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Astron. Astrophys. 355, 113-120 (2000)

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1. Introduction

The correlation between the ROSAT All Sky Survey (RASS) and the IRAS Point Source Catalog unveiled the existence of many galaxies with X-ray luminosity in the range [FORMULA]-[FORMULA] erg s-1, which had not been previously classified as Active Galactic Nuclei (AGN) (Boller et al. 1992). Subsequent optical observations indicated that a large fraction of them were previously unrecognized Seyfert galaxies (Moran et al. 1994, 1996; Dennefeld et al. in preparation). However, the nature of a few of them is far from being understood in terms of simple nuclear activity and deserves further investigations. IRAS12393+3520 (NGC 4619) is one of those, which we started to study in more details in 1992.

It is a nearby ([FORMULA]) barred spiral galaxy of morphological type SB(r)b, with a luminous nucleus and relatively weak spiral arms. Our first optical spectrum (Boller et al. 1993) was obtained at low resolution in March 1992 at the 1.93m telescope of Haute-Provence Observatory (OHP) and is displayed in Fig. 1.

[FIGURE] Fig. 1. March 1992 low resolution spectrum of IRAS12393+3520.

It shows a red continuum, typical of IRAS galaxies, with many stellar absorption features and conspicuous emission only in the red part of the spectrum ([FORMULA], [NII ]). [FORMULA] is in absorption, and the [OIII] emission barely visible. The existence of a strong NaD absorption line is an indication that significant internal extinction is present. No clear evidence is seen of a broad [FORMULA] component: a hint of it may be seen on the red side of the narrow emission line, but the presence of the atmospheric B band does not allow a proper estimate of the level of the continuum; this prompted further observations.

In the mean time, other spectra were published by Moran et al. (1994) and Mas-Hesse et al. (1996, hereinafter M96), showing a broad [FORMULA] component attributed to a Seyfert 1 nucleus. While their two low-dispersion spectra are compatible with each other, their strong, broad [FORMULA] line is not seen on ours. A high-resolution spectrum taken by M96 only 25 days after our first spectrum displays a faint, [FORMULA] broad component, but apparently much fainter than in their low-dispersion spectrum taken about two years later. M96 noted also that the NII /[FORMULA] ratio in IRAS12393+3520 was reminiscent of a LINER (Filippenko 1993 and references therein), but that, apart of it, the spectrum was remarkably featureless. From the lack of any detectable broad component of the [FORMULA], they estimated an amount of extinction [FORMULA], corresponding to [FORMULA] cm-2 (Lequeux et al. 1981; Prehdel & Schmitt 1995); such a value is not inconsistent with the reddening estimated from the narrow components. The UV spectra obtained by M96 show also broad [FORMULA] and MgII emission, as well as some absorption lines. The [FORMULA] inferred from the UV measurements is [FORMULA]0.20.

The optical to X-ray Spectral Energy Distribution (SED) of IRAS12393+3520 has been studied in detail by M96. They pointed out that a scenario consisting of intense starburst can well explain the infrared, optical and UV emissions and the optical emission lines, but underestimates the soft X-rays. On the other hand, if UV and X-rays are dominated by a non-stellar contribution, the FIR emission is then highly in excess over the one typically observed in Seyfert galaxies. The presence of both sources, with comparable amount of energy output, seemed the most viable solution for this object. Secular optical variability in itself is not discriminating between the two mechanisms if timescales are not determined, as Terlevich et al. (1992) have argued that massive star formation could also account both for variability and broad lines.

IRAS12393+3520 is bright in soft X-rays [[FORMULA]], with a photon spectral index [FORMULA] (Moran et al. 1996). Studies in the X-rays are in principle of the uppermost importance to address properly the true origin of the output power, because they can pierce throughout the innermost regions of the galaxy, and unveil the high-energy processes that occur in the immediate proximity of the nucleus. An ASCA observational program was started to search for evidence of nuclear activity in a sub-set of the Boller et al. (1992) soft X-ray luminous galaxies sample. In this paper, we report the results on IRAS12393+3520 (the only target for which time was allocated), which provided the first hard X-ray measure for this object. The results are presented together with a re-analysis of a pointed archival ROSAT/PSPC observation and with new optical data obtained in the mean time. We will assume [FORMULA] km s-1 Mpc- 1. Energies are quoted in the source rest frame and errors are at 90% confidence level for one interesting parameter ([FORMULA]).

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Online publication: March 17, 2000