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Astron. Astrophys. 348, 63-70 (1999)

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5. Comparison with previous observations

5.1. ASCA observations

As already mentioned, a long standing problem with this source is the determination of its optical extinction and the column [FORMULA] along the line of sight. The value that we found with the Beppo SAX observations is in agreement with the sum of the estimated absorption caused by the column of neutral galactic hydrogen [[FORMULA] cm-2], plus that due to the outer edge of the molecular cloud Barnard 30 in the [FORMULA] Orion rings of clouds, [[FORMULA] cm-2] (Sambruna et al. 1997).

In March 1995, ASCA observed PKS 0528+134 three times, within 2 weeks, during an active state of the source. The last of these observations was analyzed by Sambruna et al. (1997), finding a best fit value of [FORMULA] cm-2, in good agreement with the value found in our Beppo SAX observations. The entire set of the three observations was instead analyzed by Kubo et al. (1998), but assuming a column density fixed at the neutral galactic hydrogen value [[FORMULA] cm-2], without considering the extra absorption due to the Barnard cloud.

We therefore decided to re-analyze the full ASCA data set. We first fitted the 3 observations separately, finding that, despite a flux change of a factor 1.5, the three best fit values of the spectral index were nearly equal: with free [FORMULA] the values were within the 68% confidence uncertainties. We then fitted the sum of the 3 ASCA observations and the increased signal-to-noise ratio allowed to obtain the best measure of the column so far: [FORMULA] cm-2 (see Table 3 for the fit parameters).


Table 3. Single power law fits to ASCA data with fixed [FORMULA]

In Fig. 3 we show the confidence contours in the photon index-column plane, for the sum of the August 1995 ASCA observations and the sum of the 8 Beppo SAX observations.

[FIGURE] Fig. 3. Confidence contours (68%, 90% and 99%) for fits with a single power law plus absorption for the sum of the three August 1995 ASCA observations and the sum of the eight Beppo SAX observations.

We also searched for the possible presence of an Fe [FORMULA] line at 6.4 keV, finding an upper limit to its equivalent width of 67 eV in the summed 1995 spectrum (calculated in the rest frame of the source). This limit is only marginally consistent with the value of [FORMULA] eV reported by Reeves et al. (1997) analyzing the 1994 ASCA spectrum, when the source was fainter.

5.2. Historical X-ray and [FORMULA]-ray light curves

In order to understand the relationship between X-ray and [FORMULA]ray emission, we compared both the light curves in the last 7 years and the spectral indices behaviour in these two bands.

In Fig. 4 (upper panel) we show the light curves of all the observations in the X-ray band after 1991. For consistency, we have re-analyzed all these data (taken from the archives) with the same [FORMULA] cm-2. In 1995 the source varied by 50% in 2 weeks. Between the 1995 ASCA and our 1997 Beppo SAX observations, when the source was at its faintest historical level, the flux decreased by a factor [FORMULA]7. The lower panel shows the light curve of the [FORMULA]-ray flux, as observed by EGRET (Mukherjee et al. 1996, 1997a, 1997b). In [FORMULA]-rays the source was observed more often, with a variability of a factor 13 in 2 months, and a factor [FORMULA]2 in 2 days during the 1993 flare (see Mukherjee et al. 1996).

[FIGURE] Fig. 4. Historical light curves in the X-ray and [FORMULA]-ray bands, after 1991. All X-ray data have been reanalysed assuming a fixed [FORMULA] cm-2.

The lack of simultaneity of previous observations and the paucity of X-ray data do not allow to establish any (or absence of) correlation. It should be however noticed that for the three epochs of simultaneous observations (Aug. 1994, Mar. 1995 and Feb.-Mar. 1997), the X-ray and [FORMULA]-ray flux levels follow the same trend.

5.3. Flux-spectral index correlations?

In Fig. 5 (upper panel) we report the X-ray spectral indices determined by Beppo SAX and ASCA data vs the 1 keV flux. While all ASCA points are consistent with the same spectral index, the spectral shape determined by Beppo SAX observations is significantly flatter (see also Fig. 3), and corresponds to the fainter flux.

[FIGURE] Fig. 5. Spectral indices vs flux in the [2-10] keV and [0.05-1] GeV bands. All X-ray data have been reanalysed assuming a fixed [FORMULA] cm-2. [FORMULA]-ray data from Mukherjee et al. 1996, 1997a, 1997b, 1999.

In the lower panel of Fig. 5 we show the [FORMULA]-ray spectral index as a function of the [FORMULA]-ray flux (data from Mukherjee et al. 1996, 1997a, 1997b, 1999). The large error bars of [FORMULA], especially at faint fluxes, do not allow any conclusion about the presence of a trend: although the linear correlation coefficient is [FORMULA]-0.6 (random probability=0.02), a [FORMULA]-test on a fit with a constant gives 15% probability that the two quantities are not correlated.

Although we cannot draw any firm conclusion about the flux-spectral index correlations, note that during the Beppo SAX observations the source, in its faintest state, had the flattest spectral index. This is an unusual behavior for blazars (even if some other example exist, see Ulrich et al. 1997), and likely to yield important information and/or constraints on the emission models, as discussed below.

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© European Southern Observatory (ESO) 1999

Online publication: July 16, 1999