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\IBVShead{5915}{18 December 2009}

\IBVStitle{The Recovery Phase after the 2009.0-event of {\LARGE $\eta$} Carin\AE}

\IBVSauth{Fern\'andez-Laj\'us, E.$^{1,2,3}$; Fari\~na, C.$^{1,2}$; Schwartz, M. A.$^1$; Giudici, F.$^1$; \break Salerno, N.$^1$; Scalia, M. C.$^1$; Peri, C.$^1$; von Essen, C.$^1$; Calder\'on, J. P.$^1$}

\IBVSinst{Facultad de Ciencias Astron\'omicas y Geof\'{\i}sicas - 
          Universidad Nacional de La Plata}
\IBVSinst{Instituto de Astrof\'{\i}sica de La Plata (CCT La Plata - CONICET/UNLP)}
\IBVSinst{e-mail: eflajus@fcaglp.unlp.edu.ar}

\IBVStyp{ LBV }
\SIMBADobjAlias{eta Car}{HD 93308}
\IBVSkey{photometry}
\IBVSabs{The 5.5 years periodicity of eta Carinae is manifested in the}
\IBVSabs{``events'' observed in different wavelength ranges.}
\IBVSabs{The last of these ``events'' occurred as expected in 2009.0.}
\IBVSabs{Our ground-based photometric campaign registered this ``event''} 
\IBVSabs{which showed an ``eclipse-like'' feature. After that, we continued}
\IBVSabs{the observations in order to get information about the post-event}
\IBVSabs{behaviour. Our monitoring program consists of daily CCD photometry,}
\IBVSabs{obtained using the 0.8 m ``Virpi S. Niemela'' telescope at}
\IBVSabs{La Plata Observatory, and a BVRI and H_alpha filter set.}
\IBVSabs{The light curves and photometric data collected during the second} 
\IBVSabs{half of our 2009 observing season are presented in this paper.}


\begintext

It is currently accepted that the massive and luminous star $\eta$~Carin\ae\ 
is comprised of at least a binary system, with a 5.5 years orbital period 
(e.g. Damineli et al., 2000).
This assertion was firstly based on the evidence of many optical 
spectroscopic events registered since 1948 (Gaviola, 1953) 
up to the present (see Damineli et al., 2008 and references therein).
Observations performed in other wavelengths from X-rays
to mm-wavelengths confirmed that related events occurred with the same 
period and almost at the same orbital phase.
Particularly, we reported the occurrence of a photometric event
at the time of an expected spectroscopic event for 2003.5 
(Fern\'andez-Laj\'us et al., 2003).

At the beginning of 2009, $\eta$~Car experienced a new ``event'', 
which was detected in different spectral regions
(Corcoran,~2009; 
Fern\'andez-Laj\'us et al., 2010, 
hereafter Paper~II; Abraham, Breaklini and Miceli, 2009).
This event indicates the starting of ``cycle 12'', enumerating the cycles 
since 1948 (Groh and Damineli, 2004).
In Paper II, we presented the CCD optical ground-based 
photometry of the complete event, showing an ``eclipse-like'' feature 
similar to that we recorded in 2003.5.
After the 2009.0 event, we continued the observations to register the
light curves behaviour during the recovery phase after the ``eclipse-like'' 
event and to compare the evolution at the same orbital phases of the previous 
cycle reported by Fern\'andez-Laj\'us et al. (2009, hereafter Paper I).

In this paper, we present the $BVRI$ and H$\alpha$ CCD photometry of the 
recovery phase after the 2009.0 event, up to the end of our $\eta$~Car 2009 
observing season. 
The CCD images were acquired using the ``Virpi S. Niemela''  telescope at 
La Plata Observatory, with the same instrumental configuration already 
described in detail in Papers I and II. 
More than 8500 images were obtained during 79 nights between March 31 and 
August 25, 2009. 
As stated in Paper I, bias, dark and `twilight' flat-field frames 
were meant to be acquired every night. 
However, on several occasions bad weather conditions prevented us
from obtaining good quality flat-fields images. 
Even though we attempted to process images with Master calibrations 
frames, the resulting light curves presented a large number of points
out of scale as a consequence of many bad quality flat-fields.
For this reason, we decided to present only the data obtained from the 
uncalibrated images, considering that the light curves were practically 
the same as those obtained with the calibrated images.
However, the calibration images were used to check the overall behaviour 
of the telescope and detector.

The resulting $BVRI$ and H$\alpha$ light curves are depicted in Fig.~1.
The $\eta$~Car differential magnitudes, their mean values and its rms 
errors were determined as explained in Paper I.
The typical rms errors of our data (also represented in Fig.~1) are:
$\epsilon_B=0.015$, $\epsilon_V=0.006$, $\epsilon_R=0.010$, 
$\epsilon_I=0.015$ and $\epsilon_{H\alpha}=0.015$ mag.
The light curves are smoothed using natural cubic splines. 
These splines were extended backward using the data published in Paper~II, 
in order to display the 2009.0 ``eclipse-like'' event observed during 
our 2009 observing season.
The orbital phases are marked in the top axis. 
They are determined using the ephemeris given in Paper II and adding '11', 
which is the number of cycles from the first event registered in 1948 
up to the date of the event (JD 2452819.8 or $\phi = 0$) related with that 
ephemeris (see Damineli et al., 2008).

