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ASCA Observation of the Lyman-limit Quasar PKS2145+067

X-ray observation of a famous Lyman-limit quasar PKS2145+067at z_em= 0.990 was carried with ASCA. The source showed a 2-10 keV flux of 1.3 E-11 erg cm^-2 sec^-1 (L_X = 2.5 E46 erg sec^-1 for H_0=50 km s^-1 Mpc^-1) described by a power-law spectrum with a photon index Gamma=1.63 +/- 0.04 . In the ASCA energy band, no excess absorption was detected implying the absorption column density at z_ ab=0.791 was less than 1.6 E21 cm^-2 if absorbing medium had a metal abundance of 0.5 solar. Comparison with previous Einstein and ROSAT observations shows that PKS2145+067 has increased its luminosity by a factor of 2--3 between 1991 and 1998.

The source is a radio-loud QSO and is classified as a flat-spectrum radio quasars (FSRQ) (see ex. Padovani 1992).
The polarization of this quasar is as low as 1.0±0.6 % (Visvanathan & Wills 1998), and a submilliarcsecond imaging by the Very Long Baseline Array (VLBA) at 15GHz does not detect any jet-like structure (Kellermann et al. 1998). The Lyman limit system exhibits a neutral hydrogen column density as large as N (HI)=3.2×10 17 cm −2 (1.6×10 16 ≤ N (HI)≤ 6.3×10 17 cm −2 ) with a Doppler parameter b of 35 ≤ b ≤ 63 km s −1 (Bergeron et al. 1994). An 2 N.Y. Yamasaki,T. Ishikawa & T.Ohashi [Vol. , associated galaxy is identified at 55h −1 0 kpc away from the line of sight (Bergeron & Boissè 1991) and is considered to be in star formation activity. The co-existence of the absorption lines from a low-ionized element Mg II and a high-ionized O VI suggest that there are at least two states of clouds with different densities. Based on the detailed observational results, Bergeron et al. (1994) constructed a two-phase photo-ionized cloud model. The inner core zone which exhibits Mg II absorption doublet has an intermediate dimension (∼ 7 kpc) with a density of n H ∼6×10 −3 cm −3 and a temperature of T ∼ 1.2×10 4 K. The outer O VI phase is homogeneous, with a lower density n H ∼ 3×10 −4 cm −3 , with a high temperature of T ∼ 2.5×10 4 K and of a large extent (∼ 70 kpc). The total mass of the gas is 10 9 M • ⊙ and the common metal abundance is about half the solar value. The average neutral hydrogen fraction is HI/H =3×10 −3 at the core and the neutral and ionized hydrogen column density is N H =1.7×10 20 cm −2 .
Considering the similarity in the size, temperature and high metal abundance, a link between the O VI phase gas and a group of galaxies is suggested (Bergeron et al. 1994). A Gunn-Peterson test using HST FOS detect an absorption trough with τ GP−HI = 0.12∼0.14, which corresponds to an over-density by a factor of 3-7 in the intergalactic medium near z ∼0.8 (Khersonsky et al. 1997).
If there is such a hot and dense intervening cloud in the line of sight, ionized heavy elements could produce absorption features in the X-ray spectrum. We observed PKS2145+067 with ASCA (Tanaka et al. 1994) to search for an evidence of this hot intervening cloud using X-ray absorption features. The observation was performed on 5th December 1998 with a net exposure time of 36 ksec with GIS and 29 ksec with SIS, respectively. The observation mode was nominal PH mode for GIS (Ohashi et al. 1996, Makishima et al. 1996 and 1CCD FAINT mode for SIS. In this paper, we assume H 0 =50 km sec −1 Mpc −1 and q 0 =0.5 and the solar number abundances of the elements are followed by Anders & Grevesse (1989).

