34 research outputs found
Galaxy-Targeting Approach Optimized for Finding the Radio Afterglows of Gravitational Wave Sources
Kilonovae and radio afterglows of neutron star merger events have been
identified as the two most promising counterparts, of these gravitational wave
sources, that can provide arcsecond localization. While several new and
existing optical search facilities have been dedicated to finding kilonovae,
factors such as dust obscuration and the daytime sky may thwart these searches
in a significant fraction of gavitational wave events. Radio-only searches,
being almost immune to these factors, are equally capable of finding the
counterparts and in fact offer a complementary discovery approach, despite the
modest fields of view for many of the present-day radio interferometers. Such
interferometers will be able to carry out competitive searches for the
electromagnetic counterparts through the galaxy targeting approach. Adapting
and improving on an existing algorithm by Rana et al. 2017. we present here a
method that optimizes the placement of radio antenna pointings, integration
time, and antenna slew. We simulate 3D gravitational wave localizations to find
the efficacy of our algorithm; with substantial improvements in slew overhead
and containment probability coverage, our algorithm performs significantly
better than simple galaxy-rank-ordered observations. We propose that telescopes
such as the Very Large Array, MeerKAT, Australia Telescope Compact Array and
the Gaint Meterwave Radio Telescope, having fields of view 1 deg
and searching for the counterparts of nearby GW events over tens of square
degrees or larger, will especially benefit from this optimized galaxy-targeting
approach for electromagnetic counterpart searches.Comment: 13 pages, 7 figures, Submitted to Ap
B- and A-Type Stars in the Taurus-Auriga Star Forming Region
We describe the results of a search for early-type stars associated with the
Taurus-Auriga molecular cloud complex, a diffuse nearby star-forming region
noted as lacking young stars of intermediate and high mass. We investigate
several sets of possible O, B and early A spectral class members. The first is
a group of stars for which mid-infrared images show bright nebulae, all of
which can be associated with stars of spectral type B. The second group
consists of early-type stars compiled from (i) literature listings in SIMBAD;
(ii) B stars with infrared excesses selected from the Spitzer Space Telescope
survey of the Taurus cloud; (iii) magnitude- and color-selected point sources
from the 2MASS; and (iv) spectroscopically identified early-type stars from the
SDSS coverage of the Taurus region. We evaluated stars for membership in the
Taurus-Auriga star formation region based on criteria involving: spectroscopic
and parallactic distances, proper motions and radial velocities, and infrared
excesses or line emission indicative of stellar youth. For selected objects, we
also model the scattered and emitted radiation from reflection nebulosity and
compare the results with the observed spectral energy distributions to further
test the plausibility of physical association of the B stars with the Taurus
cloud. This investigation newly identifies as probable Taurus members three
B-type stars: HR 1445 (HD 28929), tau Tau (HD 29763), 72 Tau (HD 28149), and
two A-type stars: HD 31305 and HD 26212, thus doubling the number of stars A5
or earlier associated with the Taurus clouds. Several additional early-type
sources including HD 29659 and HD 283815 meet some, but not all, of the
membership criteria and therefore are plausible, though not secure, members.Comment: 31 pages, 18 figures, 6 tables. Accepted for publication in The
Astrophysical Journa
An upper-limit on the linear polarization fraction of the GW170817 radio continuum
We present late-time radio observations of GW170817, the first binary neutron
star merger discovered through gravitational waves by the advanced LIGO and
Virgo detectors. Our observations, carried out with the Karl G. Jansky Very
Large Array, were optimized to detect polarized radio emission, and thus to
constrain the linear polarization fraction of GW170817. At an epoch of ~244
days after the merger, we rule out linearly polarized emission above a fraction
of ~12% at a frequency of 2.8 GHz (99% confidence). Within the structured jet
scenario (a.k.a. successful jet plus cocoon system) for GW170817, the derived
upper-limit on the radio continuum linear polarization fraction strongly
constrains the magnetic field configuration in the shocked ejecta. We show that
our results for GW170817 are compatible with the low level of linear
polarization found in afterglows of cosmological long gamma-ray bursts.
