Monitoring

To prepare a "target of opportunity" (ToO) observation of Centaurus A in a very bright state in X-rays/gamma-rays, several instruments have been and are used to monitor the X-ray emission of Centaurus A.

Swift Monitoring @ 15 - 50 keV (2005-02-13 to present)

BAT Cen A monitor data.

(click for full resolution; plot from direct link to Swift Cen A monitoring page)
ToO trigger theshold :
The ToO condition is met if Cen A exceeds

0.05 cts cm^-2 sec^-1

in the 15 - 50 keV energy band for more than 30 consecutive days.

Comments :
MJD 55600 = 2011-02-08
MJD 55800 = 2011-08-27
MJD 55805 = 2011-09-01
MJD 55927 = 2012-01-01
MJD 56000 = 2012-03-14

MJD - date converter

Fermi GBM Monitoring @ 12 - 300 keV in four energy bands (2008-08-12 to present)

Fermi GBM Earth Occultation Monitoring Data.
Four energy bands: 12 - 25 keV, 25 - 50 keV, 50 - 100 keV, and 100 - 300 keV.

(plot from direct link to Fermi GBM Cen A monitoring page)

From the description:
Red diamonds denote > 5-sigma statistical significance (possible detections), blue diamonds denote < -3-sigma statistical significance (usually an indication of interference or background problems). Each plot shows the long-term average as a dashed line, which is shown in pink if the long-term average statistical significance is > 10 sigma.
Comments :
MJD 54800 = 2008-11-30
MJD 55800 = 2011-08-27

MJD - date converter

RXTE Monitoring @ 2 - 10 keV (1996-02-20 to 2012-01-04)

ASM Cen A monitor data.
(The red data points in the plots are > 3 sigma detections.)

RXTE was shut down on January 5, 2012 after a successful 16 year mission.

(plot from direct link to RXTE Cen A monitoring page)
ToO trigger theshold was:
The ToO condition is met if the Cen A RXTE/ASM flux averaged over 10 days exceeds

3.5 cts/s

over a period of 30 days (see also figure under "Light Curves").

Comments :
Since mid June 2011, the RXTE data on Cen A are intermittent.

Light Curves

This are all BATSE Cen A monitoring data (green; 20-200 keV (1991-04-29 to 2000-05-25)) avaialble up to TJD 11690 (2000-05-25, the day all scientific instruments aboard CGRO were switched off prior to the deliberate destruction of CGRO by NASA), combined with all RXTE ASM data (red) available in April 2011.
(For comparison with RXTE: TJD = MJD - 40000)

(data: 1991-04-29 to 2011-04-01)

Note:
Benlloch et al. (2001) mention, that: "...apparent fluctuations in the (RXTE) ASM flux at the end of each year coinciding with the closest angular separation between the sun to Cen A. This variability is due to scattering of solar X-rays off the ASM collimators onto the ASM detectors and is not due to source flux variations (Remillard, priv. comm.).".

Long Term X-ray Light Curve (@100 keV)

A good overview of the long term behaviour of Cen A around 100 keV is Fig. 5 from Bond et al. (1996).
This light curve overlaps with the above BATSE/RXTE light curve for almost 4 years and the persistent "low state" of Cen A since 1993 is evident.

Earlier X-ray Light Curves

10 year (1969 - 1979) Vela 5b Centaurus A lightcurve (1 - 12 keV) from
J. Terrell (1986).

Compilation of a 26 year (1969 - 1994) lightcurve (3 - 12 keV) from
Turner et al. (1986).

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