Energy Spectrum of PKS2145+067
The source has been clearly detected with all the four detectors of ASCA. To make the energy spectra, we applied the standard data reduction criteria for the data and integrated the photons within a radius of 6 arcmin from the source. Then we subtracted recent blank sky data in the same observation mode to correct for both the cosmic X-ray background and non X-ray background. For the GIS background, a 80 ksec deep survey observation of the FAINT mode data taken in the L1157 observation in September 1998 (PI:T. Furusho), in which L1157 dark cloud containing a class 0 proto-star has not been detected in X-ray band (Furusho et al. 2000). Since the background data were taken within 3 months before the QSO observation, the long-term variation of the detector background and performances does not cause a problem in the subtraction process. In the L1157 data, selection of the same event discrimination level yields a net exposure time of the background to be 10.9 ksec. However, the data still have better photon statistics than using the annular region of the on-source SIS data because we can maximize the on-source integration area. In both GIS and SIS cases, S/N ratios are larger than 5 in all energy bands, and the total flux uncertainty due to the fluctuation of the background is less than 1%.
The derived spectra after the background subtraction are shown in figure 1. To avoid the uncertainty in the SIS response matrix which tends to cause a systematic excess absorption of a few times 10 20 cm −2 (Cappi et al. 1998) due to the drop of the detection efficiency below 1 keV (see web pages of ASCA GOF, http://heasarc.gsfc.nasa.goc/docs/asca/watchout.html), we use only the data between 1.0 keV and 10.0 keV for SIS, while the GIS data are used between 0.6 keV and 10.0 keV. Spectral fitting package XSPEC ver 10.0 is used for the model fitting throughout this paper. We fit the SIS and GIS data simultaneously and minimize the sum of the χ 2 assuming an absorbed power-law model.
As for the absorber model, we first assume the interstellar matter in our Galaxy represented by a XSPEC "wabs" model at z = 0, which is the photoelectric absorption with a metal abundance of one solar (Morrison & McCammon 1983). All the data from SIS and GIS are consistent with each other and well described by a power-law with interstellar absorption. The resultant parameters are summarized in table 1. The absorption column density N H is consistent with the Galactic value of 4.8×10 20 cm −2 (Dickey & Lockman 1990). A 90 % upper limit for the equivalent width of an iron-K emission line, assuming a narrow (σ < 500 eV ) Gaussian line at 6.4 keV in the quasar frame (z =0.990) is 50.4 eV. The total flux between 2 and 10 keV in the observer frame is 1.3×10 −11 erg cm −2 sec −1 which corresponds to the luminosity L X:2−10keV = 2.5×10 46 erg sec −1 assuming H 0 =50 km sec −1 Mpc −1 and

Upper limit for the absorption at z=0.791
We added an additional absorption component at z =0.791 in the model and fit the spectra again. The assumed model is described by a formula; wabs × zwabs × a power-law. The "wabs " model again represents the absorption by the interstellar matter in our Galaxy and the column density N H is fixed to the Galactic value of N H = 4.8×10 20 cm −2 . The "zwabs" model is a redshifted "wabs" model, which stands for the absorption by neutral matter with an abundance of 1 solar. In this fit, we fixed the redshift of "zwabs" to be z =0.791 assuming its association to the For the next step, we assumed that the metal abundance in the absorbing matter at z = 0.791 is less than the solar level. We adopted a variable abundance photoelectric absorber using the XSPEC model of "zvphabs" ASCA Observation of PKS2145+067 5 upper limit of the X-ray absorption column is 1.6×10 21 cm −2 . This does not contradict with the two-phase model (N H = 1.7 × 10 20 cm −2 ) by Bergeron et al. (1994).
We further searched for absorption features due to discrete elements, for which ASCA data have good sensitivity.
We added a redshifted absorption edge of Si. In the photo-ionized cloud model of Bergeron et al. (1994), the most dominant population of Si is Si VII in the hot halo and Si III in the core. These ions produce a Si III edge at 1852 eV (in the rest frame) of Si III and a Si VII edge at 2001 eV which can be tested in the ASCA sensitivity band.
Again, we fitted the energy spectra with a absorbed power-law and an edge structure model given by the following formula ; wabs (N H =4.8×10 20 cm −2 fixed) × "zedge" (redshifted K-edge) × power-law . The 90 % upper limits for the optical depth of the edges are τ <0.15 for 1852 eV (at z =0.791) Si III edge and τ <0.08 for 2001 eV Si VII edge. These optical depths correspond to column densities of 9.7×10 17 cm −2 and 6.3×10 17 cm −2 for Si III and Si VII ions (Verner et al. 1994), or assuming 0.5 solar abundances, the upper limits for the total hydrogen column density of 5.4 × 10 22 cm −2 and 3.5 × 10 22 cm −2 , respectively.
These flux comparison suggests two possibilities; the first is that the X-ray luminosity of PKS2145+067 has become higher in 10 years by a factor of 2∼3. The radio flux of the source was monitored at 318 and 430 MHz between 1980 and 1994 and showed little variation only by 7 % rms (Salgado et al.1999). Sambruna (1997) compile ROSAT results and report that typical X-ray flux variation of flat-spectrum radio quasars (FSRQs) on timescales of months/years does not exceed a factor of 2, characterized by a typical amplitude of the order of 10 -30 % with no accompanying spectral changes. If the X-ray flux of PKS2145+067 has really varied by a factor of 3, this amplitude is the largest among the reported FSRQ variations.
The second possibility is that the energy spectrum is strongly cut off below the ASCA energy limit of 0.6 keV. Unfortunately, the spectral information from ROSAT and Einstein observations is not available. We added an excess absorption in the fitting model to suppress the extrapolated flux in the low energy band to be consistent with the previous results. In this case, however, the required excess absorption becomes larger than 10 22 cm −2 using the zvphabs model at z = 0.791 with a metal abundance of 0.5 solar. This is significantly larger than our upper limit of 1.6×10 21 cm −2 . So it seems unlikely that the flux difference between ASCA and the previous soft X-ray observations is only caused by a strong absorption associated with the Lyman limit clouds. It is, therefore, more plausible that the intrinsic luminosity of PKS2145+067 has increased by a factor of 2∼3 in these 10 years.