\IBVSfig{21.0cm}{5915-f1.eps}{$BVRI$ and H$\alpha$ light curves of $\eta$~Car
  observed between March 31 and August 25, 2009. Bars represent the standard
  deviations of the mean values. The data were smoothed using splines. Along
  the top axis, orbital phases are indicated according to Paper II. The
  previous cycle light curves (dashed lines) were y-shifted to display them
  together for comparison.}
\IBVSfigKey{5915-f1.eps}{eta Car}{light curve}

The light curves of Fig.~1 show that the system keeps a brightening trend 
in the five photometric bands.
It is remarkable that the brightening rate during this phase is higher
than 0.4 mag year$^{-1}$ in the $BVR$ bands, 0.3 mag year$^{-1}$ in $I$, 
and almost 0.9 mag year$^{-1}$ in H$\alpha$. 
Thus, $\eta$~Car has reached the magnitude $V \sim 4.6$, the maximum 
brightness achieved in the last one and a half century.

The differential photometry data of $\eta$~Car and the two other nearby stars
in the field, namely CPD\,-59\,2627 and CPD\,-59\,2628, are available online 
as an electronic table ({\tt 5915-t1.txt}) at the IBVS website.
An example of the table is shown in Table 2 in Paper I.

The new time-series presented in this paper together with those 
already published in Papers I and II constitute the longest based-time 
of self-consistent photometric data of $\eta$~Car currently available, 
which are useful for a better understanding of the present state of this 
object.

\vspace*{0.5cm}
The authors acknowledge the authorities of the Facultad de Ciencias
Astron\'omicas y Geof\'{\i}sicas - Universidad Nacional de La Plata, for
the observational facilities at the Observatorio Astron\'omico de La Plata.
We want to thank the participation of Maximiliano Haucke during the observations,
and the technical staff for the maintenance of the telescope and equipment.
We are grateful to A. Cuestas and G. Bosch for the English review and suggestions.

\newpage
\references

Abraham Z., Beaklini P.P., Miceli C., 2009, private communication, see presentation 
at IAU Joint Discussion 13, ``Eta Carinae in the Context of the Most Massive Stars'', 
13-14 August 2009, Rio de Janeiro, Brazil.

Corcoran, M. F., 2009, private communication, see\\
        {\tt http{}://asd.gsfc.nasa.gov/Michael.Corcoran/}\\ 
\hphantom{\ \ \ \ \ } {\tt eta\_car/etacar\_rxte\_lightcurve/index.html}
\IBVSlink{http://asd.gsfc.nasa.gov/Michael.Corcoran/eta_car/etacar_rxte_lightcurve/index.html}

Damineli, A., Kaufer, A., Wolf, B., Stahl, O., Lopes, D. \& de Ara\'ujo, F., 
2000, {\it ApJ}, {\bf 528}, L101 %\BIBCODE{2000ApJ...528L.101D}

Damineli A., Hillier D. J., Corcoran M. F., Stahl O., Levenhagen R. S., 
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Fern\'andez-Laj\'us, E., Gamen, R., Schwartz, M., Salerno, N., 
Llinares, C., Fari\~na, C., Amor\'{\i}n, R., Niemela, V., 
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%\BIBCODE{2003IBVS.5477....1F}

Fern\'andez-Laj\'us, E., Fari\~na, C., Torres, A.F., Schwartz, M.A., 
Salerno, N., Calder\'on, J.P., von Essen, C., Calcaferro, L.M., Giudici, F., 
Llinares, C., Niemela, V., 
2009, {\it A\&A}, {\bf 493}, 1093 (Paper I) 
%\BIBCODE{2009A&A...493.1093F}

Fern\'andez-Laj\'us, E., Fari\~na, C., Calder\'on, J. P., Salerno, N., 
Torres, A. F., Schwartz. M. A., von Essen, C., Giudici, F., Bareilles, F. A., 
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%\BIBCODE{2010NewA...15..108F}

Gaviola, E., 1953, {\it ApJ}, {\bf 118}, 234.

Groh, J.H., Damineli, A., 2004, {\it IBVS}, 5492.

\endreferences

\IBVSedata{5915-t1.txt}
\IBVSdataKey{5915-t1.txt}{eta Car}{photometry}

\vfill

\IBVSerratum{5838}

In IBVS 5838, first page, third paragraph, the comparison star
of V841 Cen is mentioned with two different HD numbers.
The correct name of the star is HD 128227.

\bigskip

The Editors


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