Vol. 58 (2008) pp. 187–192
A. U d a l s k i
Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
system has been designed to provide continuous real time photometric monitoring of the optical
counterparts of X-ray sources while RCOM system provides real time photometry of R Coronae
Borealis variable stars located in the OGLE-III ﬁelds. Both systems can be used for triggering follow-
up observations in crucial phases of variability episodes of monitored objects.
Key words: Surveys – Techniques: photometric – X-rays: stars – Stars: AGB and post-AGB
The Optical Gravitational Lensing Experiment (OGLE) is a long term large
scale sky survey regularly monitoring the most dense stellar ﬁelds in the sky (Udal-
ski et al. 1992, Udalski, Kubiak and Szyma´nski 1997, Udalski 2003). The OGLE
project started originally in 1992 as a ﬁrst generation microlensing survey and con-
tributed in its subsequent phases to many ﬁelds of modern astrophysics like stel-
lar astrophysics, extrasolar planet searches, gravitational lensing and others. Huge
databases of photometric measurements of hundreds of millions stars spanning sev-
eral years provide a unique opportunity for data mining, performing statistical an-
alyzes of huge samples of particular objects or conducting analysis of long term
behavior of selected classes of stars.
One of the most important results of the OGLE survey was the implementation
of real time data analysis systems that allow monitoring of selected variable objects
in almost real time. Advantages of this approach are obvious, especially in the case
of transient or non-periodic variable objects. For instance, one can carefully prepare
different kind of follow-up observations, knowing current photometric behavior of
a selected object. Microlensing ﬁeld is the best example here. The OGLE project
was the ﬁrst to implement the so called Early Warning System (EWS, Udalski et
phases. Information on such events was made public. Several microlensing follow-
up teams were formed in the several past years to observe intensively the already
discovered ongoing microlensing phenomena. This strategy turned out to be very
successful leading to the important discoveries like microlensing extrasolar planets
(Udalski et al. 2005, Beaulieu et al. 2006, Gould et al. 2006, Gaudi et al. 2008) or
Magellanic Cloud microlensings (Afonso et al. 2000, Dong et al. 2007).
OGLE-III phase of the OGLE project, that started on June 12, 2001 and has
been conducted up to now, was a signiﬁcant extension of the OGLE survey. Much
larger observing capabilities made it possible to cover practically entire area of the
LMC and SMC and large fraction of the Galactic bulge. Also new data analysis
systems were implemented during this phase (Udalski 2003). Beside the EWS
system allowing the discovery of about 600 microlensing events every year, two
new systems were developed after the ﬁrst four seasons of the OGLE-III phase:
EEWS and NOOS.
The EEWS system was designed to detect in the real time anomalies of mi-
crolensing events from a single mass microlensing. The implementation of this
system became an important step – thanks to it the vast majority of non-standard
microlensing events, including many planetary microlensings, were detected in al-
most real time and the information was passed to other microlensing groups. EEWS
system also allowed to switch the OGLE observing mode from the standard survey
mode to follow up mode where the observations of a particular object were done
with much higher cadence – dependent on the variability rate.
The second real time system NOOS (Udalski 2003) was designed to detect in
real time the transient stellar objects that brighten strongly enough to be seen in
the OGLE images for some time. This class of objects include supernovae (SNe),
long term variable stars, microlensing of very faint stars (non-detectable in the
regular OGLE photometry range) etc. A few new SNe were detected soon after the
implementation of this system (Udalski 2004).
Finally, the real time monitoring of the Einstein Cross gravitational lens (QSO
2237+0305) provides the real time photometry of four images of the quasar. This
object is one of the most important gravitational lenses and unique OGLE photo-
metric dataset (Wo´zniak et al. 2000, Udalski et al. 2006) was often used for its
modeling. Continuous monitoring of the quasar images allows early detection of
potential caustic crossings or cusp approaches in this lens. These events are cru-
cial for proper modeling and understanding the gravitational lenses and provide an
opportunity to estimate the quasar size.