Discussion & Conclusion
ASCA observation of PKS2145+067 shows that the 0.6-10 keV spectrum is well described by a power-law model (Γ=1.6) absorbed by the Galactic interstellar absorption. An upper limit for the z = 0.791 absorbing cloud is These results are consistent with the two-phase ionized cloud model by Bergeron et al. (1996). However, to obtain a simple view of the absorbing cloud, we calculated simple one-phase photoionization models using CLOUDY ver 94.00 (Ferland 1993, van Hoof et al. 2000. We studied the requirement for the UV radiation field which satisfies the upper limit of the total hydrogen column density. We assume the absorber cloud to be a plane-parallel slab of constant density, illuminated on both sides by an ionizing radiation field. The density of the cloud is varied between 1.0×10 −3 and 1.0×10 −5 cm −3 and the calculation is stopped when the neutral hydrogen column density reaches N H = 3.2 × 10 17 cm −2 . The metal abundance is assumed to be 0.5 solar. Considering the uncertainty of the UV radiation field at z = 0.791, the intensity of the UV flux at 912A is assumed to be 0.3, 1, 3 × 10 −22 erg cm −2 sec −1 sr −1 Hz −1 (Okoshi & Ikeuchi 1996), and the energy index α is assumed to be −0.5 and −1.0 . In these condition, the ionizing parameter U (= n γ /n H ) takes the values between −2.72 < log(U ) < 0.28. There are 4 cases of strong radiation field, for the 912Å intensity of 1 and 3 × 10 −22 erg cm −2 sec −1 sr −1 Hz −1 and the index α of −0.5 and −1. In these cases, the neutral hydrogen fraction becomes too low and the inferred total hydrogen column density N H exceeds 1.7×10 20 cm −2 for the fixed neutral hydrogen of N H = 3.2 × 10 17 cm −2 . In weaker radiation field cases, the cloud size becomes smaller than 1 Mpc only when the hydrogen density is larger than 4×10 −4 cm −3 , or log(U ) < −1.9. The electron temperature is 1.4 × 10 4 < T e < 2.2 × 10 4 K and the dominant spices of Oxygen are O III and O IV. As the hydrogen density is larger than 4×10 −4 cm −3 , which is close to the typical values at the No. ] ASCA Observation of PKS2145+067 7 center of clusters and groups of galaxies, it is possible that the cloud undergoes a gravitational collapse. If collisional ionization is going on in the absorbing cloud, the fraction of highly ionized ions would become large and they could be detected as X-ray absorption structures.
Comparison with the previous Einstein IPC and ROSAT PSPC observations indicates that the X-ray luminosity of PKS2145+067 has increased by a factor of 2∼3 between 1991 and 1998 and that this discrepancy is not due to an excess absorption. Sambruna (1997) studied time variability of 10 FSRQs in timescales of months-years and found that the maximum flux change is a factor of 2 drop in 0836+710 during ∼ 8 months. The large flux change in PKS2145+067 shows that the X-ray luminosity of FSRQs can vary by more than a factor of 2 in a long timescale of the order of 10 years.
The present ASCA observation have shown that an improved sensitivity for the X-ray absorbing matter would enable us to study optically observed Lyman limit or metal line systems and that a wide-band spectroscopy with good energy resolution will bring us a unique science. We hope that new X-ray instruments with superior energy resolution on board XMM-Newton and Chandra will be able to detect the ionizing hot intervening cloud directly by absorption lines and edges and reveal the physical conditions such as density, temperature, metal abundances of the inter-galactic medium.
The authors thank Dr. Y. Ishisaki and Ms. T. Furusho for their kind allowance to use their data for the background study.