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The Optical Counterpart of NGC6946 ULX-1

The Optical Counterpart of NGC6946 ULX-1. (a peculiar ULX nebula MF16). P. Abolmasov, S. Fabrika, O. Sholukhova, V. Afanasiev Special Astrophysical Observatory, Russia. ULX nebulae:. Shock excitation ([SII], [NII], [OI] etc. ) Large sizes, ~50-500pc HeII λ 4686 emission (stellar?).

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The Optical Counterpart of NGC6946 ULX-1

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  1. The Optical Counterpart of NGC6946 ULX-1 (a peculiar ULX nebula MF16) P. Abolmasov, S. Fabrika, O. Sholukhova, V. Afanasiev Special Astrophysical Observatory, Russia

  2. ULX nebulae: • Shock excitation ([SII], [NII], [OI] etc. ) • Large sizes, ~50-500pc • HeII λ4686 emission (stellar?) What are they? MH9/10, the optical counterpart of HoIX X-1 (Grise et al., 2006) • SNRs? (too powerful) • Superbubbles? (lack of young stars) • X-ray ionized nebulae? (requires too much X-rays in some cases!)

  3. from Blair&Fesen, 1994 MF16 (counterpart of NGC6946 ULX-1): 34pc (~1,˝5) (HST ACS, filter F658N) X-ray source coordinates: 20h 35m 00s.75 +60˚ 11' 30".9 But: HeIIλ4686 / Hβ ~ 0.2 !! 20pc (~1˝) Radio isophotes (VLA) LX ≈ 2.5 1039 erg/s (0.5-8.0 keV) L(H) ≈ 2 1038 erg/s L(optical lines, total) ≈ 2 1039 erg/s Also a bright radio source

  4. Observations with the 6m SAO telescope Long Slit MPFS FOV 1˝ Spectrograph wavelength range,Å spectral seeing, resolution arcsec MPFS (panoramic) 4000-7000 ~1000 1,5 SCORPIO (long-slit) 4000-5700 ~2000 1,5

  5. Two-component lines: HeII4686 H [OIII]4959 [OIII]5007 V2-V1 120-130 km/s [OIII]5007 Broader component width ~ 200-300km/s H

  6. Total luminosity in H (if the shock surface area is given); • Shock velocity estimate from the component ratio: For a single H line (from Dopita&Sutherland, 1996)

  7. Single Hβ line kinematics nH, cm-3 L(Hβ) = (7,2±0,4) 1037 erg/s Narrow-to-broad components ratio 1,85±0,19 Additional source of excitation/ionization?

  8. MF16 MF16 Photoionization by power law continuum (labelled by ionization parameter logarithm lgU) from Evans et al., 1999  SHOCK WAVES OR PHOTOIONIZATION? Only lgU~-5 can be provided by the extrapolated model for X-ray spectrum (Chandra data, taken from Roberts&Colbert, 2003), lgU~-2.7 needed B/n1/2 =

  9. HeII4686 MF16 L(HeII)=1.6 1037 erg/s Cannot be explained by a shock wave, and an EUV source needed with L<228Å≥ 1039erg/s Or X-ray source with L ~ 1041 erg/s or larger! Photoionization by power law continuum (labelled by ionization parameter logarithm lgU)

  10. … and other high-excitation lines

  11. Pure photoionization models (CLOUDY 96.01 modeling results): Central source emitting: fixed X-ray spectrum (best-fit from Roberts&Colbert, 2003), with dilution corresponding to 13pc + black body with temperatures (3-30) 104K and integral flux densities 0.3-7.0 erg/cm2/s Photoionization best-fit: lgT(K) = 5.15±0.05 (T~120 000K) F = 0.6 ± 0.1 erg/cm2/s => UV luminosity ~1040erg/s

  12. Conclusions: • MF16 remains an enigma, but it is clearly much more than a SNR • We estimate the expansion velocity for the nebula as 50-100 km/s, that is close to the late-Sedov SNR expansion rate in a moderately-dense ISM (nH~1 cm-3) • Shock waves seem to be short in explaining the observed optical emission-line spectrum • We suggest a photoionizaing source in H-ionizing UV (~1040erg/s) and He+-ionizing EUV (>1039erg/s), needed to explain the high-excitation emission line luminosities