In this note we present two new real time OGLE-III data analysis systems
implemented recently: XROM and RCOM. They allow real time monitoring of
selected classes of highly variable optical objects. The photometry provided by
these systems is available to the astronomical community from the OGLE Internet
X-ray astronomy is one of the most rapidly developing branches of modern as-
trophysics. New space missions provide more and more exciting data in this wave-
lenght range and the data ﬂow accelerates. Nevertheless in the majority of cases
the proper interpretation of observed X-ray behavior of detected objects requires
observations in other wavelengths as well, including the optical range.
The dense OGLE-III ﬁelds like the Magellanic Clouds or Galactic bulge in-
clude many X-ray sources. Part of them has been successfully identiﬁed with the
optical counterparts. For example, the SMC contains a large sample of X-ray pul-
sars discovered during the past few years (Coe et al. 2005). OGLE data has already
been used for interpretation of some of these objects (Coe et al. 2005, McGowan et
Continuous optical monitoring of counterparts of X-ray sources is very impor-
tant, as many of them undergo large optical variations, eruptions etc. likely related
to the X-ray activity. Therefore, many planned X-ray follow-up observations may
be much better tuned-up when the current optical state and behavior of these objects
Fig. 1. Light curve of the X-ray pulsar SXP 756.
The OGLE-III XROM system provides continuous photometric coverage of
a selected sample of known optical counterparts of X-ray sources located in the
OGLE-III ﬁelds. The initial sample contains 52 objects. It can be easily extended
with other or newly detected objects. Photometry of the XROM objects is typically
updated after each clear night. Fig. 1 presents the OGLE-III light curve of one of
such objects: SXP 756.
The interactive access to the XROM objects is provided via the main OGLE
The structure of the page is similar to other OGLE real time system pages. After
selecting an object, the object page is invoked providing the basic information: its
OGLE identiﬁcation, RA/DEC coordinates, ﬁnding chart and two light curve plots:
one showing the entire light curve and the second one showing the last 60 days.
The photometry is obtained through the I-ﬁlter and it is only roughly calibrated
with accuracy of the zero points of ±0
The photometry can be download from the OGLE archive:
RCOM: OGLE Real Time Monitoring of R CrB Variable Stars
R CrB stars form a group of stars that reveal dramatic variability episodes.
Their brightness can fade by a few magnitudes in the time scale of several days.
These fading episodes are unpredictable and can last for months. After that period
the brightness of these stars gradually recovers to the original state.
HJD - 2450000
Fig. 2. Light curve of the R CrB type star, MACHO-051551.8-691008, monitored by the OGLE-III
It is believed that fading is related to the formation of dust clouds over the
surface of these stars. When they disperse the brightness returns to the unobscured
group of stars – only about 50 is known in the Galaxy (Tisserand et al. 2008) and
about 20 in the Magellanic Clouds (Alcock et al. 2001, Tisserand et al. 2004).
R CrB stars provide an opportunity of studying the late stages of stellar evo-
lution. To clarify their evolutionary status extensive follow-up observations are
needed in the most dramatic fading or rising phases of the variability episodes. As
the episodes are unpredictable only continuous observations of R CrB variables
may trigger such follow-up programs.
OGLE-III ﬁelds are ideal for the real time data analysis system monitoring
R CrB variables. They contain many R CrB stars, both in the Magellanic Clouds
and the Galactic bulge. The RCOM system was designed to continuously monitor
a sample of R CrB stars from the OGLE-III ﬁelds. Initially the sample consists of
23 objects but it can be extended when the new objects are detected. Unfortunately
most of the known R CrB stars in the Galactic bulge are saturated in the OGLE-
III reference images, therefore their photometry is not available. Fig. 2 shows an
example of the light curve of one of the R CrB objects monitored by the CROM
The interactive access to the RCOM objects is provided via the main OGLE
The structure of the page and provided information are identical as for the
The photometry can be download from the OGLE archive:
grant N20303032/4275. We thank Drs. Malcolm Coe, Matthew Schurch and Peter
Cottrell for encouraging us to design and implement presented systems.
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