766 research outputs found
325 MHz VLA Observations of Ultracool Dwarfs TVLM 513-46546 and 2MASS J0036+1821104
We present 325 MHz (90 cm wavelength) radio observations of ultracool dwarfs
TVLM 513-46546 and 2MASS J0036+1821104 using the Very Large Array (VLA) in June
2007. Ultracool dwarfs are expected to be undetectable at radio frequencies,
yet observations at 8.5 GHz (3.5 cm) and 4.9 GHz (6 cm) of have revealed
sources with > 100 {\mu}Jy quiescent radio flux and > 1 mJy pulses coincident
with stellar rotation. The anomalous emission is likely a combination of
gyrosynchrotron and cyclotron maser processes in a long-duration, large-scale
magnetic field. Since the characteristic frequency for each process scales
directly with the magnetic field magnitude, emission at lower frequencies may
be detectable from regions with weaker field strength. We detect no significant
radio emission at 325 MHz from TVLM 513-46546 or 2MASS J0036+1821104 over
multiple stellar rotations, establishing 2.5{\sigma} total flux limits of 795
{\mu}Jy and 942 {\mu}Jy respectively. Analysis of an archival VLA 1.4 GHz
observation of 2MASS J0036+1821104 from January 2005 also yields a
non-detection at the level of < 130 {\mu}Jy . The combined radio observation
history (0.3 GHz to 8.5 GHz) for these sources suggests a continuum emission
spectrum for ultracool dwarfs which is either flat or inverted below 2-3 GHz.
Further, if the cyclotron maser instability is responsible for the pulsed radio
emission observed on some ultracool dwarfs, our low-frequency non-detections
suggest that the active region responsible for the high-frequency bursts is
confined within 2 stellar radii and driven by electron beams with energies less
than 5 keV.Comment: 11 pages, 5 figures, submitted to A
Constraints on the Growth and Spin of the Supermassive Black Hole in M32 From High Cadence Visible Light Observations
We present 1-second cadence observations of M32 (NGC221) with the CHIMERA
instrument at the Hale 200-inch telescope of the Palomar Observatory. Using
field stars as a baseline for relative photometry, we are able to construct a
light curve of the nucleus in the g-prime and r-prime band with 1sigma=36
milli-mag photometric stability. We derive a temporal power spectrum for the
nucleus and find no evidence for a time-variable signal above the noise as
would be expected if the nuclear black hole were accreting gas. Thus, we are
unable to constrain the spin of the black hole although future work will use
this powerful instrument to target more actively accreting black holes. Given
the black hole mass of (2.5+/-0.5)*10^6 Msun inferred from stellar kinematics,
the absence of a contribution from a nuclear time-variable signal places an
upper limit on the accretion rate which is 4.6*10^{-8} of the Eddington rate, a
factor of two more stringent than past upper limits from HST. The low mass of
the black hole despite the high stellar density suggests that the gas liberated
by stellar interactions was primarily at early cosmic times when the low-mass
black hole had a small Eddington luminosity. This is at least partly driven by
a top-heavy stellar initial mass function at early cosmic times which is an
efficient producer of stellar mass black holes. The implication is that
supermassive black holes likely arise from seeds formed through the coalescence
of 3-100 Msun mass black holes that then accrete gas produced through stellar
interaction processes.Comment: 8 pages, 3 figures, submitted to the Astrophysical Journal, comments
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On associating Fast Radio Bursts with afterglows
A radio source that faded over six days, with a redshift of
host, has been identified by Keane et al. (2016) as the transient afterglow to
a fast radio burst (FRB 150418). We report follow-up radio and optical
observations of the afterglow candidate and find a source that is consistent
with an active galactic nucleus. If the afterglow candidate is nonetheless a
prototypical FRB afterglow, existing slow-transient surveys limit the fraction
of FRBs that produce afterglows to 0.25 for afterglows with fractional
variation, , and 0.07 for , at 95%
confidence. In anticipation of a barrage of bursts expected from future FRB
surveys, we provide a simple framework for statistical association of FRBs with
afterglows. Our framework properly accounts for statistical uncertainties, and
ensures consistency with limits set by slow-transient surveys.Comment: Accepted version (ApJL
Periodic Radio and H-alpha Emission from the L Dwarf Binary 2MASSW J0746425+200032: Exploring the Magnetic Field Topology and Radius of an L Dwarf
[Abridged] We present an 8.5-hour simultaneous radio, X-ray, UV, and optical
observation of the L dwarf binary 2MASSW J0746+20. We detect strong radio
emission, dominated by short-duration periodic pulses at 4.86 GHz with
P=124.32+/-0.11 min. The stability of the pulse profiles and arrival times
demonstrates that they are due to the rotational modulation of a B~1.7 kG
magnetic field. A quiescent non-variable component is also detected, likely due
to emission from a uniform large-scale field. The H-alpha emission exhibits
identical periodicity, but unlike the radio pulses it varies sinusoidally and
is offset by exactly 1/4 of a phase. The sinusoidal variations require