Finally, we discuss our findings in the context of future expectations for the
study of radio counterparts of binary neutron star mergers identified by
ground-based gravitational-wave detectors.Comment: 5 pages, 2 figures, 1 tabl
A Hubble constant measurement from superluminal motion of the jet in GW170817
The Hubble constant () measures the current expansion rate of the
Universe, and plays a fundamental role in cosmology. Tremendous effort has been
dedicated over the past decades to measure . Notably, Planck cosmic
microwave background (CMB) and the local Cepheid-supernovae distance ladder
measurements determine with a precision of and
respectively. A - level of discrepancy exists between the two
measurements, for reasons that have yet to be understood. Gravitational wave
(GW) sources accompanied by electromagnetic (EM) counterparts offer a
completely independent standard siren (the GW analogue of an astronomical
standard candle) measurement of , as demonstrated following the discovery
of the neutron star merger, GW170817. This measurement does not assume a
cosmological model and is independent of a cosmic distance ladder. The first
joint analysis of the GW signal from GW170817 and its EM localization led to a
measurement of km/s/Mpc (median and symmetric
credible interval). In this analysis, the degeneracy in the GW signal between
the source distance and the weakly constrained viewing angle dominated the
measurement uncertainty. Recently, Mooley et al. (2018) obtained tight
constraints on the viewing angle using high angular resolution imaging of the
radio counterpart of GW170817. Here we obtain a significantly improved
measurement km/s/Mpc by using these new radio
observations, combined with the previous GW and EM data. We estimate that 15
more localized GW170817-like events (comparable signal-to-noise ratio,
favorable orientation), having radio images and light curve data, will
potentially bring resolution to the tension between the Planck and
Cepheid-supernova measurements, as compared to 50-100 GW events without such
data.Comment: 29 pages, 9 figure
Caltech-NRAO Stripe 82 Survey (CNSS) Paper V: AGNs that transitioned toradio-loud state
A recent multi-year Caltech-NRAO Stripe 82 Survey (CNSS) revealed a group of objects that appeared as new radio sources after >5--20 years of absence. They are transient phenomena with respect to the Faint Images of the Radio Sky at Twenty Centimeters (FIRST) survey and constitute the first unbiased sample of renewed radio activity. Here we present the follow-up, radio, optical and X-ray study of them. The group consist of 12 sources, both quasars and galaxies with wide redshift (0.04 24.5) distribution. Their radio properties in the first phase of activity, namely the convex spectra and compact morphology, allow them all to be classified as gigahertz-peaked spectrum (GPS) sources. We conclude that the spectral changes are a consequence of the evolution of newly-born radio jets. Our observations show that over the next few years of activity the GPS galaxies keep the convex shape of the spectrum, while GPS quasars rapidly transform into flat-spectrum sources, which may result in them not being recognized as young sources. The wide range of bolometric luminosities, black hole masses and jet powers among the transient sources indicates even greater population diversity in the group of young radio objects. We also suggest that small changes of the accretion disc luminosity (accretion rate) may be sufficient to ignite low-power radio activity that evolves on the scale of decades
A Strategy for LSST to Unveil a Population of Kilonovae without Gravitational-wave Triggers
We present a cadence optimization strategy to unveil a large population of kilonovae using optical imaging alone. These transients are generated during binary neutron star and potentially neutron star–black hole mergers and are electromagnetic counterparts to gravitational-wave signals detectable in nearby events with Advanced LIGO, Advanced Virgo, and other interferometers that will be online in the near future. Discovering a large population of kilonovae will allow us to determine how heavy-element production varies with the intrinsic parameters of the merger and across cosmic time. The rate of binary neutron star mergers is still uncertain, but only few (≾ 15) events with associated kilonovae may be detectable per year within the horizon of next-generation ground-based interferometers. The rapid evolution (~days) at optical/infrared wavelengths, relatively low luminosity, and the low volumetric rate of kilonovae makes their discovery difficult, especially during blind surveys of the sky. We propose future large surveys to adopt a rolling cadence in which g-i observations are taken nightly for blocks of 10 consecutive nights. With the current baseline2018a cadence designed for the Large Synoptic Survey Telescope (LSST), l≾ 7.5 poorly sampled kilonovae are expected to be detected in both the Wide Fast Deep (WFD) and Deep Drilling Fields (DDF) surveys per year, under optimistic assumptions on their rate, duration, and luminosity. We estimate the proposed strategy to return up to ~272 GW170817-like kilonovae throughout the LSST WFD survey, discovered independently from gravitational-wave triggers
SPIRITS 16tn in NGC 3556: A heavily obscured and low-luminosity supernova at 8.8 Mpc
We present the discovery by the SPitzer InfraRed Intensive Transients Survey
(SPIRITS) of a likely supernova (SN) in NGC 3556 at only 8.8 Mpc, which,
despite its proximity, was not detected by optical searches. A luminous
infrared (IR) transient at mag (Vega), SPIRITS 16tn is
coincident with a dust lane in the inclined, star-forming disk of the host.
Using IR, optical, and radio observations, we attempt to determine the nature
of this event. We estimate 8 - 9 mag of extinction, placing it
among the three most highly obscured IR-discovered SNe to date. The [4.5] light
curve declined at a rate of 0.013 mag day, and the color
grew redder from 0.7 to 1.0 mag by 184.7 days post discovery.
Optical/IR spectroscopy shows a red continuum, but no clearly discernible
features, preventing a definitive spectroscopic classification. Deep radio
observations constrain the radio luminosity of SPIRITS 16tn to erg s Hz between 3 - 15 GHz, excluding many
varieties of radio core-collapse SNe. A type Ia SN is ruled out by the observed
red IR color, and lack of features normally attributed to Fe-peak elements in
the optical and IR spectra. SPIRITS 16tn was fainter at [4.5] than typical
stripped-envelope SNe by 1 mag. Comparison of the spectral energy
distribution to SNe II suggests SPIRITS 16tn was both highly obscured, and
intrinsically dim, possibly akin to the low-luminosity SN 2005cs. We infer the
presence of an IR dust echo powered by a peak luminosity of the transient of erg s erg s,
consistent with the observed range for SNe II. This discovery illustrates the
power of IR surveys to overcome the compounding effects of visible extinction
and optically sub-luminous events in completing the inventory of nearby SNe.Comment: 25 pages, 10 figures, submitted to Ap
An Upper Limit on the Linear Polarization Fraction of the GW170817 Radio Continuum
We present late-time radio observations of GW170817, the first binary neutron-star (NS) merger discovered through gravitational waves (GWs) by the advanced Laser Interferometer Gravitational-wave Observatory (LIGO) and Virgo detectors. Our observations, carried out with the Karl G. Jansky Very Large Array (VLA), were optimized to detect polarized radio emission, and thus to constrain the linear polarization fraction of GW170817. At an epoch of ≈244 days after the merger, we rule out linearly polarized emission above a fraction of ≈12% at a frequency of 2.8 GHz (99% confidence). Within the structured jet scenario (a.k.a. successful jet plus cocoon system) for GW170817, the derived upper limit on the radio continuum linear polarization fraction strongly constrains the magnetic field configuration in the shocked ejecta. We show that our results for GW170817 are compatible with the low level of linear polarization found in afterglows of cosmological long γ-ray bursts (GRBs). Finally, we discuss our findings in the context of future expectations for the study of radio counterparts of binary NS mergers identified by ground-based GW detectors