GRBs) are the brightest explosions in the universe, yet the properties of their energy sources are far from understood. Very important clues, however, can be deduced by studying the afterglows of these events. We present observations of GRB 130831A and its afterglow obtained with Swift, Chandra, and multiple ground-based observatories. This burst shows an uncommon drop in the X-ray light curve at about 100 ks after the trigger, with a decay slope of α 7. The standard Forward Shock (FS) model offers no explanation for such a behaviour. Instead, a model in which a newly born magnetar outflow powers the early X-ray emission is found to be viable. After the drop, the X-ray afterglow resumes its decay with a slope typical of FS emission. The optical emission, on the other hand, displays no clear break across the X-ray drop and its decay is consistent with that of the late X-rays. Using both the X-ray and optical data, we show that the FS model can explain the emission after 100 ks. We model our data to infer the kinetic energy of the ejecta and thus estimate the efficiency of a magnetar “central engine” of a GRB. Furthermore, we break down the energy budget of this GRB into prompt emission, late internal dissipation, kinetic energy of the relativistic ejecta, and compare it with the energy of the accompanying supernova, SN 2013fu. Copyright © 2018 by the Editors.All rights reserved.Peer reviewe
Very Early Optical Afterglows of Gamma-Ray Bursts: Evidence for Relative Paucity of Detection
Very early observations with the Swift satellite of gamma-ray burst (GRB)
afterglows reveal that the optical component is not detected in a large number
of cases. This is in contrast to the bright optical flashes previously
discovered in some GRBs (e.g. GRB 990123 and GRB 021211). Comparisons of the
X-ray afterglow flux to the optical afterglow flux and prompt gamma-ray fluence
is used to quantify the seemingly deficient optical, and in some cases X-ray,
light at these early epochs. This comparison reveals that some of these bursts
appear to have higher than normal gamma-ray efficiencies. We discuss possible
mechanisms and their feasibility for explaining the apparent lack of early
optical emission. The mechanisms considered include: foreground extinction,
circumburst absorption, Ly-alpha blanketing and absorption due to high
redshift, low density environments, rapid temporal decay, and intrinsic
weakness of the reverse shock. Of these, foreground extinction, circumburst
absorption, and high redshift provide the best explanations for most of the
non-detections in our sample. There is tentative evidence of suppression of the
strong reverse shock emission. This could be because of a
Poynting-flux-dominated flow or a pure non-relativistic hydrodynamical reverse
shock.Comment: 22 pages, 5 figures. Accepted for publication in Ap