50 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 Green Bank Northern Celestial Cap Pulsar Survey II: The Discovery and Timing of Ten Pulsars
We present timing solutions for ten pulsars discovered in 350 MHz searches
with the Green Bank Telescope. Nine of these were discovered in the Green Bank
Northern Celestial Cap survey and one was discovered by students in the Pulsar
Search Collaboratory program in analysis of drift-scan data. Following
discovery and confirmation with the Green Bank Telescope, timing has yielded
phase-connected solutions with high precision measurements of rotational and
astrometric parameters. Eight of the pulsars are slow and isolated, including
PSR J09302301, a pulsar with nulling fraction lower limit of 30\% and
nulling timescale of seconds to minutes. This pulsar also shows evidence of
mode changing. The remaining two pulsars have undergone recycling, accreting
material from binary companions, resulting in higher spin frequencies. PSR
J05572948 is an isolated, 44 \rm{ms} pulsar that has been partially recycled
and is likely a former member of a binary system which was disrupted by a
second supernova. The paucity of such so-called `disrupted binary pulsars'
(DRPs) compared to double neutron star (DNS) binaries can be used to test
current evolutionary scenarios, especially the kicks imparted on the neutron
stars in the second supernova. There is some evidence that DRPs have larger
space velocities, which could explain their small numbers. PSR J1806+2819 is a
15 \rm{ms} pulsar in a 44 day orbit with a low mass white dwarf companion. We
did not detect the companion in archival optical data, indicating that it must
be older than 1200 Myr.Comment: 9 pages, 5 figure
The Galactic Nova Rate: Estimates from the ASAS-SN and Gaia Surveys
We present the first estimate of the Galactic nova rate based on optical
transient surveys covering the entire sky. Using data from the All-Sky
Automated Survey for Supernovae (ASAS-SN) and \textit{Gaia} -- the only two
all-sky surveys to report classical nova candidates -- we find 39 confirmed
Galactic novae and 7 additional unconfirmed candidates discovered from
2019--2021, yielding a nova discovery rate of yr. Using
accurate Galactic stellar mass models, three-dimensional dust maps, and
incorporating realistic nova light curves, we have built a sophisticated
Galactic nova model that allows an estimate of the recovery fraction of
Galactic novae from these surveys over this time period. The observing
capabilities of each survey are distinct: the high cadence of ASAS-SN makes it
sensitive to fast novae, while the broad observing filter and high spatial
resolution of \textit{Gaia} make it more sensitive to highly reddened novae
across the entire Galactic plane and bulge. Despite these differences, we find
that ASAS-SN and \textit{Gaia} give consistent Galactic nova rates, with a
final joint nova rate of yr. This inferred nova rate is
substantially lower than found by many other recent studies. Critically
assessing the systematic uncertainties in the Galactic nova rate, we argue that
the role of faint fast-fading novae has likely been overestimated, but that
subtle details in the operation of transient alert pipelines can have large,
sometimes unappreciated effects on transient recovery efficiency. Our predicted
nova rate can be directly tested with forthcoming red/near-infrared transient
surveys in the southern hemisphere.Comment: 24 pages, 9 figure
Early spectral evolution of classical novae: consistent evidence for multiple distinct outflows
The physical mechanism driving mass ejection during a nova eruption is still
poorly understood. Possibilities include ejection in a single ballistic event,
a common envelope interaction, a continuous wind, or some combination of these
processes. Here we present a study of 12 Galactic novae, for which we have
pre-maximum high-resolution spectroscopy. All 12 novae show the same spectral
evolution. Before optical peak, they show a slow P Cygni component. After peak
a fast component quickly arises, while the slow absorption remains superimposed
on top of it, implying the presence of at least two physically distinct flows.
For novae with high-cadence monitoring, a third, intermediate-velocity
component is also observed.
