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)

20p

c (~

1˝)

LLX X ≈≈ 2.5 10 2.5 103939 erg/s erg/s (0.5-8.0 keV)(0.5-8.0 keV)

L(HL(H) ) ≈≈ 2 10 2 103838 erg/s erg/s

L(optical lines, total) L(optical lines, total) ≈≈ 2 10 2 103939 erg/s erg/s

Also a bright radio sourceAlso 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.75 +60˚ 11' 30".9

Observations with the 6m SAO telescopeSpectrograph wavelength range,Å spectral seeing,

resolution arcsec

MPFS (panoramic) 4000-7000 ~1000 1,5

SCORPIO (long-slit) 4000-5700 ~2000 1,5

MPFS FOV

Long Slit

1˝

[OIII]5007

H

HeII4686H

[OIII]4959[OIII]5007

Two-component lines:

V2-V1 120-130 km/s

Broader component width ~ 200-300km/s

(from Dopita&Sutherland, 1996)

,

,

7.69

866 H precursorS

H shock

FkmV s F

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) 1037 erg/s

Narrow-to-broad components ratio 1,85±0,19kinematics

Additional source of excitation/ionization?

Single Hβ line

nH, cm-3

SHOCK WAVES OR PHOTOIONIZATION?

MF16

MF16

from Evans et al., 1999

Photoionization by power law continuum (labelled by ionization parameter logarithm lgU)

13.6

1

eV

FU d

cn h

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 =

L(HeII)=1.6 1037 erg/sHeII4686

Cannot be explained by a shock wave, and an EUV source needed with

L<228Å ≥ 1039erg/s

MF16

Photoionization by power law continuum (labelled by ionization parameter logarithm lgU)

Or X-ray source with L ~ 1041 erg/s or larger!

… and other high-excitation lines

Pure photoionization models (CLOUDY 96.01 modeling results):

Photoionization best-fit:

lgT(K) = 5.15±0.05

(T~120 000K)

F = 0.6 ± 0.1 erg/cm2/s

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

=> UV luminosity ~1040erg/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 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

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 ~ 2 1039 erg/s in optical emission lines only) emission line region 1×1.5arcsec (2034 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 > 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

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 5829s 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)

20h 35m 00s.75

+60˚ 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](5007+4959) / H

[SII](6717+6731)/ H

[SII]6717 / [SII]6731

AV

ne, cm-3

H

[SII] 6717+6731[OIII] 5007+4959

SCORPIO higher-resolution data (R~1000):

H [OIII]5007+4959

WEWE

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

LH = 6 1037 erg/s,

Lemission lines / LH ~ 400

n > 240cm-3 (!)

Pure shock+precursor model (Dopita&Sutherland 1995)

Vshock~400km/s

Shock power must be than:

3 2

383

1

2

101

pre shock shock

pre shock

L V R

nerg s

cm

R=10pc4

Vsh

ock, k

m/s

B/n1/2 =

Vshock , km/s

B/n1/2 =

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=2.5 1039 erg/s (0.5-8.0 keV)

Сhandra observations of NGC 6946 ULX-1 (Roberts, Colbert, 2003)

HeII

H

[OIII]

H+ radio 20cm

Hα

Hβ [SII](6717+6731)

[OIII](5007+4959)

HeII4686 [NII](6548+6583)

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˝