  13. Thank you for attention!

  14. References: • Holt, S. S., Schlegel E. M., Hwang, U., & Petre, R. 2003, ApJ, 588, 792 • Dunne, C. D., Gruendl, R. A., & Chu, Y.-H. 2001, AJ, 119, 1172 • Liu, Z.-F. & Bregman, J. N., 2005, ApJSS, 157, 59 • Roberts, T. P. & Colbert, E. J. M., 2003, MNRAS, 341, L49 • Blair, W. P., Fesen, R. A., Schlegel, E. M., 2001, AJ, 121, 1497 • Schlegel, E. M., Blair, W. P., Fesen, R. A., 2000, AJ, 120, 791 • Blair, W. P., Fesen, R. A., 1994, ApJ, 424, L103 • Lira, P., Lawrence, R. A., Johnson, A., 2000, MNRAS, 319, 17 • Pakull, M. W., Mirioni, L., 2003 RevMexAA, 15, 197 • Van Dyk, S. D., 1994, ApJ, 425, L77 • Lehmann, I. et al., 2005, A&A, 431, 847

  15. MF16 – main properties: • total x-ray luminosity > 1039 erg/s (>> Cas A) • “ULX-like” X-ray spectrum, i e, soft MCD+PL • short-timescale variability (minutes) • point source (with Chandra resolution, d < 0.7 arcsec) in X-rays X-rays not from the SNR! – Roberts & Colbert, 2003 Bright (L ~ 2 1039 erg/s in optical emission lines only) emission line region 1×1.5arcsec (2034 pc) with a SNR-like spectrum [SII] 6717,6731 / H ~ 1; bright [NII] 6548,6583, [OI] 6300, 6364 from Blair&Fesen, 1994 

  16. W50 (VLA) HST images B  H+[NII]

  17. NGC6946, U-band, CFHT, MegaPrime  Observations held 17/18 January 2005, with MultiPupil Fiber Spectrograph on 6m telescope BTA, 8 exposures, total exposure 5829s seeing ~1".5, spectral resolution R~650 (~8Å at H wavelength) (red circle denotes the coordinates of the X-ray source, according to Roberts, Colbert, 2003) 20h 35m 00s.75 +60˚ 11' 30".9

  18. Data reduction: MPFS-data reduction system developed in IDL environment, prior versions written by V. L. Afanasiev, A. V. Moiseev Reconsidered and rewritten: • Extraction using 2 etalon files (different for object and spectral standard) • Cosmic hits removal (because of non-negligible probability of cosmic particles’ overlapping for different frames for new 2k×2k CCD) • Added flat field extraction (data cubes rather than initial frames are corrected by flat fields) Included: • Atmospheric dispersion correction (better than 0,1 arcsec accuracy, using spectral standard) • Arbitrary correction for interstellar absorption, using CCM (Cardelli, Clayton & Mathis, 1998) absorption curve • Some other features…

  19. H / H [OIII](5007+4959) / H AV ne, cm-3 [SII](6717+6731)/ H [SII]6717 / [SII]6731

  20. [OIII] 5007+4959 [SII] 6717+6731 H

  21. SCORPIO higher-resolution data (R~1000): Component fluxes and radial velocities * - red, + - blue [OIII]5007+4959 H E W E W

  22. MF16 NGC 6946 galaxy: D = 5,5 (from 5,1 to 5,9) Mpc Scd, active star formation

  23. Vshock~400km/s Shock power must be than: MF16 4 R=10pc Pure shock+precursor model (Dopita&Sutherland 1995) Vshock, km/s B/n1/2 = B/n1/2 = Vshock, km/s • LH = 6 1037 erg/s, • Lemission lines / LH ~ 400 • n > 240cm-3 (!)

  24. MCD (kT=0.15 keV) + PL (Г = 2.5) LX=2.5 1039 erg/s (0.5-8.0 keV)  Сhandra observations of NGC 6946 ULX-1 (Roberts, Colbert, 2003) The ULX (NGC6946 X-1, X-8, X-11 or ULX-1, ULX-3):

  25. H HeII [OIII] H+ radio 20cm

  26. [OIII] (5007+4959) Hα Barycenter shift: line   H : -0,111 -0,088 ±0,013 H : -0,11 -0,10 ±0,02 HeII : 0,72 -0,02 ±0,02 [OIII] : 0,058 0,011 ±0,015 [SII] : -0,116 -0,081 ±0,014 [NII] : -0,061 -0,064 ±0,013 Hβ [NII] (6548+6583) HeII4686 Emission lines’ mapping: 1˝ [SII] (6717+6731)

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