The First Reported X-ray Transient in Centaurus A



In a letter to Astronomy & Astrophysics ( Astron. Astrophys. Lett. 357, L57-L60 (2000), preprint), H. Steinle, K. Dennerl, and J. Englhauser from MPE report their detection of a bright x-ray transient (designated 1RXH J132519.8-430312) towards the direction of Centaurus A in ROSAT HRI data taken already several years ago.

In July 1995 a multiwavelength campaign took place to observe Cen A (NGC 5128) from radio to gamma rays. During 5 ROSAT HRI observations, a bright x-ray source was detected 2'.5 south-west of the nucleus of NGC 5128 at the outer regions of the elliptical galaxy. At that time it was the brightest point source in the ROSAT HRI field-of-view.

All other ROSAT HRI observations made in 1990, 1994, and 1998 show no trace, even if combined, of the source, suggesting that this is a highly variable object.

Chandra observations of Centaurus A made in 1999 and 2000 (Kraft et al. 2001) show this object to vary between the observations by a factor of 500.

Given the high luminosity and variability of the object, it may be able to mimic variations of the whole galaxy in X-rays if not spatially resolved from the nucleus (as was the case in early-days X-ray instruments).

(All images are shown at the same scale. They show the central part of NGC 5128 in the center and the x-ray jet pointing towards north-east. The location of the transient is marked with arrow and circle.)

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Combined ROSAT HRI observations from 1990, 1994, and 1998 which show no source at the position of the transient.
(Image size: 12' x 12'; total exposure 102500 s)
(Credit: H. Steinle, MPE; from public data) 		Combined 5 ROSAT HRI observations from the 1995 multiwavelength campaign which show the strong transient (maked hcs113).
(Image size: 12' x 12'; total exposure 24400 s)
(Credit: H. Steinle, MPE)

The derived coordinates (uncertainty 5") are:

Published Chandra images of Cen A taken in October 1999 show a weak source at the ROSAT position, which may be the persistent counterpart of the transient.

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Simultaneous optical (B) image of Cen A taken during the 1995 campaign at the ESO La Silla 2.2m telecope. No object at the the position of the transient is visible.
(Image size: 7' x 7'; exposure 40 s)
(Credit: H. Steinle, MPE) 		Chandra image of Cen A. At the position of the transient, marked with hcs113, a weak source is visible.
(Image size: 12' x 12'; exposure 23000 s)
(Credit: NASA/CXC/SAO - image modified)

If at 3.0 Mpc (the distance of Cen A), the luminosity of 1RXH J132519.8-430312 in the energy band 0.1-2.4 keV would be 3 x 1039 erg s-1, assuming a power-law spectrum with photon index -2 and no additional intrinsic absorption in NGC 5128. This is above the Eddington luminosity of most x-ray binaries, but still within the observed range of luminosities. Therefore it cannot be ruled out that the transient is located in NGC 5128.

In a funny coincidence, I recently found a space art picture which may be a portrait of the so far unknown object:

The painting is by Joe Tucciarone and is printed here with his permission.

Last update: 2008-08-18     |     Copyright © Helmut Steinle, MPE |     Impressum

 

 Centaurus A