65 research outputs found
Optical variability of the BL Lacertae object GC 0109+224. Multiband behaviour and time scales from a 7-years monitoring campaign
We present the most continuous data base of optical
observations ever published on the BL Lacertae object GC 0109+224, collected
mainly by the robotic telescope of the Perugia University Observatory in the
period November 1994-February 2002. These observations have been complemented
by data from the Torino Observatory, collected in the period July 1995-January
1999, and Mt. Maidanak Observatory (December 2000). GC 0109+224 showed rapid
optical variations and six major outbursts were observed at the beginning and
end of 1996, in fall 1998, at the beginning and at the end of 2000, and at the
beginning of 2002. Fast and large-amplitude drops characterized its flux
behaviour. The magnitude ranged from 13.3 (16.16 mJy) to 16.46 (0.8 mJy),
with a mean value of 14.9 (3.38 mJy). In the periods where we collected
multi-filter observations, we analyzed colour and spectral indexes, and the
variability patterns during some flares. The long-term behaviour seems
approximatively achromatic, but during some isolated outbursts we found
evidence of the typical loop-like hysteresis behaviour, suggesting that rapid
optical variability is dominated by non-thermal cooling of a single emitting
particle population. We performed also a statistical analysis of the data,
through the discrete correlation function (DCF), the structure function (SF),
and the Lomb-Scargle periodogram, to identify characteristic times scales, from
days to months, in the light curves, and to quantify the mode of variability.
We also include the reconstruction of the historical light curve and a
photometric calibration of comparison stars, to favour further extensive
optical monitoring of this interesting blazar.Comment: 13 pages, 11 PS figures, 1 EPS figure, 3 tables, accepted by
Astronomy and Astrophysics. Uses A&A documentclass aa.cls, and the package
graphicx.st
Recommended from our members
Big Data and the Transformation of Operations Models: A Framework and A New Research Agenda
Big Data has been hailed as the ânext big thingâ to drive business value, transform organisations and industries, and âreveal secrets to those with the humility, willingness and tools to listenâ (Mayer-Schönberger and Cukier, 2013: 5). However, despite growing interest from organisations across industry sectors, Big Data applications appear to have concentrated on delivering incremental change and operational efficiency improvements, with little evidence on using Big Data to facilitate strategic, transformational change. In this paper, we explore how Big Data is actually being can be used across different sectors in leading organisations and examine the ways in which it is fostering change in the core operations models of organisations. A definition of âoperations modelâ is developed and the core components dimensions of an operations model are then examined, namely capacity, supply network, process and technology, and people development and organisation. Through a series of case studies, we examine the role of Big Data in affecting some, or all, of these components dimensions in order to generate value for the organisation by optimising, adapting or radically transforming the operations model. Following our analysis, we develop a tentative framework which can be used both for understanding how Big Data affects operations models, and for planning changes in operations models through Big Data. We set out a new research agenda to systematically understand the full potential of Big Data in transforming operations models
Day-Scale Variability of 3C 279 and Searches for Correlations in Gamma-Ray, X-Ray, and Optical Bands
Light curves of 3C 279 are presented in optical (R-band), X-rays (RXTE/PCA),
and gamma rays (CGRO/EGRET) for 1999 Jan-Feb and 2000 Jan-Mar. During both of
those epochs the gamma-ray levels were high, and all three observed bands
demonstrated substantial variation, on time scales as short as one day.
