30 research outputs found
The Central Engine Structure of 3C120: Evidence for a Retrograde Black Hole or a Refilling Accretion Disk
The broad-line radio galaxy 3C120 is a powerful source of both X-ray and
radio emission including superluminal jet outflows. We report on our reanalysis
of 160 ks of Suzaku data taken in 2006, previously examined by Kataoka et al.
(2007). Spectral fits to the XIS and HXD/PIN data over a range of 0.7-45 keV
reveal a well-defined iron K line complex with a narrow Ka core and
relativistically broadened features consistent with emission from the inner
regions of the accretion disk. Furthermore, the inner region of the disk
appears to be truncated with an inner radius of r_in = 11.7^{+3.5}_{-5.2} r_g.
If we assume that fluorescent iron line features terminate at the inner-most
stable circular orbit (ISCO), we measure a black hole spin of a < -0.1 at a 90%
confidence level. A rapidly spinning prograde black hole (a > 0.8) can be ruled
out at the 99% confidence level. Alternatively, the disk may be truncated well
outside of the ISCO of a rapid prograde hole. The most compelling scenario is
the possibility that the inner regions of the disk were destroyed/ejected by
catastrophic instabilities just prior to the time these observations were made.Comment: Preprint Formatting. 14 pages with 3 figures and 2 tables. Accepted
for publication in ApJ Letter
SN 2016iet: The Pulsational or Pair Instability Explosion of a Low Metallicity Massive CO Core Embedded in a Dense Hydrogen-Poor Circumstellar Medium
We present optical photometry and spectroscopy of SN 2016iet, an
unprecedented Type I supernova (SN) at with no obvious analog in the
existing literature. The peculiar light curve has two roughly equal brightness
peaks ( mag) separated by 100 days, and a subsequent slow decline
by 5 mag in 650 rest-frame days. The spectra are dominated by emission lines of
calcium and oxygen, with a width of only km s, superposed on a
strong blue continuum in the first year, and with a large ratio of at late times. There is no clear evidence
for hydrogen or helium associated with the SN at any phase. We model the light
curves with several potential energy sources: radioactive decay, central
engine, and circumstellar medium (CSM) interaction. Regardless of the model,
the inferred progenitor mass near the end of its life (i.e., CO core mass) is
M and up to M, placing the event in the
regime of pulsational pair instability supernovae (PPISNe) or pair instability
supernovae (PISNe). The models of CSM interaction provide the most consistent
explanation for the light curves and spectra, and require a CSM mass of
M ejected in the final decade before explosion. We further
find that SN 2016iet is located at an unusually large offset ( kpc) from
its low metallicity dwarf host galaxy ( Z, M), supporting the PPISN/PISN interpretation. In the final
spectrum, we detect narrow H emission at the SN location, likely due to
a dim underlying galaxy host or an H II region. Despite the overall consistency
of the SN and its unusual environment with PPISNe and PISNe, we find that the
inferred properties of SN\,2016iet challenge existing models of such events.Comment: 26 Pages, 17 Figures, Submitted to Ap
The low-frequency radio catalog of flat spectrum sources
A well known property of the gamma-ray sources detected by COS-B in the
1970s, by the Compton Gamma-ray Observatory in the 1990s and recently by the
Fermi observations is the presence of radio counterparts, in particular for
those associated to extragalactic objects. This observational evidence is the
basis of the radio-gamma-ray connection established for the class of active
galactic nuclei known as blazars. In particular, the main spectral property of
the radio counterparts associated with gamma-ray blazars is that they show a
flat spectrum in the GHz frequency range. Our recent analysis dedicated to
search blazar-like candidates as potential counterparts for the unidentified
gamma-ray sources (UGSs) allowed us to extend the radio-gamma-ray connection in
the MHz regime. We also showed that below 1 GHz blazars maintain flat radio
spectra. Thus on the basis of these new results, we assembled a low-frequency
radio catalog of flat spectrum sources built by combining the radio
observations of the Westerbork Northern Sky Survey (WENSS) and of the
Westerbork in the southern hemisphere (WISH) catalog with those of the NRAO
Very Large Array Sky survey (NVSS). This could be used in the future to search
for new, unknown blazar-like counterparts of the gamma-ray sources. First we
found NVSS counterparts of WSRT radio sources and then we selected flat
spectrum radio sources according to a new spectral criterion specifically