chromospheric emission from a large-scale field structure, with the radio
pulses likely emanating from the magnetic poles. While both light curves can be
explained by a rotating mis-aligned magnetic field, the 1/4 phase lag rules out
a symmetric dipole topology since it would result in a phase lag of 1/2
(poloidal field) or zero (toroidal field). We therefore conclude that either
(i) the field is dominated by a quadrupole configuration, which can naturally
explain the 1/4 phase lag; or (ii) the H-alpha and/or radio emission regions
are not trivially aligned with the field. Regardless of the field topology, we
use the measured period along with the known rotation velocity (vsini=27 km/s),
and the binary orbital inclination (i=142 deg), to derive a radius for the
primary star of 0.078+/-0.010 R_sun. This is the first measurement of the
radius of an L dwarf, and along with a mass of 0.085+/-0.010 M_sun it provides
a constraint on the mass-radius relation below 0.1 M_sun. We find that the
radius is about 30% smaller than expected from theoretical models, even for an
age of a few Gyr.Comment: Submitted to Ap
The Ecology and Evolution of Patience in Two New World Monkeys
Decision making often involves choosing between small, short-term rewards and large, long-term rewards. All animals, humans included, discount future rewards-the present value of delayed rewards is viewed as less than the value of immediate rewards. Despite its ubiquity, there exists considerable but unexplained variation between species in their capacity to wait for rewards-that is, to exert patience or self-control. Using two closely related primates-common marmosets (Callithrix jacchus) and cotton-top tamarins (Saguinus oedipus)-we uncover a variable that may explain differences in how species discount future rewards. Both species faced a self-control paradigm in which individuals chose between taking an immediate small reward and waiting a variable amount of time for a large reward. Under these conditions, marmosets waited significantly longer for food than tamarins. This difference cannot be explained by life history, social behaviour or brain size. It can, however, be explained by feeding ecology: marmosets rely on gum, a food product acquired by waiting for exudate to flow from trees, whereas tamarins feed on insects, a food product requiring impulsive action. Foraging ecology, therefore, may provide a selective pressure for the evolution of self-control.Psycholog
First Detection of a Strong Magnetic Field on a Bursty Brown Dwarf: Puzzle Solved
We report the first direct detection of a strong, 5 kG magnetic field on the
surface of an active brown dwarf. LSR J1835+3259 is an M8.5 dwarf exhibiting
transient radio and optical emission bursts modulated by fast rotation. We have
detected the surface magnetic field as circularly polarized signatures in the
819 nm sodium lines when an active emission region faced the Earth. Modeling
Stokes profiles of these lines reveals the effective temperature of 2800 K and
log gravity acceleration of 4.5. These parameters place LSR J1835+3259 on
evolutionary tracks as a young brown dwarf with the mass of 554 M and age of 224 Myr. Its magnetic field is at least 5.1 kG and covers
at least 11% of the visible hemisphere. The active region topology recovered
using line profile inversions comprises hot plasma loops with a vertical
stratification of optical and radio emission sources. These loops rotate with
the dwarf in and out of view causing periodic emission bursts. The magnetic
field is detected at the base of the loops. This is the first time that we can
quantitatively associate brown dwarf non-thermal bursts with a strong, 5 kG
surface magnetic field and solve the puzzle of their driving mechanism. This is
also the coolest known dwarf with such a strong surface magnetic field. The
young age of LSR J1835+3259 implies that it may still maintain a disk, which
may facilitate bursts via magnetospheric accretion, like in higher-mass T
Tau-type stars. Our results pave a path toward magnetic studies of brown dwarfs
and hot Jupiters.Comment: ApJ, in pres
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Living With Everyday Evaluations on Social Media Platforms
The Like button is simultaneously a means of social interaction and a tool to evaluate social media content. Likes are built into the infrastructure of platforms, fueling personalized searches, recommendations, and targeted advertisements while contributing to the economic valuation of platforms and users alike. Working at the intersection of infrastructure and practice, we analyze the experience of everyday evaluations on social media to understand how platforms shape what people care about. Based on in-depth interviews with 25 artists who use Instagram, we identify three overlapping orientations to the Like button: affective, relational, and infrastructural. We show how the flexibility of the button creates ambiguity around the meaning of a Like which drives practices of metaevaluation, and argue that the platform architecture incentivizes an economic approach to evaluation that crowds out other value schemas, shaping how artists use the platform, make art, and even understand themselves. We conclude with a discussion of the broader implications of everyday evaluations, algorithms, and platforms.
