The Optical Counterpart of NGC6946 ULX-1 P. Abolmasov, S. Fabrika, O. Sholukhova, V. Afanasiev Special Astrophysical Observatory, Russia (a peculiar ULX nebula MF16)
ULX nebulae: MH9/10, the optical counterpart of HoIX X-1 (Grise et al., 2006) -Shock excitation ([SII], [NII], [OI] etc. ) -Large sizes, ~50-500pc -HeII λ4686 emission (stellar?) -SNRs? (too powerful) -Superbubbles? (lack of young stars) -X-ray ionized nebulae? (requires too much X-rays in some cases!) What are they?
34pc (~1,˝5) 20pc (~1˝) L X ≈ erg/s ( keV) L(H ) ≈ erg/s L(optical lines, total) ≈ erg/s Also a bright radio source from Blair&Fesen, 1994 MF16 (counterpart of NGC6946 ULX-1): But: HeIIλ4686 / Hβ ~ 0.2 !! (HST ACS, filter F658N) Radio isophotes (VLA) X-ray source coordinates: 20h 35m 00s ˚ 11' 30".9
Observations with the 6m SAO telescope Spectrographwavelength range,Åspectralseeing, resolutionarcsec MPFS (panoramic) ~10001,5 SCORPIO (long-slit) ~20001,5 MPFS FOV Long Slit 1˝1˝
[OIII]5007 HH HeII 4686 H [OIII] 4959 [OIII] 5007 Two-component lines: V 2 -V 1 km/s Broader component width ~ km/s
(from Dopita&Sutherland, 1996) Total luminosity in H (if the shock surface area is given); Shock velocity estimate from the component ratio: For a single H line
L(Hβ) = (7,2±0,4) erg/s Narrow-to-broad components ratio 1,85±0,19 kinematics Additional source of excitation/ionization? Single Hβ line n H, cm -3
SHOCK WAVES OR PHOTOIONIZATION? MF16 from Evans et al., 1999 Photoionization by power law continuum (labelled by ionization parameter logarithm lgU) 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/n 1/2 =
L(HeII)= erg/s HeII 4686 Cannot be explained by a shock wave, and an EUV source needed with L <228Å ≥ erg/s MF16 Photoionization by power law continuum (labelled by ionization parameter logarithm lgU) Or X-ray source with L ~ erg/s or larger!
… and other high-excitation lines
Pure photoionization models (CLOUDY modeling results): Photoionization best-fit: lgT(K) = 5.15±0.05 (T~ K) F = 0.6 ± 0.1 erg/cm 2 /s fixed X-ray spectrum (best-fit from Roberts&Colbert, 2003), with dilution corresponding to 13pc + black body with temperatures (3-30) 10 4 K and integral flux densities erg/cm 2 /s => UV luminosity ~10 40 erg/s Central source emitting:
Conclusions: ● MF16 remains an enigma, but it is clearly much more than a SNR ● We estimate the expansion velocity for the nebula as km/s, that is close to the late-Sedov SNR expansion rate in a moderately-dense ISM (n H ~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 (~10 40 erg/s) and He + - ionizing EUV (>10 39 erg/s), needed to explain the high-excitation emission line luminosities
Thank you for attention!
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
MF16 – main properties: X-rays not from the SNR! – Roberts & Colbert, 2003 Bright (L ~ 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 ● total x-ray luminosity > 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
HST images B H + [NII] W50 (VLA)
Observations held 17/18 January 2005, with MultiPupil Fiber Spectrograph on 6m telescope BTA, 8 exposures, total exposure 5829 s seeing ~1".5, spectral resolution R~650 ( ~8Å at H wavelength) NGC6946, U-band, CFHT, MegaPrime ( red circle denotes the coordinates of the X-ray source, according to Roberts, Colbert, 2003 ) 20 h 35 m 00 s ˚ 11' 30".9
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…
H / H [OIII] ( ) / H [SII] ( ) / H [SII]6717 / [SII]6731 AVAV n e, cm -3
HH [SII] [OIII]
SCORPIO higher-resolution data (R~1000): H [OIII] WWEE WWEE Component fluxes and radial velocities * - red, + - blue
MF16 NGC 6946 galaxy: D = 5,5 (from 5,1 to 5,9) Mpc Scd, active star formation
MF16 L H = erg/s, L emission lines / L H ~ 400 n > 240cm -3 (!) Pure shock+precursor model (Dopita&Sutherland 1995) V shock ~400km/s Shock power must be than: R=10pc 4 V shock, km/s B/n 1/2 = V shock, km/s B/n 1/2 =
The ULX (NGC6946 X-1, X-8, X-11 or ULX-1, ULX-3): MCD (kT=0.15 keV) + PL (Г = 2.5) L X = erg/s ( keV) Сhandra observations of NGC 6946 ULX-1 (Roberts, Colbert, 2003)
HeII HH [OIII] H + radio 20cm
HαHα HβHβ [SII] ( ) [OIII] ( ) HeII4686 [NII] ( ) Emission lines’ mapping: 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 1˝1˝