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The warm absorber in NGC 5548

The warm absorber in NGC 5548. Jelle Kaastra / Elisa Costantini SRON Katrien Steenbrugge CfA. Overview of the talk. Introduction to NGC 5548 Short description of WA modeling Velocity structure of the outflow How many absorption components? Emission features Time variability

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The warm absorber in NGC 5548

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  1. The warm absorber in NGC 5548 Jelle Kaastra / Elisa Costantini SRON Katrien Steenbrugge CfA

  2. Overview of the talk • Introduction to NGC 5548 • Short description of WA modeling • Velocity structure of the outflow • How many absorption components? • Emission features • Time variability • Preliminary results of the 2005 observation • Conclusions

  3. 1. Introduction to NGC 5548 • One of the brightest S1 in UV and X-ray Well studied in UV and X-rays • Low galactic absorption  ideal for spectroscopy • Moderately deep warm absorber  blending not too large, but still strong WA signature

  4. History 1: the low-resolution era • ASCA data (Reynolds 1997) • Spectrum: power law • Additional Fe-line • Warm absorber (modeled with continuum edges) Fe-K emission Warm absorber

  5. History 2: high spectral resolution: lines! • NGC 5548 first Seyfert ever observed at high spectral resolution (dec 1999, Chandra LETGS) • Lots of absorption lines from different ions • Shows importance of high resolution Kaastra et al. 2000

  6. 2. Overview of available data

  7. High-resolution X-ray and UV observations of NGC 5548 • This presentation is based on three groups of spectra: • 1. LETGS/HETGS/RGS single observations in 1999/2000 (see next slide) • 2. Large X-ray/UV campaign in 2002 (see later this talk) • 3. 150 ks LETGS observation in 2005 (end of the talk, preliminary results)

  8. NGC 5548 - need for campaign • Chandra LETGS: Dec 1999 - 86 ks • Chandra HETGS: Feb 2000 – 83 ks • XMM-Newton : Dec 2000 – 28 ks • XMM-Newton : Jul 2001 – 137 ks • None with simultaneous UV • HST/GHRS : Feb/Aug 1996 – 18.2 ks • HST/STIS : Mar 1998 – 8.9 ks • FUSE : Jun 2000 – 25 ks • None with simultaneous X-ray

  9. Multi-λ campaign in 2002 • Approved Chandra/HST/FUSE campaign •  Observed Jan 2002 (without FUSE, due to technical problems) • HST/STIS: 21 ks – Crenshaw et al. 2003 • Chandra LETGS+HETGS: 510 ks • Kaastra et al. 2004 - Time variability • Steenbrugge et al. 2005 - Spectra

  10. 3. Short description of WA modeling

  11. X-ray analysis • Fit spectra using a power law + modified blackbody continuum • Where needed, add emission lines: relativistic, BLR or NLR X-ray lines • Fit warm absorber using a model (see next slide)  ionic or total column densities • Using photo-ionisation model, derive NH and ξ distribution • Spectral fits done with SPEX, global fits

  12. Photoionisation models • Models for transmission of a thin slab • Continuum & line absorption calculated • slab model: ion columns independent • xabs model: ion columns coupled through xstar/cloudy runs • warm model: continuous distribution of NH(ξ)

  13. What is in the photo-ionisation models? • Continuum opacities from Verner et al. 95 • Line absorption from many different sources: Ferland & Verner, HULLAC (Fe-L & Fe-M, O-K inner shell) etc. • Take account of line profile (Voigt) • Allow for turbulent motion and systematic outflow velocities • Self-consistent photo-ionisation model is under development

  14. More info about SPEX • See the web page: www.sron.nl/divisions/hea/spex/index.html

  15. 4. Velocity structure of the outflow

  16. X-ray line profiles: 1999 spectrum • Lines are broadened: 270±100 km/s • Lines are blueshifted: 280±100 km/s • Lines have tendency for extended blue wing • Some lines (O VII res, O VIII Lyα) have P Cygni -like profile •  outflowing, photoionised wind model

  17. A comparison with UV lines • High spectral resolution in UV • Only few UV lines (H I, C IV, N V and O VI) • At least 5 components seen in absorption, superimposed on broad emission lines • UV lines narrow and resolved: σv= 20-80 km/s • R.M.S. width of ensemble: 160-260 km/s, consistent with LETGS • UV lines are blueshifted, range 160-1060 km/s •  UV & X-ray lines two manifestations of same phenomenon

  18. Velocity structure in UV lines • STIS spectra (Crenshaw & Kraemer 1999) show 5 velocity components: • Nr 1 high -1040 km/s • Nr 2 med -667 km/s • Nr 3 med -530 km/s • Nr 4 med -336 km/s • Nr 5 low -160 km/s Radial velocity (km/s)

  19. X-ray line profiles • At long λ, LETGS has higher resolution: C VI profile not fully resolved but consistent with UV structure

  20. Outflow velocity versus ξ • UV shows 5 velocity components • X-ray resolution insufficient to resolve them • But high average v at high ξ

  21. Velocity structure • Strongest lines fit in three components with fixed v: high, med and low v • High v column density increases more rapidly with ξ than med or low: UV/HST Simultaneous UV/FUSE Non-simultaneous.

