XMM-NEWTON AND FUSE TENTATIVE EVIDENCE FOR A WHIM FILAMENT ALONG THE LINE OF SIGHT TO PKS 0558-504

We present a possible O viii X-ray absorption line at z = 0.117 ± 0.001 which, if confirmed, will be the first one associated with a broad H i Lyβ (BLB: FWHM = 160+50−30 km s−1) absorber. The absorber lies along the line of sight to the nearby (z = 0.1372) Seyfert 1 galaxy PKS 0558-504, consistent with being a WHIM filament. The X-ray absorber is marginally detected in two independent XMM-Newton spectra of PKS 0558-504, a long ∼600 ks guest-observer observation and a shorter, ∼300 ks total, calibration observation, with a combined single line statistical significance of 2. 8σ (2.7σ and 1.2σ in the two spectra, respectively). When fitted with our self-consistent hybrid-photoionization WHIM models, the combined XMM-Newton spectrum is consistent with the presence of O viii Kα at z = (0.117 ± 0.001). This model gives best-fitting temperature and equivalent H column density of the absorber of log T = 6.56+0.19−0.17 K, and log NH = (21.5 ± 0.3)(Z/Z0.01☉)−1 cm−2, and predicts the marginal contribution of only two more lines within the XMM-Newton Reflection Grating Spectrometer band pass, Ne ix Kα (λ = 13.45 Å) and Fe xvii L (λ = 15.02 Å), both with equivalent widths well within the 1σ sensitivity of the combined XMM-Newton spectrum of PKS 0558-504 (EW1σ < 3 mÅ). The lack of detection of associated O vi in the archival FUSE spectrum of PKS 0558-504 allows us to infer a tighter lower limit on the temperature, of log T>6.52 K (at 1σ). The statistical significance of this single X-ray detection is increased by the detection of BLB and complex H i Lyβ absorption in archival FUSE spectra of PKS 0558-504, at redshifts z = 0.1183 ± 0.0001 consistent with the best-fitting redshift of the X-ray absorber. The FUSE spectrum shows a broad (FWHM = 160+50−30 km s−1) absorption complex, which we identify as H i Lyβ zBLB = (0.1183 ± 0.0001). The single line statistical significance of this line is 4. 1σ (3.7σ if systematics are considered). A possible H i Lyα is marginally hinted in an archival low-resolution (Δλ ∼ 6 Å) International Ultraviolet Explorer (IUE) spectrum of PKS 0558-504, at a redshift of z = (0.119 ± 0.001) and with single line significance of 1. 7σ. Thus, the combined significance of the three (XMM-Newton, FUSE, and IUE) independent tentative detections, is 5. 2σ (5.0σ if the H i Lyα is not considered, and 4.6σ if the systematics in FUSE are considered). The detection of both metal and H lines at a consistent redshift, in this hot absorbing system, allows us to speculate on its metallicity. By associating the bulk of the X-ray absorber with the BLB line detected in the FUSE spectrum at zBLB = 0.1183 ± 0.0001, we obtain a metallicity of 1%–4% Solar. Although the absorber is only blueshifted by ∼−6000 km s−1 from the systemic redshift of PKS 0558-504, the identification of the absorbing gas with a high velocity nuclear ionized outflow, is unlikely. The physical, chemical, and dynamical properties of the detected absorber are all quite different from those typically found in the warm absorber (WA) outflows, commonly detected in Seyferts and higher luminosity quasars. WA outflow velocities typically span a range of few hundreds to ∼1–2 thousands km s−1; WA metallicities, when measured, are typically found to be at least Solar; high-ionization WAs are virtually always found to coexist with lower-ionization X-ray and UV phases. All this strongly suggests that the absorber, if confirmed, is an intervening WHIM system.