1,856 research outputs found
Radio Monitoring of the Tidal Disruption Event Swift J164449.3+573451. I. Jet Energetics and the Pristine Parsec-Scale Environment of a Supermassive Black Hole
We present continued radio observations of the tidal disruption event
SwiftJ164449.3+573451 extending to \sim216 days after discovery. The data are
part of a long-term program to monitor the expansion and energy scale of the
relativistic outflow, and to trace the parsec-scale environment around a
previously-dormant supermassive black hole (SMBH). The new observations reveal
a significant change in the radio evolution starting at \sim1 month, with a
brightening at all frequencies that requires an increase in the energy by about
an order of magnitude, and an overall density profile around the SMBH of rho
\propto r^{-3/2} (0.1-1.2 pc) with a significant flattening at r\sim0.4-0.6 pc.
The increase in energy cannot be explained with continuous injection from an L
\propto t^{-5/3} tail, which is observed in the X-rays. Instead, we conclude
that the relativistic jet was launched with a wide range of Lorentz factors,
obeying E(>Gamma) \propto Gamma^{-2.5}. The similar ratio of duration to
dynamical timescale for Sw1644+57 and GRBs suggests that this result may be
applicable to GRBs as well. The radial density profile may be indicative of
Bondi accretion, with the inferred flattening at r\sim0.5 pc in good agreement
with the Bondi radius for a \sim10^6 M_sun black hole. The density at \sim0.5
pc is about a factor of 30 times lower than inferred for the Milky Way galactic
center, potentially due to a smaller number of mass-shedding massive stars.
From our latest observations (\sim216 d) we find that the jet energy is
E_{iso}\sim5x10^{53} erg (E_j\sim2.4x10^{51} erg for theta_j=0.1), the radius
is r\sim1.2 pc, the Lorentz factor is Gamma\sim2.2, the ambient density is
n\sim0.2 cm^{-3}, and the projected size is r_{proj}\sim25 microarcsec.
Assuming no future changes in the observed evolution we predict that the radio
emission from Sw1644+57 should be detectable with the EVLA for several decades,
and will be resolvable with VLBI in a few years.Comment: Submitted to ApJ; 22 pages, 2 tables, 9 figure
Short-Baseline Electron Neutrino Disappearance, Tritium Beta Decay and Neutrinoless Double-Beta Decay
We consider the interpretation of the MiniBooNE low-energy anomaly and the
Gallium radioactive source experiments anomaly in terms of short-baseline
electron neutrino disappearance in the framework of 3+1 four-neutrino mixing
schemes. The separate fits of MiniBooNE and Gallium data are highly compatible,
with close best-fit values of the effective oscillation parameters Delta m^2
and sin^2 2 theta. The combined fit gives Delta m^2 >~ 0.1 eV^2 and 0.11 <
sin^2 2 theta < 0.48 at 2 sigma. We consider also the data of the Bugey and
Chooz reactor antineutrino oscillation experiments and the limits on the
effective electron antineutrino mass in beta-decay obtained in the Mainz and
Troitsk Tritium experiments. The fit of the data of these experiments limits
the value of sin^2 2 theta below 0.10 at 2 sigma. Considering the tension
between the neutrino MiniBooNE and Gallium data and the antineutrino reactor
and Tritium data as a statistical fluctuation, we perform a combined fit which
gives Delta m^2 \simeq 2 eV and 0.01 < sin^2 2 theta < 0.13 at 2 sigma.
Assuming a hierarchy of masses m_1, m_2, m_3 << m_4, the predicted
contributions of m_4 to the effective neutrino masses in beta-decay and
neutrinoless double-beta-decay are, respectively, between about 0.06 and 0.49
and between about 0.003 and 0.07 eV at 2 sigma. We also consider the
possibility of reconciling the tension between the neutrino MiniBooNE and
Gallium data and the antineutrino reactor and Tritium data with different
mixings in the neutrino and antineutrino sectors. We find a 2.6 sigma
indication of a mixing angle asymmetry.Comment: 14 pages; final version published in Phys.Rev.D82:053005,201
A Relativistic Type Ibc Supernova Without a Detected Gamma-ray Burst
Long duration gamma-ray bursts (GRBs) mark the explosive death of some
massive stars and are a rare sub-class of Type Ibc supernovae (SNe Ibc). They
are distinguished by the production of an energetic and collimated relativistic
outflow powered by a central engine (an accreting black hole or neutron star).
