166 research outputs found
Optically variable active galactic nuclei in the 3 yr VST survey of the COSMOS field
The analysis of the variability of active galactic nuclei (AGNs) at different
wavelengths and the study of possible correlations among different spectral
windows are nowadays a major field of inquiry. Optical variability has been
largely used to identify AGNs in multivisit surveys. The strength of a
selection based on optical variability lies in the chance to analyze data from
surveys of large sky areas by ground-based telescopes. However the
effectiveness of optical variability selection, with respect to other
multiwavelength techniques, has been poorly studied down to the depth expected
from next generation surveys. Here we present the results of our r-band
analysis of a sample of 299 optically variable AGN candidates in the VST survey
of the COSMOS field, counting 54 visits spread over three observing seasons
spanning > 3 yr. This dataset is > 3 times larger in size than the one
presented in our previous analysis (De Cicco et al. 2015), and the observing
baseline is ~8 times longer. We push towards deeper magnitudes (r(AB) ~23.5
mag) compared to past studies; we make wide use of ancillary multiwavelength
catalogs in order to confirm the nature of our AGN candidates, and constrain
the accuracy of the method based on spectroscopic and photometric diagnostics.
We also perform tests aimed at assessing the relevance of dense sampling in
view of future wide-field surveys. We demonstrate that the method allows the
selection of high-purity (> 86%) samples. We take advantage of the longer
observing baseline to achieve great improvement in the completeness of our
sample with respect to X-ray and spectroscopically confirmed samples of AGNs
(59%, vs. ~15% in our previous work), as well as in the completeness of
unobscured and obscured AGNs. The effectiveness of the method confirms the
importance to develop future, more refined techniques for the automated
analysis of larger datasets.Comment: 21 pages, 10 figures; accepted for publication in A&
Optimizing Cadences with Realistic Light Curve Filtering for Serendipitous Kilonova Discovery with Vera Rubin Observatory
Current and future optical and near-infrared wide-field surveys have the potential of finding kilonovae, the optical and infrared counterparts to neutron star mergers, independently of gravitational-wave or high-energy gamma-ray burst triggers. The ability to discover fast and faint transients such as kilonovae largely depends on the area observed, the depth of those observations, the number of re-visits per field in a given time frame, and the filters adopted by the survey; it also depends on the ability to perform rapid follow-up observations to confirm the nature of the transients. In this work, we assess kilonova detectability in existing simulations of the LSST strategy for the Vera C. Rubin Wide Fast Deep survey, with focus on comparing rolling to baseline cadences. Although currently available cadences can enable the detection of more than 300 kilonovae out to 1400 Mpc over the ten-year survey, we can expect only 3-32 kilonovae similar to GW170817 to be recognizable as fast-evolving transients. We also explore the detectability of kilonovae over the plausible parameter space, focusing on viewing angle and ejecta masses. We find that observations in redder izy bands are crucial for identification of nearby (within 300 Mpc) kilonovae that could be spectroscopically classified more easily than more distant sources. Rubin's potential for serendipitous kilonova discovery could be increased by gain of efficiency with the employment of individual 30s exposures (as opposed to 2x15s snap pairs), with the addition of red-band observations coupled with same-night observations in g- or r-bands, and possibly with further development of a new rolling-cadence strategy
The VMC survey – XXIII. Model fitting of light and radial velocity curves of Small Magellanic Cloud classical Cepheids
We present the results of the χ2 minimization model fitting technique applied to optical and near-infrared photometric and radial velocity data for a sample of nine fundamental and three first overtone classical Cepheids in the Small Magellanic Cloud (SMC). The near-infrared photometry (JK filters) was obtained by the European Southern Observatory (ESO) public survey ‘VISTA near-infrared Y, J, Ks survey of the Magellanic Clouds system’ (VMC). For each pulsator, isoperiodic model sequences have been computed by adopting a non-linear convective hydrodynamical code in order to reproduce the multifilter light and (when available) radial velocity curve amplitudes and morphological details. The inferred individual distances provide an intrinsic mean value for the SMC distance modulus of 19.01 mag and a standard deviation of 0.08 mag, in agreement with the literature. Moreover, the intrinsic masses and luminosities of the