37 research outputs found
Detection of Radio Emission from the Hyperactive L Dwarf 2MASS J13153094-2649513AB
We report the detection of radio emission from the unusually active L5e + T7
binary 2MASS J13153094-2649513AB made with the Australian Telescope Compact
Array. Observations at 5.5 GHz reveal an unresolved source with a continuum
flux of 370+/-50 microJy, corresponding to a radio luminosity of L_rad = nuL_nu
= (9+/-3)x10^23 erg/s and log10(L_rad/L_bol) = -5.44+/-0.22. No detection is
made at 9.0 GHz to a 5 sigma limit of 290 microJy, consistent with a power law
spectrum S_nu ~ nu^-a with a > 0.5. The emission is quiescent, with no evidence
of variability or bursts over 3 hr of observation, and no measurable
polarization (V/I < 34%). 2MASS J1315-2649AB is one of the most radio-luminous
ultracool dwarfs detected in quiescent emission to date, comparable in strength
to other cool sources detected in outburst. Its detection indicates no decline
in radio flux through the mid-L dwarfs. It is unique among L dwarfs in having
strong and persistent Halpha and radio emission, indicating the coexistence of
a cool, neutral photosphere (low electron density) and a highly active
chromosphere (high electron density and active heating). These traits, coupled
with the system's mature age and substellar secondary, makes 2MASS J1315-2649AB
an important test for proposed radio emission mechanisms in ultracool dwarfs.Comment: 5 pages, 4 figures, accepted for publication in ApJ Letter
The Intermediate Mass Black Hole Candidate in the Center of NGC 404: New Evidence from Radio Continuum Observations
We present the results of deep, high-resolution, 5 GHz Expanded Very Large
Array (EVLA) observations of the nearby, dwarf lenticular galaxy and
intermediate mass black hole candidate (M ~4.5 x 10^5 M_sun), NGC 404. For the
first time, radio emission at frequencies above 1.4 GHz has been detected in
this galaxy. We found a modestly resolved source in the NGC 404 nucleus with a
total radio luminosity of 7.6 +/- 0.7 x 10^17 W/Hz at 5 GHz and a spectral
index from 5 to 7.45 GHz of alpha = -0.88 +/- 0.30. NGC 404 is only the third
central intermediate mass black hole candidate detected in the radio regime
with subarcsecond resolution. The position of the radio source is consistent
with the optical center of the galaxy and the location of a known, hard X-ray
point source (Lx ~1.2 x 10^37 erg/s). The faint radio and X-ray emission could
conceivably be produced by an X-ray binary, star formation, a supernova remnant
or a low-luminosity AGN powered by an intermediate mass black hole. In light of
our new EVLA observations, we find that the most likely scenario is an
accreting intermediate mass black hole, with other explanations incompatible
with the observed X-ray and/or radio luminosities or statistically unlikely.Comment: Accepted for publication to Ap
Radio Properties of Tidal Disruption Events
Radio observations of tidal disruption events (TDEs) probe material ejected
by the disruption of stars by supermassive black holes (SMBHs), uniquely
tracing the formation and evolution of jets and outflows, revealing details of
the disruption hydrodynamics, and illuminating the environments around
previously-dormant SMBHs. To date, observations reveal a surprisingly diverse
population. A small fraction of TDEs (at most a few percent) have been observed
to produce radio-luminous mildly relativistic jets. The remainder of the
population are radio quiet, producing less luminous jets, non-relativistic
outflows or, possibly, no radio emission at all. Here, we review the radio
observations that have been made of TDEs to date and discuss possible
explanations for their properties, focusing on detected sources and, in
particular, on the two best-studied events: Sw J1644+57 and ASASSN-14li. We
also discuss what we have learned about the host galaxies of TDEs from radio
observations and review constraints on the rates of bright and faint radio
outflows in TDEs. Upcoming X-ray, optical, near-IR, and radio surveys will
greatly expand the sample of TDEs, and technological advances open the exciting
possibility of discovering a sample of TDEs in the radio band unbiased by host
galaxy extinction.Comment: Resubmitted for publication in Springer Space Science Reviews
following referee comments. Chapter in ISSI review "The Tidal Disruption of