Observationally, this outflow is manifested in the pulse of gamma-rays and a
long-lived radio afterglow. To date, central engine-driven SNe have been
discovered exclusively through their gamma-ray emission, yet it is expected
that a larger population goes undetected due to limited satellite sensitivity
or beaming of the collimated emission away from our line-of-sight. In this
framework, the recovery of undetected GRBs may be possible through radio
searches for SNe Ibc with relativistic outflows. Here we report the discovery
of luminous radio emission from the seemingly ordinary Type Ibc SN 2009bb,
which requires a substantial relativistic outflow powered by a central engine.
The lack of a coincident GRB makes SN 2009bb the first engine-driven SN
discovered without a detected gamma-ray signal. A comparison with our extensive
radio survey of SNe Ibc reveals that the fraction harboring central engines is
low, ~1 percent, measured independently from, but consistent with, the inferred
rate of nearby GRBs. Our study demonstrates that upcoming optical and radio
surveys will soon rival gamma-ray satellites in pinpointing the nearest
engine-driven SNe. A similar result for a different supernova is reported
independently.Comment: To appear in Nature on Jan 28 2010. Embargoed for discussion in the
press until 13:00 US Eastern Time on Jan 27 (Accepted version, 27 pages,
Manuscript and Suppl. Info.
GRB060218 and GRBs associated with Supernovae Ib/c
We plan to fit the complete gamma- and X-ray light curves of the long
duration GRB060218, including the prompt emission, in order to clarify the
nature of the progenitors and the astrophysical scenario of the class of GRBs
associated to SNe Ib/c. The initial total energy of the electron-positron
plasma E_{e^\pm}^{tot}=2.32\times 10^{50} erg has a particularly low value
similarly to the other GRBs associated with SNe. For the first time we observe
a baryon loading B=10^{-2} which coincides with the upper limit for the
dynamical stability of the fireshell. The effective CircumBurst Medium (CBM)
density shows a radial dependence n_{cbm} \propto r^{-\alpha} with
1.0<\alpha<1.7 and monotonically decreases from 1 to 10^{-6} particles/cm^3.
Such a behavior is interpreted as due to a fragmentation in the fireshell.
Analogies with the fragmented density and filling factor characterizing Novae
are outlined. The fit presented is particularly significant in view of the
complete data set available for GRB060218 and of the fact that it fulfills the
Amati relation. We fit GRB060218, usually considered as an X-Ray Flash (XRF),
as a "canonical GRB" within our theoretical model. The smallest possible black
hole, formed by the gravitational collapse of a neutron star in a binary
system, is consistent with the especially low energetics of the class of GRBs
associated with SNe Ib/c. We give the first evidence for a fragmentation in the
fireshell. Such a fragmentation is crucial in explaining both the unusually
large T_{90} and the consequently inferred abnormal low value of the CBM
effective density.Comment: 4 pages, 3 figures, to appear in A&A Letter
The Optical SN 2012bz Associated with the Long GRB 120422A
The association of Type Ic SNe with long-duration GRBs is well established.
We endeavor, through accurate ground-based observational campaigns, to
characterize these SNe at increasingly high redshifts. We obtained a series of
optical photometric and spectroscopic observations of the Type Ic SN2012bz
associated with the Swift long-duration GRB120422A (z=0.283) using the 3.6-m
TNG and the 8.2-m VLT telescopes. The peak times of the light curves of
SN2012bz in various optical filters differ, with the B-band and i'-band light
curves reaching maximum at ~9 and ~23 rest-frame days, respectively. The
bolometric light curve has been derived from individual bands photometric
measurements, but no correction for the unknown contribution in the
near-infrared (probably around 10-15%) has been applied. Therefore, the present
light curve should be considered as a lower limit to the actual UV-optical-IR
bolometric light curve. This pseudo-bolometric curve reaches its maximum (Mbol
= -18.56 +/- 0.06) at 13 +/- 1 rest-frame days; it is similar in shape and
luminosity to the bolometric light curves of the SNe associated with z<0.2 GRBs
and more luminous than those of SNe associated with XRFs. A comparison with the
model generated for the bolometric light curve of SN2003dh suggests that
SN2012bz produced only about 15% less 56Ni than SN2003dh, about 0.35 Msol.
