959 research outputs found
VLBI detection of an AGN pair in the binary black hole candidate SDSS J1536+0441
We present first pc-scale radio imaging of the radio-quiet candidate binary
black hole system SDSS J1536+0441. The observations were carried out by the
European VLBI Network at the frequency of 5 GHz and allowed to image SDSS
J1536+0441 with a resolution of about 10 mas (50 pc). Two compact radio cores
are detected at the position of the kpc-scale components VLA-A and VLA-B,
proving the presence of two compact active nuclei with radio luminosity about
10^{40} erg/s, thus ruling out the possibility that the two radio sources are
both powered by one 0.1 pc binary black hole. From a comparison with published
8.5 GHz flux densities we derived an estimate of the radio spectral index of
the two pc-scale cores. Both cores have flat or inverted spectral index and, at
least for the case of VLA-A, we can rule out the possibility that synchrotron
self-absorption is responsible for the inverted radio spectrum. We suggest that
thermal free-free emission from an X-ray heated disk wind may be powering the
radio emission in VLA-A.Comment: Accepted for publication in ApJ Letters
A VLA Study of High-redshift GRBs I - Multi-wavelength Observations and Modeling of GRB 140311A
We present the first results from a recently concluded study of GRBs at
with the Karl G. Jansky Very Large Array (VLA). Spanning to
GHz and 7 epochs from 1.5 to 82.3 d, our observations of GRB 140311A are
the most detailed joint radio and millimeter observations of a GRB afterglow at
to date. In conjunction with optical/near-IR and X-ray data, the
observations can be understood in the framework of radiation from a single
blast wave shock with energy erg
expanding into a constant density environment with density, . The X-ray and radio observations require a jet break at d, yielding an opening angle of and a beaming-corrected blast wave kinetic energy of
erg. The results from our radio follow-up
and multi-wavelength modeling lend credence to the hypothesis that detected
high-redshift GRBs may be more tightly beamed than events at lower redshift. We
do not find compelling evidence for reverse shock emission, which may be
related to fast cooling driven by the moderately high circumburst density.Comment: 16 pages, 13 figures, submitted to Ap
GRB 021004: A Possible Shell Nebula around a Wolf-Rayet Star Gamma-Ray Burst Progenitor
The rapid localization of GRB 021004 by the HETE-2 satellite allowed nearly
continuous monitoring of its early optical afterglow decay, as well as
high-quality optical spectra that determined a redshift of z=2.328 for its
host, an active starburst galaxy with strong Lyman-alpha emission and several
absorption lines. Spectral observations show multiple absorbers blueshifted by
up to 3,155 km/s relative to the host galaxy Lyman-alpha emission.We argue that
these correspond to a fragmented shell nebula, gradually enriched by a
Wolf-Rayet wind over the lifetime of a massive progenitor bubble. In this
scenario, the absorbers can be explained by circumstellar material that have
been radiatively accelerated by the GRB emission. Dynamical and photoionization
models are used to provide constraints on the radiative acceleration from the
early afterglow.Comment: 5 pages, 3 figures, to appear in the proceedings of the 2003 GRB
Conferenc
The superluminous supernova SN 2017egm in the nearby galaxy NGC 3191: a metal-rich environment can support a typical SLSN evolution
At redshift z=0.03, the recently-discovered SN 2017egm is the nearest Type I
superluminous supernova (SLSN) to date, and first near the center of a massive
spiral galaxy (NGC 3191). Using SDSS spectra of NGC 3191, we find a metallicity
~2 Z at the nucleus and ~1.3 Z for a star forming region at a
radial offset similar to SN 2017egm. Archival radio-to-UV photometry reveals a
star-formation rate ~15 M yr (with ~70% dust-obscured), which
can account for a Swift X-ray detection, and stellar mass ~
M. We model the early UV-optical light curves with a magnetar
central-engine model, using the Bayesian light curve fitting tool MOSFiT. The
fits indicate ejecta mass 2-4 M, spin period 4-6 ms, magnetic field
(0.7-1.7)G, and kinetic energy 1-2 erg. These
parameters are consistent with the overall distributions for SLSNe, modeled by
Nicholl et al (2017), although the derived mass and spin are towards the low
end, possibly indicating enhanced loss of mass and angular momentum before
explosion. This has two implications: (i) SLSNe can occur at solar metallicity,
although with a low fraction ~10%; and (ii) metallicity has at most a modest
effect on their properties. Both conclusions are in line with results for long
gamma-ray bursts. Assuming a monotonic rise gives an explosion date MJD
. However, a short-lived excess in the data relative to the
best-fitting models may indicate an early-time `bump'. If confirmed, SN 2017egm
would be the first SLSN with a spectrum during the bump-phase; this shows the
same O II lines seen at maximum light, which may be an important clue for
explaining these bumps.Comment: Accepted for publication in ApJ
Probing the nature of the massive black hole binary candidate SDSS J1536+0441
We present an imaging study of the black hole binary candidate SDSS
J1536+0441 (z=0.3893), based on deep, high resolution VzK images collected at
the ESO/VLT. The images clearly show an asymmetric elongation, indicating the
presence of a companion source at ~1" (~5 kpc projected distance) East from the
quasar. The host galaxy of the quasar is marginally resolved. We find that the
companion source is a luminous galaxy, the light profile of which suggests the
presence of an unresolved, faint nucleus (either an obscured AGN or a compact
stellar bulge). The study of the environment around the quasar indicates the
occurrence of a significant over-density of galaxies with a redshift compatible
with z~0.4. This suggests that it resides in a moderately rich cluster of
galaxies.Comment: 4 figures, 1 table. Accepted for publication in ApJ Letter
A Hydrogen-Poor Superluminous Supernova with Enhanced Iron-Group Absorption: A New Link Between SLSNe and Broad-Lined Type Ic SNe
We present optical observations of the Type I superluminous supernova
(SLSN-I) SN2017dwh at , which reached
mag at peak. Spectra taken a few days after peak show an unusual and strong
absorption line centered near 3200\AA\ that we identify with Co II, suggesting
a high fraction of synthesized Ni in the ejecta. By month
after peak, SN2017dwh became much redder than other SLSNe-I, instead strongly
resembling broad-lined Type Ic supernovae (Ic-BL SNe) with clear suppression of
the flux redward of \AA, providing further evidence for a large
mass of Fe-group elements. Late-time upper limits indicate a Ni mass of
M, leaving open the possibility that SN2017dwh produced
a Ni mass comparable to SN1998bw ( M). Fitting the
light curve with a combined magnetar and Ni model using ,
we find that the light curve can easily accommodate such masses without
affecting the inferred magnetar parameters. We also find that SN2017dwh
occurred in the least-luminous detected host galaxy to date for a SLSN-I, with
mag and an implied metallicity of .
The spectral properties of SN2017dwh provide new evidence linking SLSNe-I with
Type Ic-BL SNe, and in particular the high Fe-group abundance may be due to
enhanced Ni production or mixing due to asphericity. Finally, we find
that SN2017dwh represents the most extreme end of a correlation between
continuum shape and Co II absorption strength in the near-peak spectra of
SLSNe-I, indicating that Fe-group abundance likely accounts for some of the
variation in their spectral shapes.Comment: 16 pages, 7 figures, Submitted to Ap
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