Authors: A.Bliss, D.M.Worrall, M.Birkinshaw (Bristol), H.Tananbaum, S.Murray (Harvard-Smithsonian C.f.A.)

Fig.1: Combined IVU band image, J2310-437 circled

REFERENCES 1. D. M. Worrall et al, ApJ, 19992. G. Abell, ApJS, 19583. S. S. Murray et al, in prep.4. H. Tananbaum et al, ApJ, 1997

UNMASKING THE AGN IN J2310-437

Fig.2: 8.6GHz on 0.3-7keV X-ray

OPTICAL SPECTRUM (Fig.4)At this galaxy’s redshift of 0.086, the CaII break which is used to distinguish between BL Lac objects and elliptical galaxies should be about 4350Å. However, the short-wavelength cutoff on this spectrum is around 4700Å which is more typical of an elliptical galaxy. There are also no Hß or [OIII] emission lines which rules out the possible presence of a Seyfert galaxy or quasar (4).

INTRODUCTIONJ2310-437 (Fig.1) is hosted by an elliptical galaxy of z=0.0886, at the centre of a cluster of Abell richness class 0 (Ref.1), where Abell richness is the number of galaxies in a cluster that lie within the magnitude range m3 to m3+2 (m3 is the magnitude of the third brightest member of the cluster). An Abell richness of class 0 indicates that there are 30-49 galaxies in the cluster within this range (2). It is unusual in that it appears as a bright (1044 erg s-1) X-ray source (3), and radio mapping shows a moderately bright object with a small jet (Fig.2). However, in the optical there is no unequivocal evidence for an AGN, and the spectrum shows no emission lines. This ongoing project uses high-quality ESO NTT imaging to investigate the level of jet activity in five optical bands and look for evidence of a compact core in these images, with a view to confirming the interpretation of J2310-437 as an anomalous BL Lac object with a faint optical core and bright host galaxy. This is similar to the optically dull, X-ray and radio-loud galaxy 3C 264 (4), raising the question whether these two sources are extreme BL Lac objects or part of a distinct population.

Fig.3: Optical images withradio (8.6GHz) contoursoverlaid

OPTICAL IMAGESThe imaging data was from the U, B, V, R and I bands, measured over 3 nights using the SUSI2 instrument on the ESO NTT .These images were reduced using IRAF, and Fig.3 shows optical images as colour scale with radio contours overlaid to show the position and angle of the jet. The B band (421.20nm) shows the best jet in the optical, with a small hook seen in the lower left quadrant. The jet is seen less defined in the U band as a diffuse fan.

POSSIBLE OPTICAL CORE?Using the ELLIPSE command in IRAF the host galaxy was subtracted from the R band (641.58nm) optical image. This left a bright object (shown in Fig.5) with what appears to be a small-scale jet, corresponding to the radio contours. There is also an area which could be interpreted as a bright disk although this may be a feature caused by ELLIPSE.

CONCLUSIONS AND FURTHER WORKResults so far seem to support the interpretation of J2310-437 as an anomalous BLLac object; radio and X-ray luminous, whilst optically dull with no emission lines and a spectrum typical of an elliptical galaxy. Galactic subtraction of the images will be continued to better determine the presence of a disk and compact optical core.

Fig.5: Optical image with galaxy subtracted Fig.4 (1)