Similarly the VLT spectra of SN2012bz, after correction for Galactic extinction
and for the contribution of the host galaxy, suggest comparable explosion
parameters with those observed in SN2003dh (EK~3.5 x 10^52 erg, Mej~7 Msol) and
a similar progenitor mass (~25-40 Msol). GRB120422A is consistent with the
Epeak-Eiso and the EX,iso-Egamma,iso-E_peak relations. GRB120422A/SN2012bz
shows the GRB-SN connection at the highest redshift so far accurately monitored
both photometrically and spectroscopically.Comment: 7 pages, 6 figures, 2 tables, accepted for publication in Astronomy &
Astrophysic
SN 2010ay is a Luminous and Broad-lined Type Ic Supernova within a Low-metallicity Host Galaxy
We report on our serendipitous pre-discovery detection and detailed follow-up
of the broad-lined Type Ic supernova (SN) 2010ay at z = 0.067 imaged by the
Pan-STARRS1 3pi survey just ~4 days after explosion. The SN had a peak
luminosity, M_R ~ -20.2 mag, significantly more luminous than known GRB-SNe and
one of the most luminous SNe Ib/c ever discovered. The absorption velocity of
SN 2010ay is v_Si ~ 19,000 km/s at ~40 days after explosion, 2-5 times higher
than other broad-lined SNe and similar to the GRB-SN 2010bh at comparable
epochs. Moreover, the velocity declines ~2 times slower than other SNe Ic-BL
and GRB-SNe. Assuming that the optical emission is powered by radioactive
decay, the peak magnitude implies the synthesis of an unusually large mass of
56 Ni, M_Ni = 0.9 M_solar. Modeling of the light-curve points to a total ejecta
mass, M_ej ~ 4.7 M_sol, and total kinetic energy, E_K ~ 11x10^51 ergs. The
ratio of M_Ni to M_ej is ~2 times as large for SN 2010ay as typical GRB-SNe and
may suggest an additional energy reservoir. The metallicity (log(O/H)_PP04 + 12
= 8.19) of the explosion site within the host galaxy places SN 2010ay in the
low-metallicity regime populated by GRB-SNe, and ~0.5(0.2) dex lower than that
typically measured for the host environments of normal (broad-lined) Ic
supernovae. We constrain any gamma-ray emission with E_gamma < 6x10^{48} erg
(25-150 keV) and our deep radio follow-up observations with the Expanded Very
Large Array rule out relativistic ejecta with energy, E > 10^48 erg. We
therefore rule out the association of a relativistic outflow like those which
accompanied SN 1998bw and traditional long-duration GRBs, but place
less-stringent constraints on a weak afterglow like that seen from XRF 060218.
These observations challenge the importance of progenitor metallicity for the
production of a GRB, and suggest that other parameters also play a key role.Comment: 19 pages, 10 figures, V3 has revisions following referee's report;
more information at
http://www.cfa.harvard.edu/~nsanders/papers/2010ay/summary.htm
Relativistic ejecta from XRF 060218 and the rate of cosmic explosions
Over the last decade, long-duration gamma-ray bursts (GRBs) including the
subclass of X-ray flashes (XRFs) have been revealed to be a rare variety of
Type Ibc supernova (SN). While all these events result from the death of
massive stars, the electromagnetic luminosities of GRBs and XRFs exceed those
of ordinary Type Ibc SNe by many orders of magnitude. The essential physical
process that causes a dying star to produce a GRB or XRF, and not just an SN,
remains the crucial open question. Here we present radio and X-ray observations
of XRF 060218 (associated with SN 2006aj), the second nearest GRB identified
to-date, which allow us to measure its total energy and place it in the larger
context of cosmic explosions. We show that this event is 100 times less
energetic but ten times more common than cosmological GRBs. Moreover, it is
distinguished from ordinary Type Ibc SNe by the presence of 10^48 erg coupled
to mildly-relativistic ejecta, along with a central engine (an accretion-fed,
rapidly rotating compact source) which produces X-rays for weeks after the
explosion. This suggests that the production of relativistic ejecta is the key
physical distinction between GRBs/XRFs and ordinary SNe, while the nature of
the central engine (black hole or magnetar) may distinguish typical bursts from
low-luminosity, spherical events like XRF 060218.Comment: To appear in Nature on August 31 2006 (15 pages, 3 figures, 1 table,
including Supplementary Information
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