5 research outputs found
Shocks and dust formation in nova V809 Cep
The discovery that many classical novae produce detectable GeV -ray
emission has raised the question of the role of shocks in nova eruptions. Here
we use radio observations of nova V809 Cep (Nova Cep 2013) with the Jansky Very
Large Array to show that it produced non-thermal emission indicative of
particle acceleration in strong shocks for more than a month starting about six
weeks into the eruption, quasi-simultaneous with the production of dust.
Broadly speaking, the radio emission at late times -- more than a six months or
so into the eruption -- is consistent with thermal emission from of freely expanding, ~K ejecta. At 4.6 and 7.4 GHz, however, the
radio light-curves display an initial early-time peak 76 days after the
discovery of the eruption in the optical (). The brightness temperature at
4.6 GHz on day 76 was greater than , an order of magnitude above what
is expected for thermal emission. We argue that the brightness temperature is
the result of synchrotron emission due to internal shocks within the ejecta.
The evolution of the radio spectrum was consistent with synchrotron emission
that peaked at high frequencies before low frequencies, suggesting that the
synchrotron from the shock was initially subject to free-free absorption by
optically thick ionized material in front of the shock. Dust formation began
around day 37, and we suggest that internal shocks in the ejecta were
established prior to dust formation and caused the nucleation of dust
The Gravity Collective: A Search for the Electromagnetic Counterpart to the Neutron Star-Black Hole Merger GW190814
We present optical follow-up imaging obtained with the Katzman Automatic
Imaging Telescope, Las Cumbres Observatory Global Telescope Network, Nickel
Telescope, Swope Telescope, and Thacher Telescope of the LIGO/Virgo
gravitational wave (GW) signal from the neutron star-black hole (NSBH) merger
GW190814. We searched the GW190814 localization region (19 deg for the
90th percentile best localization), covering a total of 51 deg and 94.6%
of the two-dimensional localization region. Analyzing the properties of 189
transients that we consider as candidate counterparts to the NSBH merger,
including their localizations, discovery times from merger, optical spectra,
likely host-galaxy redshifts, and photometric evolution, we conclude that none
of these objects are likely to be associated with GW190814. Based on this
finding, we consider the likely optical properties of an electromagnetic
counterpart to GW190814, including possible kilonovae and short gamma-ray burst
afterglows. Using the joint limits from our follow-up imaging, we conclude that
a counterpart with an -band decline rate of 0.68 mag day, similar to
the kilonova AT 2017gfo, could peak at an absolute magnitude of at most
mag (50% confidence). Our data are not constraining for ''red'' kilonovae and
rule out ''blue'' kilonovae with (30% confidence). We
strongly rule out all known types of short gamma-ray burst afterglows with
viewing angles 17 assuming an initial jet opening angle of
and explosion energies and circumburst densities similar to
afterglows explored in the literature. Finally, we explore the possibility that
GW190814 merged in the disk of an active galactic nucleus, of which we find
four in the localization region, but we do not find any candidate counterparts
among these sources.Comment: 86 pages, 9 figure
Classical novae at radio wavelengths
We present radio observations (1-40 GHz) for 36 classical novae, representing data from over five decades compiled from the literature, telescope archives, and our own programs. Our targets display a striking diversity in their optical parameters (e.g., spanning optical fading timescales, t 2 = 1-263 days), and we find a similar diversity in the radio light curves. Using a brightness temperature analysis, we find that radio emission from novae is a mixture of thermal and synchrotron emission, with nonthermal emission observed at earlier times. We identify high brightness temperature emission (T B > 5 104 K) as an indication of synchrotron emission in at least nine (25%) of the novae. We find a class of synchrotron-dominated novae with mildly evolved companions, exemplified by V5589 Sgr and V392 Per, that appear to be a bridge between classical novae with dwarf companions and symbiotic binaries with giant companions. Four of the novae in our sample have two distinct radio maxima (the first dominated by synchrotron and the later by thermal emission), and in four cases the early synchrotron peak is temporally coincident with a dramatic dip in the optical light curve, hinting at a common site for particle acceleration and dust formation. We publish the light curves in a machine-readable table and encourage the use of these data by the broader community in multiwavelength studies and modeling efforts