Abstract
We present the discovery that ASASSN-14ko is a periodically flaring active galactic nucleus at the center of the galaxy ESO 253-G003. At the time of its discovery by the All-Sky Automated ...Survey for Supernovae (ASAS-SN), it was classified as a supernova close to the nucleus. The subsequent 6 yr of
V
- and
g
-band ASAS-SN observations revealed that ASASSN-14ko has nuclear flares occurring at regular intervals. The 17 observed outbursts show evidence of a decreasing period over time, with a mean period of
P
0
= 114.2 ± 0.4 days and a period derivative of
. The most recent outburst in 2020 May, which took place as predicted, exhibited spectroscopic changes during the rise and had a UV bright, blackbody spectral energy distribution similar to tidal disruption events (TDEs). The X-ray flux decreased by a factor of 4 at the beginning of the outburst and then returned to its quiescent flux after ∼8 days. The Transiting Exoplanet Survey Satellite observed an outburst during Sectors 4–6, revealing a rise time of 5.60 ± 0.05 days in the optical and a decline that is best fit with an exponential model. We discuss several possible scenarios to explain ASASSN-14ko’s periodic outbursts, but currently favor a repeated partial TDE. The next outbursts should peak in the optical on UT 2020 September 7.4±1.1 and UT 2020 December 26.5±1.4.
X-ray response to disc evolution in two γ Cas stars Nazé, Yaël; Rauw, Gregor; Bohlsen, Terrence ...
Monthly notices of the Royal Astronomical Society,
05/2022, Letnik:
512, Številka:
2
Journal Article, Web Resource
Recenzirano
Odprti dostop
ABSTRACT
The Hα emission of a set of southern γ Cas stars was monitored since 2019, with the aim of detecting transition events and examining how their peculiar X-ray emission would react in such ...cases. Two stars, namely HD 119682 and V767 Cen, were found to display slowly decreasing disc emissions. These decreases were not perfectly monotonic and several temporary and limited rebuilding events were observed. For HD 119682, the emission component in Hα disappeared in 2020 mid-July. In X-rays, the X-ray flux was twice smaller than that recorded two decades ago but of a similar level as observed a decade ago. The X-ray flux decreased over the campaign by 30 per cent, but the hardness remained similar in data sets of all epochs. In particular, the γ Cas character remained as clear as before even when there was no trace of disc emission in the Hα line. For V767 Cen, the full disappearance of disc emission in Hα never occurred. We followed closely a disc rebuilding event, but no significant change in flux or hardness was detected. These behaviours are compared to those of other γ Cas stars and their consequences on the X-ray generation are discussed.
Abstract
Eta Carinae (
η
Car) exhibits a unique set of P Cygni profiles with both broad and narrow components. Over many decades, the spectrum has changed—there has been an increase in observed ...continuum fluxes and a decrease in Fe
ii
and H
i
emission-line equivalent widths. The spectrum is evolving toward that of a P Cygni star such as P Cygni itself and HDE 316285. The spectral evolution has been attributed to intrinsic variations such as a decrease in the mass-loss rate of the primary star or differential evolution in a latitudinal-dependent stellar wind. However, intrinsic wind changes conflict with three observational results: the steady long-term bolometric luminosity; the repeating X-ray light curve over the binary period; and the constancy of the dust-scattered spectrum from the Homunculus. We extend previous work that showed a secular strengthening of P Cygni absorptions by adding more orbital cycles to overcome temporary instabilities and by examining more atomic transitions.
cmfgen
modeling of the primary wind shows that a time-decreasing mass-loss rate is not the best explanation for the observations. However, models with a
small
dissipating absorber in our line of sight can explain both the increase in brightness and changes in the emission and P Cygni absorption profiles. If the spectral evolution is caused by the dissipating circumstellar medium, and not by intrinsic changes in the binary, the dynamical timescale to recover from the Great Eruption is much less than a century, different from previous suggestions.
The Halpha emission of a set of southern gamma-Cas stars was monitored since 2019, with the aim of detecting transition events and examining how their peculiar X-ray emission would react in such ...cases. Two stars, HD119682 and V767Cen, were found to display slowly decreasing disk emissions. These decreases were not perfectly monotonic and several temporary and limited re-building events were observed. For HD119682, the emission component in Halpha disappeared in mid-July 2020. In X-rays, the X-ray flux was twice smaller than recorded two decades ago but of a similar level as observed a decade ago. The X-ray flux decreased over the campaign by 30%, but the hardness remained similar in datasets of all epochs. In particular, the gamma-Cas character remained as clear as before even when there was no trace of disk emission in the Halpha line. For V767Cen, the full disappearance of disk emission in Halpha never occurred. We followed closely a disk rebuilding event, but no significant change in flux or hardness was detected. These behaviours are compared to those of other gamma-Cas stars and their consequences on the X-ray generation are discussed.