Stars by Massive Black Holes" vol. 79. Table 2 is available in
machine-readable format upon reques
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Energy Injection in Gamma-Ray Burst Afterglows
We present multi-wavelength observations and modeling of Gamma-ray Bursts (GRBs) that exhibit a simultaneous re-brightening in their X-ray and optical light curves, and are also detected at radio wavelengths. We show that the re-brightening episodes can be modeled by injection of energy into the blastwave and that in all cases the energy injection rate falls within the theoretical bounds expected for a distribution of energy with ejecta Lorentz factor. Our measured values of the circumburst density, jet opening angle, and beaming corrected kinetic energy are consistent with the distribution of these parameters for long-duration GRBs at both z ∼ 1 and z & 6, suggesting that the jet launching mechanism and environment of these events are similar to that of GRBs that do not have bumps in their light curves. However, events exhibiting re-brightening episodes have lower radiative efficiencies than average, suggesting that a majority of the kinetic energy of the outflow is carried by slow-moving ejecta, which is further supported by steep measured distributions of the ejecta energy as a function of Lorentz factor. We do not find evidence for reverse shocks over the energy injection period, implying that the onset of energy injection is a gentle process. We further show that GRBs exhibiting simultaneous X-ray and optical re-brightenings are likely the tail of a distribution of events with varying rates of energy injection, forming the most extreme events in their class. Future X-ray observations of GRB afterglows with Swift and its successors will thus likely discover several more such events, while radio follow-up and multi-wavelength modeling of similar events will unveil the role of energy injection in GRB afterglows.Astronom
Energy Injection in Gamma-ray Burst Afterglows
We present multi-wavelength observations and modeling of gamma-ray bursts (GRBs) that exhibit a simultaneous re-brightening in their X-ray and optical light curves, and are also detected at radio wavelengths. We show that the re-brightening episodes can be modeled by injection of energy into the blastwave and that in all cases the energy injection rate falls within the theoretical bounds expected for a distribution of energy with ejecta Lorentz factor. Our measured values of the circumburst density, jet opening angle, and beaming-corrected kinetic energy are consistent with the distribution of these parameters for long-duration GRBs at both z ~ 1 and z ≳ 6, suggesting that the jet launching mechanism and environment of these events are similar to that of GRBs that do not have bumps in their light curves. However, events exhibiting re-brightening episodes have lower radiative efficiencies than average, suggesting that a majority of the kinetic energy of the outflow is carried by slow-moving ejecta, which is further supported by steep measured distributions of the ejecta energy as a function of Lorentz factor. We do not find evidence for reverse shocks over the energy injection period, implying that the onset of energy injection is a gentle process. We further show that GRBs exhibiting simultaneous X-ray and optical re-brightenings are likely the tail of a distribution of events with varying rates of energy injection, forming the most extreme events in their class. Future X-ray observations of GRB afterglows with Swift and its successors will thus likely discover several more such events, while radio follow-up and multi-wavelength modeling of similar events will unveil the role of energy injection in GRB afterglows
Short GRB 130603B: Discovery of a jet break in the optical and radio afterglows, and a mysterious late-time X-ray excess
We present radio, optical/NIR, and X-ray observations of the afterglow of the
short-duration 130603B, and uncover a break in the radio and optical bands at
0.5 d after the burst, best explained as a jet break with an inferred jet
opening angle of 4-8 deg. GRB 130603B is only the third short GRB with a radio
afterglow detection to date, and the first time that a jet break is evident in
the radio band. We model the temporal evolution of the spectral energy
distribution to determine the burst explosion properties and find an