best-fitting model show that all these pulsators are brighter than the canonical evolutionary mass–luminosity relation (MLR), suggesting a significant efficiency of core overshooting and/or mass-loss. Assuming that the inferred deviation from the canonical MLR is only due to mass-loss, we derive the expected distribution of percentage mass-loss as a function of both the pulsation period and the canonical stellar mass. Finally, a good agreement is found between the predicted mean radii and current period–radius (PR) relations in the SMC available in the literature. The results of this investigation support the predictive capabilities of the adopted theoretical scenario and pave the way for the application to other extensive data bases at various chemical compositions, including the VMC Large Magellanic Cloud pulsators and Galactic Cepheids with Gaia parallaxe
Extending the variability selection of active galactic nuclei in the W-CDF-S and SERVS/SWIRE region
Variability has proven to be a powerful tool to detect active galactic nuclei (AGN) in multi-epoch surveys. The new-generation facilities expected to become operational in the next few years will mark a new era in time-domain astronomy and their wide-field multi-epoch campaigns will favor extensive variability studies.We present our analysis of AGN variability in the second half of the VST survey of the Wide Chandra Deep Field South, performed in the r band and covering a 2 sq. deg area. The analysis complements a previous work, in which the first half of the area was investigated. We provide a reliable catalog of variable AGN candidates, which will be critical targets in future variability studies
Variability and transient search in the SUDARE–VOICE field: A new method to extract the light curves
The VLT Survey Telescope (VST) Optical Imaging of the CDFS and ES1 Fields Survey, in
synergy with the SUDARE survey, is a deep optical ugri imaging of the CDFS and ES1 fields
using the VST. The observations for the CDFS field comprise about 4.38 deg2 down to r ∼
26 mag. The total on-sky time spans over 4 yr in this field, distributed over four adjacent subfields. In this paper, we use the multiepoch r-band imaging data to measure the variability of
the detected objects and search for transients. We perform careful astrometric and photometric
calibrations and point spread function modelling. A new method, referring to as differential
running-average photometry, is proposed to measure the light curves of the detected objects.
With the method, the difference of PSFs between different epochs can be reduced, and the
background fluctuations are also suppressed
Quantitative assessment of harmonic power doppler myocardial perfusion imaging with intravenous levovist™ in patients with myocardial infarction: comparison with myocardial viability evaluated by coronary flow reserve and coronary flow pattern of infarct-related artery
BACKGROUND: Myocardial contrast echocardiography and coronary flow velocity pattern with a rapid diastolic deceleration time after percutaneous coronary intervention has been reported to be useful in assessing microvascular damage in patients with acute myocardial infarction. AIM: To evaluate myocardial contrast echocardiography with harmonic power Doppler imaging, coronary flow velocity reserve and coronary artery flow pattern in predicting functional recovery by using transthoracic echocardiography. METHODS: Thirty patients with anterior acute myocardial infarction underwent myocardial contrast echocardiography at rest and during hyperemia and were quantitatively analyzed by the peak color pixel intensity ratio of the risk area to the control area (PIR). Coronary flow pattern was measured using transthoracic echocardiography in the distal portion of left anterior descending artery within 24 hours after recanalization and we assessed deceleration time of diastolic flow velocity. Coronary flow velocity reserve was calculated two weeks after acute myocardial infarction. Left ventricular end-diastolic volumes and ejection fraction by angiography were computed. RESULTS: Pts were divided into 2 groups according to the deceleration time of coronary artery flow pattern (Group A; 20 pts with deceleration time ≧ 600 msec, Group B; 10 pts with deceleration time < 600 msec). In acute phase, there were no significant differences in left ventricular end-diastolic volume and ejection fraction (Left ventricular end-diastolic volume 112 ± 33 vs. 146 ± 38 ml, ejection fraction 50 ± 7 vs. 45 ± 9 %; group A vs. B). However, left ventricular end-diastolic volume in Group B was significantly larger than that in Group A (192 ± 39 vs. 114 ± 30 ml, p < 0.01), and ejection fraction in Group B was significantly lower than that in Group A (39 ± 9 vs. 52 ± 7%, p < 0.01) at 6 months. PIR and coronary flow velocity reserve of Group A were higher than Group B (PIR, at rest: 0.668 ± 0.178 vs. 0.248 ± 0.015, p < 0.0001: during hyperemia 0.725 ± 0.194 vs. 0.295 ± 0.107, p < 0.0001; coronary flow velocity reserve, 2.60 ± 0.80 vs. 1.31 ± 0.29, p = 0.0002, respectively). CONCLUSION: The preserved microvasculature detecting by myocardial contrast echocardiography and coronary flow velocity reserve is related to functional recovery after acute myocardial infarction
