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Astron. Astrophys. 350, L31-L34 (1999)

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3. Results

The X-ray position is [FORMULA]=16h24m56.7s, [FORMULA] with a 2[FORMULA] error radius of 14". This coincides with the optical position measured by the POSS scans of the US Naval Observatory leading to the position [FORMULA] = 16h24m56.5s, [FORMULA]. We identified RX J1624.9+7554 as a galaxy with a redshift of z=0.0636[FORMULA]0.0005. No other source was found inside the 2[FORMULA] error circle, either on the POSS or on the R-image.

3.1. X-rays

During the RASS observation RX J1624.9+7554 had a count rate of 0.54[FORMULA]0.02 [FORMULA] with a hardness ratio of HR=-0.20[FORMULA]0.04 (Thomas et al. 1998).

We performed standard spectral fits to the RASS spectrum, with all parameters free or with neutral absorption fixed to the Galactic value ([FORMULA]; Dickey & Lockmann 1990). The results are listed in Table 1. The best fitting models are a power law with intrinsic [FORMULA] and a thermal Bremsstrahlung spectrum. We cannot distinguish between these two models on the basis of the spectral fitting. However, the thermal Bremsstrahlung is ruled out because the emission region size implied by the variability between the RASS and the pointed observation is small enough that the assumption of optically thin gas is violated (see e.g Elvis et al. 1991). Furthermore, we would expect to see emission lines in thermal gas, and a Raymond-Smith model provides a very poor fit. The power law spectrum is steep with an energy spectral index [FORMULA] =3.0 (see Table 1). We also tried a two power law model and a broken power law, but the same X-ray spectral slopes were obtained in the soft and hard components. A single blackbody fit to the data does not give a reasonable fit, but using a blackbody for the soft and a steep power law for the hard photons gives a good fit (see Table 1). For the power law model, we found excess absorption of cold matter above the Galactic value. This explains the relatively `hard' hardness ratio of -0.20 considering the steepness of the spectrum. While the steep power law (with intrinsic [FORMULA] cannot be ruled out statistically, it is a less favored model because it appears to be difficult to produce physically.


Table 1. Spectral fits to the RASS spectrum of RX J1624.9+7554. "[FORMULA]" is the column density given in units of [FORMULA], "Norm" is the normalization at 1.0 keV (rest frame) in [FORMULA], "[FORMULA]" the energy spectral slope, "[FORMULA]" the black body integral in [FORMULA], "Arasm" the normalization amplitude (in units if [FORMULA], see EXSAS manual, Zimmermann et al. 1998), and "[FORMULA]" and "[FORMULA]" the radiation and plasma temperatures in eV. Models are power law (powl), blackbody (bbdy), thermal Bremsstrahlung (thbr), and Raymond-Smith thermal plasma (rasm). All errors refer to the 1[FORMULA] level.

There are two pointed observations (ROR 141820 and 141829), obtained on January 13, 1992, in which RX J1624.9+7554 could have been detected. However, the source was detected in neither. We verified the attitude solution using the bright star [FORMULA] UMi that is visible in the field ROR 141829. We measured an upper limit for the count rate of RX J1624.9+7554 of 0.0023 [FORMULA] at the expected location.

[FIGURE] Fig. 1. Power law fit to the RASS spectrum of RX J1624.9+7554.

Fig. 2 displays the RASS lightcurve of RX J1624.9+7554 for all satellite orbits passing over it. A variability test leads to a [FORMULA] = 113 (71 dof) for a constant hypothesis. The source is somewhat variable around a mean count rate of 0.54 by a factor of about 2 on the timescale of a day. The count rates of the sixth day are slightly higher than the rest.

[FIGURE] Fig. 2. Light curve of RX J1624.9+7554 of the RASS data. The observation started October 07, 1990, 10:13. The errors refer to the 1[FORMULA] level.

3.2. Optical

The nuclear optical spectra from RX J1624.9+7554 taken in 1998 and 1999 are typical of a non-active galaxy with some stellar absorption lines (see Fig. 3). The only sign of activity is a weak [NII][FORMULA]6584 emission line. In the 2-D spectra extranuclear H[FORMULA] and [NII] emission is seen, probably from HII regions. The [NII] line in the nuclear spectrum may also be from an HII region. A first look at the 1998 McD spectra led to a probable mis-classification of the object as a BL Lac (Thomas et al. 1998). From the [FORMULA] slit 1998 McD spectrum we measure magnitudes V = 16.2 and R=15.7. From the R-band image a magnitude of 15.6 was obtained. The optical polarization measurements yield a degree of polarization of 0.35[FORMULA]0.31%. Therefore, the source is essentially unpolarized.

[FIGURE] Fig. 3. Combined optical spectrum of RX J1624.9+7554 obtained with the 2.4m telescope at MDM

3.3. Spectral energy distribution

Fig. 4 shows the Spectral Energy Distribution (SED) of RX J1624.9+7554. The optical data are the 1998 wide-slit McD spectrum. The X-ray spectrum is represented by a power law model with free absorption parameter [FORMULA] from the RASS observation with the limits given in Table 1. Infrared data yield detections of 50 and 90 mJy at 12 and 60 µm, respectively.

[FIGURE] Fig. 4. Spectral Energy Distribution of RX J1624.9+7554. The optical data are the broad slit McD spectrum. The infrared data are IRAS scans and the X-ray data are represented by a error bow-tie from the RASS observation and a power law fit. All data shown are in the observer's frame.

We looked for radio data and found that the nearest source in the NRAO VLA Sky Survey (NVSS) catalog is 6.73 arcminutes away from our source. This catalog contains objects with fluxes stronger than [FORMULA] mJy (Condon et al. 1998).

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© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999