These observations are consistent with a scenario where the slow component is
associated with the initial ejection of the accreted material and the fast
component with a radiation-driven wind from the white dwarf. When these flows
interact, the slow flow is swept up by the fast flow, producing the
intermediate component. These colliding flows may produce the gamma-ray
emission observed in some novae. Our spectra also show that the transient heavy
element absorption lines seen in some novae have the same velocity structure
and evolution as the other lines in the spectrum, implying an association with
the nova ejecta rather than a pre-existing circumbinary reservoir of gas or
material ablated from the secondary. While this basic scenario appears to
qualitatively reproduce multi-wavelength observations of classical novae,
substantial theoretical and observational work is still needed to untangle the
rich diversity of nova properties.Comment: 39 pages, 35 figures, submitted to Ap
The Green Bank Northern Celestial Cap Pulsar Survey. II. the Discovery and Timing of 10 Pulsars
We present timing solutions for 10 pulsars discovered in 350 MHz searches with the Green Bank Telescope. Nine of these were discovered in the Green Bank Northern Celestial Cap survey and one was discovered by students in the Pulsar Search Collaboratory program during an analysis of drift-scan data. Following the discovery and confirmation with the Green Bank Telescope, timing has yielded phase-connected solutions with high-precision measurements of rotational and astrometric parameters. Eight of the pulsars are slow and isolated, including PSR J0930-2301, a pulsar with a nulling fraction lower limit of ∼30% and a nulling timescale of seconds to minutes. This pulsar also shows evidence of mode changing. The remaining two pulsars have undergone recycling, accreting material from binary companions, resulting in higher spin frequencies. PSR J0557-2948 is an isolated, 44 ms pulsar that has been partially recycled and is likely a former member of a binary system that was disrupted by a second supernova. The paucity of such so-called \ disrupted binary pulsars\ (DRPs) compared to double neutron star (DNS) binaries can be used to test current evolutionary scenarios, especially the kicks imparted on the neutron stars in the second supernova. There is some evidence that DRPs have larger space velocities, which could explain their small numbers. PSR J1806+2819 is a 15 ms pulsar in a 44-day orbit with a low-mass white dwarf companion. We did not detect the companion in archival optical data, indicating that it must be older than 1200 Myr
Catching a nova X-ray/UV flash in the visible? Early spectroscopy of the extremely slow Nova Velorum 2022 (Gaia22alz)
We present early spectral observations of the very slow Galactic nova
Gaia22alz, over its gradual rise to peak brightness that lasted 180 days.
During the first 50 days, when the nova was only 3--4 magnitudes above its
normal brightness, the spectra showed narrow (FWHM 400 km s)
emission lines of H Balmer, He I, He II, and C IV, but no P Cygni absorption. A
few weeks later, the high-excitation He II and C IV lines disappeared, and P
Cygni profiles of Balmer, He I, and eventually Fe II lines emerged, yielding a
spectrum typical of classical novae before peak. We propose that the early
spectra of Gaia22alz are produced in the white dwarf's envelope or accretion
disk, reprocessing X-ray and ultraviolet emission from the white dwarf after a
dramatic increase in the rate of thermonuclear reactions, during a phase known
as the ``early X-ray/UV flash''. If true, this would be one of the rare times
that the optical signature of the early X-ray/UV flash has been detected. While
this phase might last only a few hours in other novae and thus be easily
missed, it was possible to detect in Gaia22alz due to its very slow and gradual
rise and thanks to the efficiency of new all-sky surveys in detecting
transients on their rise. We also consider alternative scenarios that could
explain the early spectral features of Gaia22alz and its unusually slow rise.Comment: 20 pages, 12 figures, 2 tables. Submitted to MNRA
The galactic nova rate: estimates from the ASAS-SN and Gaia Surveys
Stars and planetary systemsHigh Energy Astrophysic
The multi-wavelength view of shocks in the fastest nova V1674 Her
Classical novae are shock-powered multi-wavelength transients triggered by a
thermonuclear runaway on an accreting white dwarf. V1674 Her is the fastest
nova ever recorded (time to declined by two magnitudes is t_2=1.1 d) that
challenges our understanding of shock formation in novae. We investigate the
physical mechanisms behind nova emission from GeV gamma-rays to cm-band radio
using coordinated Fermi-LAT, NuSTAR, Swift and VLA observations supported by
optical photometry. Fermi-LAT detected short-lived (18 h) 0.1-100 GeV emission
from V1674 Her that appeared 6 h after the eruption began; this was at a level
of (1.6 +/- 0.4)x10^-6 photons cm^-2 s^-1. Eleven days later, simultaneous
NuSTAR and Swift X-ray observations revealed optically thin thermal plasma
shock-heated to kT_shock = 4 keV. The lack of a detectable 6.7 keV Fe K_alpha
emission suggests super-solar CNO abundances. The radio emission from V1674 Her
was consistent with thermal emission at early times and synchrotron at late
times. The radio spectrum steeply rising with frequency may be a result of
either free-free absorption of synchrotron and thermal emission by unshocked
outer regions of the nova shell or the Razin-Tsytovich effect attenuating
synchrotron emission in dense plasma. The development of the shock inside the
ejecta is unaffected by the extraordinarily rapid evolution and the
intermediate polar host of this nova.Comment: 20 pages, 9 figures, 3 tables. Accepted to MNRA
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