defined for radio observations performed below 1 GHz. We also described the
main properties of the catalog listing 28358 radio sources and their logN-logS
distributions. Finally a comparison with with the Green Bank 6-cm radio source
catalog has been performed to investigate the spectral shape of the
low-frequency flat spectrum radio sources at higher frequencies.Comment: 10 pages, 10 figures, 1 table, ApJS published in 2014 (pre-proof
version uploaded
Presto-Color: A Photometric Survey Cadence for Explosive Physics and Fast Transients
We identify minimal observing cadence requirements that enable photometric astronomical surveys to detect and recognize fast and explosive transients and fast transient features. Observations in two different filters within a short time window (e.g., g-and-i, or r-and-z, within 1.5 hr) are desirable for this purpose. Such an observing strategy delivers both the color and light curve evolution of transients on the same night. This allows the identification and initial characterization of fast transient—or fast features of longer timescale transients—such as rapidly declining supernovae, kilonovae, and the signatures of SN ejecta interacting with binary companion stars or circumstellar material. Some of these extragalactic transients are intrinsically rare and generally all hard to find, thus upcoming surveys like the Large Synoptic Survey Telescope (LSST) could dramatically improve our understanding of their origin and properties. We colloquially refer to such a strategy implementation for the LSST as the Presto-Color strategy (rapid-color). This cadence's minimal requirements allow for overall optimization of a survey for other science goals
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
The Tidal Disruption Event AT 2018hyz II: Light Curve Modeling of a Partially Disrupted Star
AT 2018hyz (=ASASSN-18zj) is a tidal disruption event (TDE) located in the
nucleus of a quiescent E+A galaxy at a redshift of , first
detected by the All-Sky Automated Survey for Supernovae (ASAS-SN). We present
optical+UV photometry of the transient, as well as an X-ray spectrum and radio
upper limits. The bolometric light curve of AT 2018hyz is comparable to other
known TDEs and declines at a rate consistent with a at early times,
emitting a total radiated energy of erg. An excess bump
appears in the UV light curve about 50 days after bolometric peak, followed by
a flattening beyond 250 days. The light curve shows an excess bump in the UV
about 50 days after bolometric peak lasting for at least 100 days, which may be
related to an outflow. We detect a constant X-ray source present for at least
86 days. The X-ray spectrum shows a total unabsorbed flux of erg cm s and is best fit by a blackbody plus
power-law model with a photon index of . A thermal X-ray model is
unable to account for photons keV, while the radio non-detection favors
inverse-Compton scattering rather than a jet for the non-thermal component. We
model the optical and UV light curves using the Modular Open-Source Fitter for
Transients (MOSFiT) and find a best fit for a black hole of
M partially disrupting a M star (stripping a mass of
M for the inferred impact parameter, ). The low
optical depth implied by the small debris mass may explain how we are able to
see hydrogen emission with disk-like line profiles in the spectra of AT 2018hyz
(see our companion paper, Short et al.~2020).Comment: 10 pages, 10 figures, published in MNRA
Presto-Color: A Photometric Survey Cadence for Explosive Physics and Fast Transients
We identify minimal observing cadence requirements that enable photometric astronomical surveys to detect and recognize fast and explosive transients and fast transient features. Observations in two different filters within a short time window (e.g., g-and-i, or r-and-z, within 1.5 hr) are desirable for this purpose. Such an observing strategy delivers both the color and light curve evolution of transients on the same night. This allows the identification and initial characterization of fast transient—or fast features of longer timescale transients—such as rapidly declining supernovae, kilonovae, and the signatures of SN ejecta interacting with binary companion stars or circumstellar material. Some of these extragalactic transients are intrinsically rare and generally all hard to find, thus upcoming surveys like the Large Synoptic Survey Telescope (LSST) could dramatically improve our understanding of their origin and properties. We colloquially refer to such a strategy implementation for the LSST as the Presto-Color strategy (rapid-color). This cadence's minimal requirements allow for overall optimization of a survey for other science goals