Correlation analyses provided no consistent pattern, although a rather
significant optical/gamma-ray correlation was seen in 1999, with a gamma-ray
lag of ~2.5 days, and there are other suggestions of correlations in the light
curves. For comparison, correlation analysis is also presented for the
gamma-ray and X-ray light curves during the large gamma ray flare in 1996 Feb
and the two gamma-bright weeks leading up to it; the correlation at that time
was strong, with a gamma-ray/X-ray offset of no more than 1 day.Comment: 20 pages, including 7 figures; accepted by The Astrophysical Journa
Optical and radio behaviour of the BL Lacertae object 0716+714
Eight optical and four radio observatories have been intensively monitoring
the BL Lac object 0716+714 in the last years: 4854 data points have been
collected in the UBVRI bands since 1994, while radio light curves extend back
to 1978. Many of these data are presented here for the first time. The
long-term trend shown by the optical light curves seems to vary with a
characteristic time scale of about 3.3 years, while a longer period of 5.5-6
years seems to characterize the radio long-term variations. In general, optical
colour indices are only weakly correlated with brightness. The radio flux
behaviour at different frequencies is similar, but the flux variation amplitude
decreases with increasing wavelength. The radio spectral index varies with
brightness (harder when brighter), but the radio fluxes seem to be the sum of
two different-spectrum contributions: a steady base level and a harder-spectrum
variable component. Once the base level is removed, the radio variations appear
as essentially achromatic, similarly to the optical behaviour. Flux variations
at the higher radio frequencies lead the lower-frequency ones with week-month
time scales. The behaviour of the optical and radio light curves is quite
different, the broad radio outbursts not corresponding in time to the faster
optical ones and the cross-correlation analysis indicating only weak
correlation with long time lags. However, minor radio flux enhancements
simultaneous with the major optical flares can be recognized, which may imply
that the mechanism producing the strong flux increases in the optical band also
marginally affects the radio one.Comment: 18 pages, 15 Postscript figures, 5 JPEG figures, accepted for
publication in A&
The correlated optical and radio variability of BL Lacertae. WEBT data analysis 1994-2005
Since 1997, BL Lacertae has undergone a phase of high optical activity, with
the occurrence of several prominent outbursts. Starting from 1999, the Whole
Earth Blazar Telescope (WEBT) consortium has organized various multifrequency
campaigns on this blazar, collecting tens of thousands of data points. One of
the main issues in the study of this huge dataset has been the search for
correlations between the optical and radio flux variations, and for possible
periodicities in the light curves. The analysis of the data assembled during
the first four campaigns (comprising also archival data to cover the period
1968-2003) revealed a fair optical-radio correlation in 1994-2003, with a delay
of the hard radio events of ~100 days. Moreover, various statistical methods
suggested the existence of a radio periodicity of ~8 years. In 2004 the WEBT
started a new campaign to extend the dataset to the most recent observing
seasons, in order to possibly confirm and better understand the previous
results. In this campaign we have collected and assembled about 11000 new
optical observations from twenty telescopes, plus near-IR and radio data at
various frequencies. Here, we perform a correlation analysis on the long-term
R-band and radio light curves. In general, we confirm the ~100-day delay of the
hard radio events with respect to the optical ones, even if longer (~200-300
days) time lags are also found in particular periods. The radio
quasi-periodicity is confirmed too, but the "period" seems to progressively
lengthen from 7.4 to 9.3 years in the last three cycles. The optical and radio
behaviour in the last forty years suggests a scenario where geometric effects
play a major role. In particular, the alternation of enhanced and suppressed
optical activity (accompanied by hard and soft radio events, respectively) canComment: 6 pages, 4 figure
Multi-Epoch Multiwavelength Spectra and Models for Blazar 3C~279
Of the blazars detected by EGRET in GeV gamma rays, 3C 279 is not only the
best-observed by EGRET, but also one of the best-monitored at lower
frequencies. We have assembled eleven spectra, from GHz radio through GeV gamma
rays, from the time intervals of EGRET observations. Although some of the data
have appeared in previous publications, most are new, including data taken
during the high states in early 1999 and early 2000. All of the spectra show
substantial gamma-ray contribution to the total luminosity of the object; in a
high state, the gamma-ray luminosity dominates over that at all other
frequencies by a factor of more than 10. There is no clear pattern of time
correlation; different bands do not always rise and fall together, even in the
optical, X-ray, and gamma-ray bands.
The spectra are modeled using a leptonic jet, with combined synchrotron
self-Compton + external Compton gamma-ray production. Spectral variability of
3C 279 is consistent with variations of the bulk Lorentz factor of the jet,
accompanied by changes in the spectral shape of the electron distribution. Our
modeling results are consistent with the UV spectrum of 3C 279 being dominated
by accretion disk radiation during times of low gamma-ray intensity.Comment: 39 pages including 13 figures; data tables not included (see ApJ web
version or contact author
A rapid and dramatic outburst in Blazar 3C 454.3 during May 2005 - Optical and infrared observations with REM and AIT
The flat-spectrum radio quasar 3C 454.3 is well known to be a highly active
and variable source with outbursts occurring across the whole electromagnetic
spectrum over the last decades. In spring 2005, 3C 454.3 has been reported to
exhibit a strong optical outburst which subsequently triggered multi-frequency
observations of the source covering the radio up to gamma-ray bands. Here, we
present first results of our near-IR/optical (V, R, I, H band) photometry
performed between May 11 and August 5, 2005 with the Rapid Eye Mount (REM) at
La Silla in Chile and the Automatic Imaging Telescope (AIT) of the Perugia
University Observatory. 3C 454.3 was observed during an exceptional and
historical high state with a subsequent decrease in brightness over our 86 days
observing period. The continuum spectral behaviour during the flaring and
declining phase suggests a synchrotron peak below the near-IR band as well as a
geometrical origin of the variations e.g. due to changes in the direction of
forward beaming.Comment: 5 pages, 3 figures, accepted for publication in A&A Letter
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