100 research outputs found
The Progenitor of Supernova 2011dh Has Vanished
We conducted Hubble Space Telescope (HST) Snapshot observations of the Type
IIb Supernova (SN) 2011dh in M51 at an age of ~641 days with the Wide Field
Camera 3. We find that the yellow supergiant star, clearly detected in pre-SN
HST images, has disappeared, implying that this star was almost certainly the
progenitor of the SN. Interpretation of the early-time SN data which led to the
inference of a compact nature for the progenitor, and to the expected survival
of this yellow supergiant, is now clearly incorrect. We also present
ground-based UBVRI light curves obtained with the Katzman Automatic Imaging
Telescope (KAIT) at Lick Observatory up to SN age ~70 days. From the
light-curve shape including the very late-time HST data, and from recent
interacting binary models for SN 2011dh, we estimate that a putative surviving
companion star to the now deceased yellow supergiant could be detectable by
late 2013, especially in the ultraviolet. No obvious light echoes are
detectable yet in the SN environment.Comment: 6 pages, new versions of the 3 figures, improved U-band SN
photometry, to appear in ApJ Letter
Observational implications of gamma-ray burst afterglow jet simulations and numerical light curve calculations
We discuss jet dynamics for narrow and wide gamma-ray burst (GRB) afterglow
jets and the observational implications of numerical simulations of
relativistic jets in two dimensions. We confirm earlier numerical results that
sideways expansion of relativistic jets during the bulk of the afterglow
emission phase is logarithmic in time and find that this also applies to narrow
jets with half opening angle of 0.05 radians. As a result, afterglow jets
remain highly nonspherical until after they have become nonrelativistic.
Although sideways expansion steepens the afterglow light curve after the jet
break, the jet edges becoming visible dominates the jet break, which means that
the jet break is sensitive to the observer angle even for narrow jets. Failure
to take the observer angle into account can lead to an overestimation of the
jet energy by up to a factor 4. This weakens the challenge posed to the
magneter energy limit by extreme events such as GRB090926A. Late time radio
calorimetry based on a spherical nonrelativistic outflow model remains relevant
when the observer is approximately on-axis and where differences of a few in
flux level between the model and the simulation are acceptable. However, this
does not imply sphericity of the outflow and therefore does not translate to
high observer angles relevant to orphan afterglows. For more accurate
calorimetry and in order to model significant late time features such as the
rise of the counterjet, detailed jet simulations remain indispensable.Comment: 7 Figures. Replaced with accepted version. Significantly expanded,
including additional discussion of time scale
A Beaming-Independent Estimate of the Energy Distribution of Long Gamma-Ray Bursts: Initial Results and Future Prospects
We present single-epoch radio afterglow observations of 24 long-duration
gamma-ray burst (GRB) on a timescale of >100 d after the burst. These
observations trace the afterglow evolution when the blastwave has decelerated
to mildly- or non-relativistic velocities and has roughly isotropized. We infer
beaming-independent kinetic energies using the Sedov-Taylor self-similar
solution, and find a median value for the sample of detected bursts of about
7x10^51 erg, with a 90% confidence range of 1.1x10^50-3.3x10^53 erg. Both the
median and 90% confidence range are somewhat larger than the results of
multi-wavelength, multi-epoch afterglow modeling (including large beaming
corrections), and the distribution of beaming-corrected gamma-ray energies.
This is due to bursts in our sample with only a single-frequency observation
for which we can only determine an upper bound on the peak of the synchrotron
spectrum. This limitation leads to a wider range of allowed energies than for
bursts with a well-measured spectral peak. Our study indicates that
single-epoch centimeter-band observations covering the spectral peak on a
timescale of ~1 yr can provide a robust estimate of the total kinetic energy
distribution with a small investment of telescope time. The substantial
increase in bandwidth of the EVLA (up to 8 GHz simultaneously with full
coverage at 1-40 GHz) will provide the opportunity to estimate the kinetic
energy distribution of GRBs with only a few hours of data per burst.Comment: Submitted to ApJ; 11 pages, 5 figures, 2 table
The Pan-STARRS1 Photometric System
The Pan-STARRS1 survey is collecting multi-epoch, multi-color observations of
the sky north of declination -30 deg to unprecedented depths. These data are
being photometrically and astrometrically calibrated and will serve as a
reference for many other purposes. In this paper we present our determination
of the Pan-STARRS photometric system: gp1, rp1, ip1, zp1, yp1, and wp1. The
Pan-STARRS photometric system is fundamentally based on the HST Calspec
spectrophotometric observations, which in turn are fundamentally based on
models of white dwarf atmospheres. We define the Pan-STARRS magnitude system,
and describe in detail our measurement of the system passbands, including both
the instrumental sensitivity and atmospheric transmission functions.
Byproducts, including transformations to other photometric systems, galactic
extinction, and stellar locus are also provided. We close with a discussion of
remaining systematic errors.Comment: 39 pages, 9 figures, machine readable table of bandpasses, accepted
for publication in Ap
Nebular spectroscopy of the nearby Type IIb supernova 2011dh
We present nebular spectra of the nearby Type IIb supernova (SN) 2011dh taken between 201 and 678 d after core collapse. At these late times, SN 2011dh exhibits strong emission lines including a broad and persistent Hα feature. New models of the nebular spectra confirm that the progenitor of SN 2011dh was a low-mass giant (M ≈ 13–15 M_⊙) that ejected ∼ 0.07 M_⊙ of ^(56)Ni and ∼ 0.27 M_⊙ of oxygen at the time of explosion, consistent with the recent disappearance of a candidate yellow supergiant progenitor. We show that light from the SN location is dominated by the fading SN at very late times (∼2 yr) and not, for example, by a binary companion or a background source. We present evidence for interaction between the expanding SN blast wave and a circumstellar medium at late times and show that the SN is likely powered by positron deposition ≳1 yr after explosion. We also examine the geometry of the ejecta and show that the nebular line profiles of SN 2011dh indicate a roughly spherical explosion with aspherical components or clumps
The Very Young Type Ia Supernova 2013dy: Discovery, and Strong Carbon Absorption in Early-Time Spectra
The Type Ia supernova (SN Ia) 2013dy in NGC 7250 (d ~ 13.7 Mpc) was
discovered by the Lick Observatory Supernova Search. Combined with a
prediscovery detection by the Italian Supernova Search Project, we are able to
constrain the first-light time of SN 2013dy to be only 0.10 +/- 0.05 d (2.4 +/-
1.2 hr) before the first detection. This makes SN 2013dy the earliest known
detection of an SN Ia. We infer an upper limit on the radius of the progenitor
star of R_0 < 0.25 R_sun, consistent with that of a white dwarf. The light
curve exhibits a broken power law with exponents of 0.88 and then 1.80. A
spectrum taken 1.63 d after first light reveals a C II absorption line
comparable in strength to Si II. This is the strongest C II feature ever
detected in a normal SN Ia, suggesting that the progenitor star had significant
unburned material. The C II line in SN 2013dy weakens rapidly and is undetected
in a spectrum 7 days later, indicating that C II is detectable for only a very
short time in some SNe Ia. SN 2013dy reached a B-band maximum of M_B = -18.72
+/- 0.03 mag ~17.7 d after first light.Comment: Accepted for Publication in ApJ Letter
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