We present the discovery that ASASSN-14ko is a periodically flaring AGN at the center of the galaxy ESO 253-G003. At the time of its discovery by the All-Sky Automated Survey for Supernovae ...(ASAS-SN), it was classified as a supernova close to the nucleus. The subsequent six years of V- and g-band ASAS-SN observations reveal that ASASSN-14ko has nuclear flares occurring at regular intervals. The seventeen observed outbursts show evidence of a decreasing period over time, with a mean period of \(P_0 = 114.2 \pm 0.4\) days and a period derivative of \(\dot{P} = -0.0017\pm0.0003\). The most recent outburst in May 2020, which took place as predicted, exhibited spectroscopic changes during the rise and a had a UV bright, blackbody spectral energy distribution similar to tidal disruption events (TDEs). The X-ray flux decreased by a factor of 4 at the beginning of the outburst and then returned to its quiescent flux after ~8 days. TESS observed an outburst during Sectors 4-6, revealing a rise time of \(5.60 \pm 0.05\) days in the optical and a decline that is best fit with an exponential model. We discuss several possible scenarios to explain ASASSN-14ko's periodic outbursts, but currently favor a repeated partial TDE. The next outbursts should peak in the optical on UT 2020-09-7.4\( \pm \)1.1 and UT 2020-12-26.5\( \pm \)1.4.
We report on the first multi-color precision light curve of the bright
Wolf-Rayet binary $\gamma^2$ Velorum, obtained over six months with the
nanosatellites in the BRITE- Constellation fleet. In ...parallel, we obtained 488
high-resolution optical spectra of the system. In this first report on the
datasets, we revise the spectroscopic orbit and report on the bulk properties
of the colliding winds. We find a dependence of both the light curve and excess
emission properties that scales with the inverse of the binary separation. When
analyzing the spectroscopic properties in combination with the photometry, we
find that the phase dependence is caused only by excess emission in the lines,
and not from a changing continuum. We also detect a narrow, high-velocity
absorption component from the He I $\lambda$5876 transition, which appears
twice in the orbit. We calculate smoothed-particle hydrodynamical simulations
of the colliding winds and can accurately associate the absorption from He I to
the leading and trailing arms of the wind shock cone passing tangentially
through our line of sight. The simulations also explain the general strength
and kinematics of the emission excess observed in wind lines such as C III
$\lambda$5696 of the system. These results represent the first in a series of
investigations into the winds and properties of $\gamma^2$ Velorum through
multi-technique and multi-wavelength observational campaigns.
From $5.5$ months of dual-band optical photometric monitoring at the $1$ mmag
level, BRITE-Constellation has revealed two simultaneous types of variability
in the O4I(n)fp star $\zeta$ Puppis: one ...single periodic non-sinusoidal
component superimposed on a stochastic component. The monoperiodic component is
the $1.78$ d signal previously detected by Coriolis/SMEI, but this time along
with a prominent first harmonic. The shape of this signal changes over time, a
behaviour that is incompatible with stellar oscillations but consistent with
rotational modulation arising from evolving bright surface inhomogeneities. By
means of a constrained non-linear light curve inversion algorithm we mapped the
locations of the bright surface spots and traced their evolution. Our
simultaneous ground-based multi-site spectroscopic monitoring of the star
unveiled cyclical modulation of its He II $\lambda4686$ wind emission line with
the $1.78$-day rotation period, showing signatures of Corotating Interaction
Regions (CIRs) that turn out to be driven by the bright photospheric spots
observed by BRITE. Traces of wind clumps are also observed in the He II
$\lambda4686$ line and are correlated with the amplitudes of the stochastic
component of the light variations probed by BRITE at the photosphere,
suggesting that the BRITE observations additionally unveiled the photospheric
drivers of wind clumps in $\zeta$ Pup and that the clumping phenomenon starts
at the very base of the wind. The origins of both the bright surface
inhomogeneities and the stochastic light variations remain unknown, but a
subsurface convective zone might play an important role in the generation of
these two types of photospheric variability.
Risk and control Maia, Isabela Ribeiro Damaso; de Cacella, Paulo Maurício F; Fernandes, José Luiz B
Central Banking,
11/2012, Letnik:
23, Številka:
2
Trade Publication Article
Based on a more strategic participation and holistic view of an organisation, risk management still has significant relevance for central banks and other entities. Risk and governance are relevant in ...the uncertain crisis environment people are living in now, especially if a change in the tasks and obligations of risk areas is made. The objective of this article is to propose a more strategic role for risk management that has an effect on the decision-making process and is based on the separation of the tasks of monitoring and control. To do so it is necessary to discuss risk and governance given the uncertain environment of today, and also review some behavioural aspects that may affect the decision-making process. We will also suggest a crucial change in the tasks and obligations of risk management areas, moving from a reactive perspective to a more strategic proactive institutional role, in line with the concept of enterprise risk management.
PDF