  22. 5. How many absorption components? • First glance at 1999 LETGS spectrum: strong lines of O VII, O VIII and others from photoionized plasma (Kaastra et al. 2000) • Is there more than 1 component?

  23. Why we can constrain the warm absorber in NGC 5548 so well K-shell region • High flux & low NH  bright in soft X-ray band • also detection of WA at long λ from L-shell transitions Mg, Si, S etc. •  redundancy in determining WA structure L-shell region Sample of 1.5-100 Å spectra (here the 2002 spectrum)

  24. The oxygen region: why important? • Good diagnostic region because: • For almost any ξ there is a diagnostic ion • Oxygen is the most abundant metal

  25. Long wavelength transitions • Our photoionisation model at long λ quite succesfull • See 2002 LETGS spectrum • Only global fitting works here: low S/N • Atomic data need update in this region

  26. A more detailed look to 1999 data: three ionization components • Ionization structure is not a simple 1-phase medium • fits to LETGS data require at least 3 ionization components: • log ξ = 0.5 • log ξ = 1.9 • log ξ = 2.9 Kaastra et al.2002

  27. Decomposition into separate ξ: evidence for 5 components • Use column densities Fe ions from RGS data • Measured Nion as sum of separate ξ components • LETGS results similar • Need at least 5 components

  28. Separate components in pressure equilibrium? • Not all components in pressure equilibrium (same Ξ~ξ/T~F/p) • Division into ξ comps often poorly defined • Continuous NH(ξ) distribution: see next slide

  29. Column density versus ξ

  30. Column density versus ξ Fe at low T: DR rates?

  31. Continuous ionization distribution • Continuous distribution over at least 3.5 orders in ξ • dNH/dlnξ~ξα, with α=0.40±0.05 • Adopt streamer-like geometry • Take dNH=n(s)ds with s distance from axis • ξ=L/n(s)r², r and L constant • n(s)~1/(1+s/s0)β, β=1/(1+α) • s»s0: n(s)~1/s0.71

  32. Mass loss through the wind

  33. 6. Emission features

  34. Relativistic lines in NGC 5548 • Evidence for relativistic lines of O VIII and N VII in 1999 spectrum • Lines weak: EW 0.6 and 1.1 Å • Significant at 3σ • Inclination 46º consistent with Fe-K (Yaqoob et al.) • Inner radius <2.6GM/c²  Kerr hole? • Also seen by BeppoSAX before? (Nicastro et al.)

  35. Broad emission lines • NGC 5548: C VI Lyα • FWHM 10000 km/s (Kaastra et al. 2002) • Also seen in O VII triplet in NGC 5548 (Steenbrugge et al. 2005) and Mrk 279 Costantini et al. 2005)

  36. Narrow emission lines • O VII forbidden line strongest narrow line • No significant red/blueshift • Low σv < 300 km/s  from distant region • Not variable between 1999 and 2002 (LETGS, RGS)

  37. 7. Time variability

  38. Long term variability of NGC 5548 • Difference between LETGS spectra: • Dec 1999 - Jan 2002 • Difference in red wing broad C VI lines (@ +2000 to 3000 km/s, FWHM=1000 km/s) • Difference in O V line→ log ξ~-0.2

  39. Short term continuum variability

  40. 8. New LETGS data: april 2005

  41. New LETGS data: april 2005 • New data 150 ks • Taken in april 2005 • NGC 5548 was in a very low state: 4.5 x weaker than in 2002 • Continuum is very hard

  42. Changes in warm absorber • Plot shows scaled spectra • Significant change in O V- OIII (see plot) • OV is deeper and broader: trend of 2002 continued • Very deep O III • Other ions no significant change

  43. Interpretation of WA changes • Luminosity drop of factor 4.5 since 2002 • But not simply scaling with ξ=L/nr² • Columns O III, O IV and O V much larger • Also larger width: σ=440 km/s versus 70-140 km/s in 2002

  44. Forbidden line O VII: time variable • Forbidden line constant during 1999-2002 Chandra/XMM observations • Fluxes inph/m2/s: • 1999: 0.81±0.19 • 2002: 0.88±0.12 • 2005: 0.35±0.08 •  forbidden line formed at pc scale

  45. Conclusions/questions • Warm absorber in NGC 5548 and other sources more consistent with continuous NH(ξ) distribution then separate components in pressure equilibrium • Outflow should occur in narrow, density stratified streamers • What determines maximum ionization parameter?

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