PKS J2310-437 is an AGN with bright X-ray emission relative to its weak radio emission and optical continuum. It is believed that its jet lies far enough from the line of sight that it is not highly relativistically beamed. It thus provides an extreme test of AGN models. We present new observations aimed at refining the measurement of the source's properties. In optical photometry with the NTT we measure a central excess with relatively steep spectrum lying above the bright elliptical galaxy emission, and we associate the excess wholly or in part with the AGN. A new full-track radio observation with the ATCA finds that the core 8.64GHz emission has varied by about 20 per cent over 38 months, and improves the mapping of the weak jet. With Chandra we measure a well-constrained power-law spectral index for the X-ray core, uncontaminated by extended emission from the cluster environment, with a negligible level of intrinsic absorption. Weak X-ray emission from the resolved radio jet is also measured. Our analysis suggests that the optical continuum in this radio galaxy has varied by at least a factor of four over a timescale of about two years, something that should be testable with further observations. We conclude that the most likely explanation for the bright central X-ray emission is synchrotron radiation from high-energy electrons.
Deep Dive into Unmasking the Active Galactic Nucleus in PKS J2310-437.
PKS J2310-437 is an AGN with bright X-ray emission relative to its weak radio emission and optical continuum. It is believed that its jet lies far enough from the line of sight that it is not highly relativistically beamed. It thus provides an extreme test of AGN models. We present new observations aimed at refining the measurement of the source’s properties. In optical photometry with the NTT we measure a central excess with relatively steep spectrum lying above the bright elliptical galaxy emission, and we associate the excess wholly or in part with the AGN. A new full-track radio observation with the ATCA finds that the core 8.64GHz emission has varied by about 20 per cent over 38 months, and improves the mapping of the weak jet. With Chandra we measure a well-constrained power-law spectral index for the X-ray core, uncontaminated by extended emission from the cluster environment, with a negligible level of intrinsic absorption. Weak X-ray emission from the resolved radio jet is als
PKS J2310-437 is hosted by an elliptical galaxy at redshift z = 0.0886, at the centre of a cluster of Abell richness 0 (Tucker et al. 1995). It is interesting and unusual in that it appears as a luminous (∼ 10 44 ergs s -1 ) X-ray source, associated with a significant radio source (Tananbaum et al. 1997;Worrall et al. 1999), but with low optical luminosity and the characteristics of a typical elliptical galaxy. It shows no optical line emission as could be expected from an AGN, or UV continuum emission as from a BL Lac nucleus. This is not believed to be due to extinction as ROSAT X-ray data demonstrate a soft spectrum showing no excess absorption by gas in the J2310-437 galaxy (Tananbaum et al. 1997), and it would be hard to argue for a dust/gas torus that absorbs the AGN optical light without diminishing the soft X-ray emission.
Any optical AGN emission must be weak. Worrall et al. (1999) give an upper limit of 32 µJy at 4400 Å, based on the size of the CaII break in a spectrum obtained in 1996, but suggested that the actual value lies close to this limit. Evidence for the presence of a weak AGN optical continuum is presented by Caccianiga and Maccacaro (1997) who find that the size of the CaII break in J2310-437 as measured through a narrow spectral slit to be 38 ±4%, which lies between the values expected of a BL lac object (≤ 25%) and that of a typical elliptical galaxy (≈ 50%). They measured a larger break when using a wider spectral slit that would incorporate the same AGN emission diluted by more starlight. However, no absolute flux calibration was available to put qualitative evidence for AGN emission onto a more quantitative footing.
The radio structure is interesting, showing a one-sided jet that extends about 10 arcsec to the south-east of the core, embedded in a large-scale double plume (Worrall et al. 1999). That relatively large radio lobes can be seen on both sides implies that J2310-437 is seen less ’end-on’ than expected of a BL Lac object. Worrall et al. (1999) suggest that the jet is 30 • to the line of sight based on a core prominence that is lower than expected for a BL Lac object.
Objects that show extreme characteristics in their multifrequency properties are likely to challenge most strongly our understanding of their emission mechanisms. In this paper we present new observations aimed at testing how well the extreme properties of J2310-437 might be fitted within the framework of emission models discussed for other AGN.
The redshift of J2310-437 is 0.0886. In this paper we adopt values for the cosmological parameters of H 0 = 70 km s -1 Mpc -1 , Ω m0 = 0.3, and Ω Λ0 = 0.7. Thus 1 arcsec corresponds to a projected distance of 1.66 kpc at the source.
We observed J2310-437 with the SUSI2 instrument on the European Southern Observatory New Technology Telescope (ESO NTT) with the intent of exploiting its excellent seeing to detect the optical core of the galaxy and image the jet. We also observed J2310-437 with Chandra to obtain high-resolution spatial and spectral data so that the core AGN emission spectrum could be resolved from the galaxy and cluster, and also to search for an X-ray jet corresponding to the radio jet. Finally, new radio data were obtained using the Australian Telescope Compact Array (ACTA). Two 12-hour tracks, 51 months apart, have improved on the earlier results reported by Worrall et al. (1999).
Data in the UBVR-and I-bands were collected with the ESO 3.5m NTT equipped with the Superb Seeing Imager -2 instrument (SUSI2) on the nights of 2004 August 7, and 2004 September 13 and 22. The SUSI2 CCD camera provides a field of view 5.5 x 5.5 arcmin 2 , with a scale of 0.16 arcsec pixel -1 (after 2x2 binning). Total integration times, central wavelengths and average seeing are given in Table 1.
The raw data were processed using IRAF to bias-subtract, flat-field and apply gradient corrections. Dome flats were used for flat-fielding. The I-band images contained strong fringing which was removed by creating and subtracting a master fringe mask. Any remaining gradient was removed by subtracting a fitted background using IRAF. The images were then flipped along a vertical axis to bring them to sky orientation. A WCS was added to the images using an R-band optical image with attached world co-ordinate system that was previously acquired from the CTIO telescope. A median average was used to combine the data to create one mosaic for each band.This proved effective at removing cosmic ray hits. The number of overlapping frames used to create the mosaics were 10, 7, 6, 6 and 7 in U, B, V, R and I respectively. The seeing was particularly bad for the R-band data (Table 1), and so data in this filter are excluded from our subsequent analysis. Flux calibration was carried out using the photometry for standard stars PG1525A and PG1525B given in the Landolt Equatorial Standards table (Landolt 1992). .
Radial profiles in each optical band were extracted from the background-subtract
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