isotropic-equivalent kinetic energy of (0.6-1.7) x 10^51 erg and a circumburst
density of 5 x 10^-3-30 cm^-3. From the inferred opening angle of GRB 130603B,
we calculate beaming-corrected energies of Egamma (0.5-2) x 10^49 erg and EK
(0.1-1.6) x 10^49 erg. Along with previous measurements and lower limits we
find a median short GRB opening angle of 10 deg. Using the all-sky observed
rate of 10 Gpc^-3 yr^-1, this implies a true short GRB rate of 20 yr^-1 within
200 Mpc, the Advanced LIGO/VIRGO sensitivity range for neutron star binary
mergers. Finally, we uncover evidence for significant excess emission in the
X-ray afterglow of GRB 130603B at >1 d and conclude that the additional energy
component could be due to fall-back accretion or spin-down energy from a
magnetar formed following the merger.Comment: Submitted to ApJ; emulateapj style; 10 pages, 1 table, 3 figure
A Resolved Ring of Debris Dust around the Solar Analog HD 107146
We present resolved images of the dust continuum emission from the debris disk around the young (80-200 Myr) solar-type star HD 107146 with CARMA at λ = 1.3 mm and the CSO at λ = 350 μ. Both images show that the dust emission extends over an approximately 10" diameter region. The high-resolution (3") CARMA image further reveals that the dust is distributed in a partial ring with significant decrease in a flux inward of 97 AU. Two prominent emission peaks appear within the ring separated by ~140° in the position angle. The morphology of the dust emission is suggestive of dust captured into a mean motion resonance, which would imply the presence of a planet at an orbital radius of ~45-75 AU
Dynamically Driven Evolution of the Interstellar Medium in M51
Massive star formation occurs in giant molecular clouds (GMCs); an understanding of the evolution of GMCs is a prerequisite to develop theories of star formation and galaxy evolution. We report the highest-fidelity observations of the grand-design spiral galaxy M51 in carbon monoxide (CO) emission, revealing the evolution of GMCs vis-a-vis the large-scale galactic structure and dynamics. The most massive GMCs (giant molecular associations (GMAs)) are first assembled and then broken up as the gas flow through the spiral arms. The GMAs and their H_2 molecules are not fully dissociated into atomic gas as predicted in stellar feedback scenarios, but are fragmented into smaller GMCs upon leaving the spiral arms. The remnants of GMAs are detected as the chains of GMCs that emerge from the spiral arms into interarm regions. The kinematic shear within the spiral arms is sufficient to unbind the GMAs against self-gravity. We conclude that the evolution of GMCs is driven by large-scale galactic dynamics—their coagulation into GMAs is due to spiral arm streaming motions upon entering the arms, followed by fragmentation due to shear as they leave the arms on the downstream side. In M51, the majority of the gas remains molecular from arm entry through the interarm region and into the next spiral arm passage
Atmospheric phase correction using CARMA-PACS: high angular resolution observations of the FU Orionis star PP 13S*
We present 0".15 resolution observations of the 227 GHz continuum emission from the circumstellar disk around
the FU Orionis star PP 13S*. The data were obtained with the Combined Array for Research in Millimeter-wave
Astronomy (CARMA) Paired Antenna Calibration System (C-PACS), which measures and corrects the atmospheric
delay fluctuations on the longest baselines of the array in order to improve the sensitivity and angular resolution of
the observations. A description of the C-PACS technique and the data reduction procedures are presented. C-PACS
was applied to CARMA observations of PP 13S*, which led to a factor of 1.6 increase in the observed peak flux
of the source, a 36% reduction in the noise of the image, and a 52% decrease in the measured size of the source
major axis. The calibrated complex visibilities were fitted with a theoretical disk model to constrain the disk surface
density. The total disk mass from the best-fit model corresponds to 0.06 M_⊙, which is larger than the median mass of a disk around a classical T Tauri star. The disk is optically thick at a wavelength of 1.3 mm for orbital radii less than 48 AU. At larger radii, the inferred surface density of the PP 13S* disk is an order of magnitude lower than that needed to develop a gravitational instability