380 research outputs found
An Updated Ultraviolet Calibration for the Swift/UVOT
We present an updated calibration of the Swift/UVOT broadband ultraviolet
(uvw1, uvm2, and uvw2) filters. The new calibration accounts for the ~1% per
year decline in the UVOT sensitivity observed in all filters, and makes use of
additional calibration sources with a wider range of colours and with HST
spectrophotometry. In this paper we present the new effective area curves and
instrumental photometric zeropoints and compare with the previous calibration.Comment: 4 pages, 3 figures, 2 tables. Presented at GRB 2010 symposium,
Annapolis, November 2010 to be published in American Institute of Physics
Conference Serie
Bolometric and UV Light Curves of Core-Collapse Supernovae
The Swift UV-Optical Telescope (UVOT) has been observing Core-Collapse
Supernovae (CCSNe) of all subtypes in the UV and optical since 2005. We present
here 50 CCSNe observed with the Swift UVOT, analyzing their UV properties and
behavior. Where we have multiple UV detections in all three UV filters (\lambda
c = 1928 - 2600 \AA), we generate early time bolometric light curves, analyze
the properties of these light curves, the UV contribution to them, and derive
empirical corrections for the UV-flux contribution to optical-IR based
bolometric light curves
Multi-wavelength Observations of the Type IIb Supernova 2009mg
We present Swift UVOT and XRT observations, and visual wavelength
spectroscopy of the Type IIb supernova (SN) 2009mg, discovered in the Sb galaxy
ESO 121-G26. The observational properties of SN 2009mg are compared to the
prototype Type IIb SNe 1993J and 2008ax, with which we find many similarities.
However, minor differences are discernible including SN 2009mg not exhibiting
an initial fast decline or u-band upturn as observed in the comparison objects,
and its rise to maximum is somewhat slower leading to slightly broader light
curves. The late-time temporal index of SN 2009mg, determined from 40 days
post-explosion, is consistent with the decay rate of SN 1993J, but inconsistent
with the decay of 56Co. This suggests leakage of gamma-rays out of the ejecta
and a stellar mass on the small side of the mass distribution. Our XRT
non-detection provides an upper limit on the mass-loss rate of the progenitor
of <1.5x10^-5 Msun per yr. Modelling of the SN light curve indicates a kinetic
energy of 0.15 (+0.02,-0.13) x10^51 erg, an ejecta mass of 0.56(+0.10,-0.26)
Msun and a 56Ni mass of 0.10\pm0.01 Msun.Comment: 10 pages, 8 figures, accepted for publication in MNRA
GRB 081203A: Swift UVOT captures the earliest ultraviolet spectrum of a gamma-ray burst
We present the earliest ultraviolet (UV) spectrum of a gamma-ray burst (GRB) as observed with the Swift Ultra-Violet/Optical Telescope (UVOT). The GRB 081203A spectrum was observed for 50 s with the UV-grism starting 251 s after the Swift-Burst-Alert-Telescope (BAT) trigger. During this time, the GRB was ≈13.4 mag (u filter) and was still rising to its peak optical brightness. In the UV-grism spectrum, we find a damped Lyα line, Lyβ and the Lyman continuum break at a redshift z= 2.05 ± 0.01. A model fit to the Lyman absorption implies a gas column density of log NH i= 22.0 ± 0.1 cm−2, which is typical of GRB host galaxies with damped Lyα absorbers. This observation of GRB 081203A demonstrates that for brighter GRBs (v≈ 14 mag) with moderate redshift (0.5 < z < 3.5) the UVOT is able to provide redshifts, and probe for damped Lyα absorbers within 4–6 min from the time of the Swift-BAT trigger
Swift UVOT Observations of Core-Collapse SNe
We review recent UV observations of core-collapse supernovae (SNe) with the
Swift Ultra-violet/Optical Telescope (UVOT) during its first two years.