Multiwavelength observations of the extraordinary accretion event AT2021lwx
We present observations from X-ray to mid-infrared wavelengths of the most
energetic non-quasar transient ever observed, AT2021lwx. Our data show a single
optical brightening by a factor to a luminosity of erg
s, and a total radiated energy of erg, both greater
than any known optical transient. The decline is smooth and exponential and the
ultra-violet - optical spectral energy distribution resembles a black body with
temperature K. Tentative X-ray detections indicate a secondary
mode of emission, while a delayed mid-infrared flare points to the presence of
dust surrounding the transient. The spectra are similar to recently discovered
optical flares in known active galactic nuclei but lack some characteristic
features. The lack of emission for the previous seven years is inconsistent
with the short-term, stochastic variability observed in quasars, while the
extreme luminosity and long timescale of the transient disfavour the disruption
of a single solar-mass star. The luminosity could be generated by the
disruption of a much more massive star, but the likelihood of such an event
occurring is small. A plausible scenario is the accretion of a giant molecular
cloud by a dormant black hole of solar masses. AT2021lwx thus
represents an extreme extension of the known scenarios of black hole accretion.Comment: 11 pages, 5 figures, Accepted for publication in MNRA
SN 2022jli: a type Ic supernova with periodic modulation of its light curve and an unusually long rise
We present multi-wavelength photometry and spectroscopy of SN 2022jli, an
unprecedented Type Ic supernova discovered in the galaxy NGC 157 at a distance
of 23 Mpc. The multi-band light curves reveal many remarkable
characteristics. Peaking at a magnitude of , the high-cadence
photometry reveals 12.5day periodic undulations superimposed on the
200 day supernova decline. This periodicity is observed in the light curves
from nine separate filter and instrument configurations with peak-to-peak
amplitudes of 0.1 mag. This is the first time that repeated periodic
oscillations, over many cycles, have been detected in a supernova light curve.
SN 2022jli also displays an extreme early excess which fades over 25
days followed by a rise to a peak luminosity of erg
s. Although the exact explosion epoch is not constrained by data, the
time from explosion to maximum light is 59 days. The luminosity can
be explained by a large ejecta mass (M)
powered by Ni but we find difficulty in quantitatively modelling the
early excess with circumstellar interaction and cooling. Collision between the
supernova ejecta and a binary companion is a possible source of this emission.
We discuss the origin of the periodic variability in the light curve, including
interaction of the SN ejecta with nested shells of circumstellar matter and
neutron stars colliding with binary companions.Comment: Accepted in ApJ
Influência da segregação granulométrica e do emprego de aditivos de moagem na adequação de cinzas de casca de arroz como coproduto
RESUMO A cinza de casca de arroz (CCA) é um resíduo proveniente da combustão da casca de arroz utilizada como biomassa na produção de energia. Esta cinza é gerada em grandes quantidades e possui baixa massa específica, o que dificulta o seu gerenciamento, pois demanda muito espaço para o devido armazenamento e descarte. A CCA possui um elevado teor de sílica em sua composição, fator este que pode torná-la um material atrativo para vários segmentos industriais. Neste contexto, este trabalho tem como objetivo avaliar a influência do beneficiamento físico da cinza de casca de arroz, por meio de processos de segregação granulométrica e moagem, com e sem o uso de aditivos, nas características deste material e na sua adequação como coproduto. A metodologia experimental utilizada para o desenvolvimento deste trabalho envolveu a segregação e moagem da CCA (com e sem aditivos de moagem), caracterização química, física e estrutural das amostras de CCA bruta, segregadas e moídas. Os resultados obtidos indicam que a segregação granulométrica se apresenta como fator determinante para a utilização da CCA como coproduto. Com relação à moagem, pode-se verificar que o diâmetro médio das partículas diminui e a massa específica das amostras aumenta, com o aumento do tempo de moagem. Entretanto, verifica-se que os aditivos usados neste trabalho, nas concentrações testadas, não influenciam significativamente na redução do diâmetro das partículas
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