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A Case Study of On-the-Fly Wide-Field Radio Imaging Applied to the Gravitational-wave Event GW 151226
We apply a newly-developed On-the-Fly mosaicing technique on the NSF's Karl
G. Jansky Very Large Array (VLA) at 3 GHz in order to carry out a sensitive
search for an afterglow from the Advanced LIGO binary black hole merger event
GW 151226. In three epochs between 1.5 and 6 months post-merger we observed a
100 sq. deg region, with more than 80% of the survey region having a RMS
sensitivity of better than 150 uJy/beam, in the northern hemisphere having a
merger containment probability of 10%. The data were processed in
near-real-time, and analyzed to search for transients and variables. No
transients were found but we have demonstrated the ability to conduct blind
searches in a time-frequency phase space where the predicted afterglow signals
are strongest. If the gravitational wave event is contained within our survey
region, the upper limit on any late-time radio afterglow from the merger event
at an assumed mean distance of 440 Mpc is about 1e29 erg/s/Hz. Approximately
1.5% of the radio sources in the field showed variability at a level of 30%,
and can be attributed to normal activity from active galactic nuclei. The low
rate of false positives in the radio sky suggests that wide-field imaging
searches at a few Gigahertz can be an efficient and competitive search
strategy. We discuss our search method in the context of the recent afterglow
detection from GW 170817 and radio follow-up in future gravitational wave
observing runs.Comment: 11 pages. 6 figures. 1 table. Accepted for publication in ApJ Letter
A mildly relativistic wide-angle outflow in the neutron-star merger event GW170817
GW170817 was the first gravitational wave detection of a binary neutron-star merger. It was accompanied by radiation across the electromagnetic spectrum and localized to the galaxy NGC 4993 at a distance of 40 megaparsecs. It has been proposed that the observed γ-ray, X-ray and radio emission is due to an ultra-relativistic jet launched during the merger, directed away from our line of sight. The presence of such a jet is predicted from models that posit neutron-star mergers as the central engines that drive short hard γ-ray bursts. Here we report that the radio light curve of GW170817 has no direct signature of an off-axis jet afterglow. Although we cannot rule out the existence of a jet pointing elsewhere, the observed γ-rays could not have originated from such a jet. Instead, the radio data require a mildly relativistic wide-angle outflow moving towards us. This outflow could be the high-velocity tail of the neutron-rich material dynamically ejected during the merger or a cocoon of material that breaks out when a jet transfers its energy to the dynamical ejecta. The cocoon model explains the radio light curve of GW170817 as well as the γ-rays and X-rays (possibly also ultraviolet and optical emission), and is therefore the model most consistent with the observational data. Cocoons may be a ubiquitous phenomenon produced in neutron-star mergers, giving rise to a heretofore unidentified population of radio, ultraviolet, X-ray and γ-ray transients in the local Universe
Periodic Bursts of Coherent Radio Emission from an Ultracool Dwarf
We report the detection of periodic (p = 1.96 hours) bursts of extremely
bright, 100% circularly polarized, coherent radio emission from the M9 dwarf
TVLM 513-46546. Simultaneous photometric monitoring observations have
established this periodicity to be the rotation period of the dwarf. These
bursts, which were not present in previous observations of this target, confirm
that ultracool dwarfs can generate persistent levels of broadband, coherent
radio emission, associated with the presence of kG magnetic fields in a
large-scale, stable configuration. Compact sources located at the magnetic
polar regions produce highly beamed emission generated by the electron
cyclotron maser instability, the same mechanism known to generate planetary
coherent radio emission in our solar system. The narrow beams of radiation pass
our line of sight as the dwarf rotates, producing the associated periodic
bursts. The resulting radio light curves are analogous to the periodic light
curves associated with pulsar radio emission highlighting TVLM 513-46546 as the
prototype of a new class of transient radio source.Comment: 12 pages, 3 figures, accepted for publication in ApJ Letter
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