Rest-frame UV photometry is useful for differentiating SN types by exploiting
the UV-optical spectral shape and more subtle UV features. This is useful for
the real-time classification of local and high-redshift SNe using only
photometry. Two remarkable SNe Ib/c were observed with UVOT -- SN2006jc was a
UV bright SN Ib. Swift observations of GRB060218/SN2006aj began shortly after
the explosion and show a UV-bright peak followed by a UV-faint SN bump. UV
observations are also useful for constraining the temperature and ionization
structure of SNe IIP. Rest-frame UV observations of all types are important for
understanding the extinction, temperature, and bolometric luminosity of SNe and
to interpret the observations of high redshift SNe observed at optical
wavelengths.Comment: Figures are enlarged and colorized from print versio
Prompt optical observations of GRB050319 with the Swift UVOT
The UVOT telescope on the Swift observatory has detected optical afterglow
emission from GRB 050319. The flux declines with a power law slope of alpha =
-0.57 between the start of observations some 230 seconds after the burst onset
(90s after the burst trigger) until it faded below the sensitivity threshold of
the instrument after ~5 x 10^4s. There is no evidence for the rapidly declining
component in the early light curve that is seen at the same time in the X-ray
band. The afterglow is not detected in UVOT shortward of the B-band, suggesting
a redshift of about 3.5. The optical V-band emission lies on the extension of
the X-ray spectrum, with an optical to X-ray slope of beta = -0.8. The
relatively flat decay rate of the burst suggests that the central engine
continues to inject energy into the fireball for as long as a few x 10^4s after
the burst.Comment: 16 pages, 4 figures, accepted by Ap
Photometric Calibration of the Swift Ultraviolet/Optical Telescope
We present the photometric calibration of the Swift UltraViolet/Optical
Telescope (UVOT) which includes: optimum photometric and background apertures,
effective area curves, colour transformations, conversion factors for count
rates to flux, and the photometric zero points (which are accurate to better
than 4 per cent) for each of the seven UVOT broadband filters. The calibration
was performed with observations of standard stars and standard star fields that
represent a wide range of spectral star types. The calibration results include
the position dependent uniformity, and instrument response over the 1600-8000A
operational range. Because the UVOT is a photon counting instrument, we also
discuss the effect of coincidence loss on the calibration results. We provide
practical guidelines for using the calibration in UVOT data analysis. The
results presented here supersede previous calibration results.Comment: Minor improvements after referees report. Accepted for publication in
MNRA
Correlated X-ray/Ultraviolet/Optical variability in the very low mass AGN NGC 4395
We report the results of a one year Swift X-ray/UV/optical programme
monitoring the dwarf Seyfert nucleus in NGC 4395 in 2008-2009. The UV/optical
flux from the nucleus was found to vary dramatically over the monitoring
period, with a similar pattern of variation in each of the observed UV/optical
bands (spanning 1900 - 5500 {\AA}). In particular, the luminosity of NGC 4395
in the 1900 {\AA} band changed by more than a factor of eight over the
monitoring period. The fractional variability was smaller in the UV/optical
bands than that seen in the X-rays, with the X-ray/optical ratio increasing
with increasing flux. Pseudo-instantaneous flux measurements in the X-ray and
each UV/optical band were well correlated, with cross correlation coefficients
of >0.7, significant at 99.9 per cent confidence. Archival Swift observations
from 2006 sample the intra-day X-ray/optical variability on NGC 4395. These
archival data show a very strong correlation between the X-ray and b bands,
with a cross-correlation coefficient of 0.84 (significant at >99 per cent
confidence). The peak in the cross correlation function is marginally resolved
and asymmetric, suggesting that X-rays lead the b band, but by 1 hour. In
response to recent (August 2011) very high X-ray flux levels from NGC4395 we
triggered Swift ToO observations, which sample the intra-hour X-ray/UV
variability. These observations indicate, albeit with large uncertainties, a
lag of the 1900 {\AA} band behind the X-ray flux of ~400 s. The tight
correlation between the X-ray and UV/optical lightcurves, together with the
constraints we place on lag time-scale are consistent with the UV/optical
variability of NGC 4395 being primarily due to reprocessing of X-ray photons by
the accretion disc.Comment: 11 pages, 9 figures, 3 tables. Accepted for publication in MNRA
INTEGRAL and Swift/XRT observations of IGR~J16460+0849
IGR J16460+0849 is reported in the 3rd IBIS catalog with the shortest
exposure of about 10 ks among all the detected sources, which makes it the most
interesting target to be investigated with a deeper exposure. We analyze all
available observations carried out by the International Gamma-Ray Astrophysics
Laboratory (INTEGRAL) on the unidentified source IGR J16460+0849. The data
were processed by using the latest version OSA 7.0. In addition we analyze also
all the available Swift/XRT data on this source. We find that IGR J16460+0849
has a detection significance of ~ 4.4 sigma in the 20-100 keV band during the
observational period between March 2003 and September 2004, when it was exposed
by ~ 19 ks. Thereafter the source was not detected anymore, despite an
additional exposure of ~ 271 ks. This suggests a flux variability on a
timescale of years. The spectral analysis shows that the IBIS/ISGRI data are
well presented by a power-law shape, with a photon index obtained as
1.45+-0.86. So far, the source has been detected consistently by IBIS/ISGRI in
the subsequent observations and in the adjacent energy bands. We have analyzed
the Swift/XRT observations on IGR J16460+0849 as well, and found no source
inside the IBIS/ISGRI error circle. The non-detection during the Swift/XRT
observation is consistent with the source having a variable nature.Comment: published in A&
- …