Free–free absorber surrounding GHz-peaked spectrum sources

https://doi.org/10.1016/S1387-6473(01)00194-4Get rights and content

Abstract

We present multi-frequency observations towards GHz-Peaked Spectrum (GPS) sources with the VLBA. Recent VSOP observations of a GPS source OQ208 have shown that its convex spectrum is caused by free–free absorption (FFA) due to external ionized gas surrounding the source. The keys are: (1) sharp cut-off at low frequency in the spectrum, and (2) asymmetric peak frequency in double lobe. In order to investigate the nature of FFA in GPS sources, we observed nine objects (five QSOs, two RGs, two Sy2s) selected from GPS catalog. We tried spectral fitting using FFA model for each source. All QSOs show asymmetric FFA opacities like OQ208 does, while the opacity of RGs and Sy2s are symmetric. This supports the unification model between RGs and QSOs.

Introduction

GPS sources are characterized by their compact overall size (<1 kpc) and convex radio spectrum peaked at GHz frequency (O’Dea et al., 1991). GPS sources show a steep rising spectrum below the peak frequency, which may be due to synchrotron self-absorption (SSA) or FFA (O’Dea, 1998). Recent VSOP observations of a GPS source OQ208 show the evidence of extrinsic free–free absorber surrounding the object (Kameno et al., 2000). To test the FFA model for GPS sources and to determine occurrence of FFA towards GPS sources, we conducted multi-frequency VLBI observations at 1.6 and 5 GHz with VSOP, and at 2, 8 and 15 GHz with the VLBA. In this paper, we report the results of the VLBA observations and discuss the nuclear environment in GPS sources.

Section snippets

Observations and results

We observed nine GPS sources (Table 1) selected from GPS catalog by de Vries et al. (1997). The sample objects are selected by the following criteria: (1) peak frequency: 1.6 GHz <νmax<15 GHz, and (2) flux density: S1.6 GHz>0.1 Jy, S5 GHz>0.5 Jy, S15 GHz>0.2 Jy. Observations were made during 24 h periods on December 15, 1998 using the VLBA of NRAO. All objects were observed 2–6 times with short-duration (22 min scan at 15 GHz and 11 min scan at 2 and 8 GHz). Fringe fitting and a priori

FFA symmetry and RG and QSO unification

Histogram of asymmetry in FFA opacity is shown in Fig. 2. FFA opacity ratio in the horizontal axis is defined as the ratio of FFA opacity on the two lobes (τlobe1/τlobe2; τlobe2>τlobe1). Clear difference is seen in FFA opacity ratio between RG/Sy2 and QSOs. The opacity in RGs and Sy2s is symmetric, while QSOs show asymmetric FFA opacity. It supports the unified model between RG and QSO (Barthel, 1989) and between Sy1 and Sy2 (Antonucci and Miller, 1985). If the jet axis is on the sky plane (the

Conclusion

The VLBA study at 2, 8 and 15 GHz towards nine GPS sources showed the clear difference in FFA opacity between RG/Sy2 and QSO, which supports the RG and QSO unification. VSOP observations will give us complete spectra of GPS sources, and allow us study FFA towards GPS sources.

The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

References (7)

  • R.J. Antonucci et al.

    ApJ

    (1985)
  • P.D. Barthel

    ApJ

    (1989)
  • G.V. Bicknell et al.

    ApJ

    (1997)
There are more references available in the full text version of this article.
View full text