---
abstract: 'We present a study of a peculiar nebula MF16 associated with an Ultraluminous X-ray Source NGC6946 ULX-1. We use integral-field and long-slit spectral data obtained with the 6-m telescope (Russia). The nebula was for a long time considered powered by strong shocks enhancing both high-excitation and low-excitation lines. However, kinematical properties point to rather moderate expansion rates ($V_S \sim 100\div 200$[$km\,s^{-1}\,$]{}). The total power of the emission-line source exceeds by one or two orders of magnitude the power observed expansion rate can provide, that points towards the existence of an additional source of excitation and ionization. Using CLOUDY96.01 photoionization code we derive the properties of the photoionizing source. Its total UV/EUV luminosity must be about $10^{40}$ erg/s.'
date: '??? and in revised form ???'
---

Introduction {#sec:intro}
============

Quite a large number of Ultraluminous X-ray Sources (ULXs) are associated with emission-line nebulae (ULX Nebulae, ULXNe), mostly large-scale bubbles powered by shock waves [@pamir (Pakull & Mirioni, 2003)]. However, several exceptions are known like the nebula associated with HoII X-1 [@lehmann (Lehmann , 2005)], that is clearly a photoionized [H [ii]{}]{} region. Another well-known example is the nebula MF16 coincident with the ULX NGC6946 ULX1.

The attention to MF16 was first drawn by [@BF_94], who identified the object as a Supernova Remnant (SNR), according to the emission-line spectrum with bright collisionally-excited lines. It was long considered an unusually luminous SNR, because of its huge optical emission-line ($L_{H\alpha} = 1.9\times10^{39}erg\,s^{-1}$, according to [@BF_94], for the tangential size $20\times34pc$) and X-ray ($L_X = 2.5\times10^{39}erg\,s^{-1}$ in the $0.5-8$keV range, according to the luminosities given by [@RoCo]).

However, it was shown by [@RoCo], that the spectral, spatial and timing properties of the X-ray source do not agree with the suggestion of a bright SNR, but rather suppose a point source with a typical “ULX-like” X-ray spectrum: cool Multicolor Disk (MCD) and a Power Law (PL) component. So, apart from the physical nature of the object, MF16 should be considered a [*ULX nebula*]{}, one of a new class of objects, described by [@pamir].

Optical Spectroscopy {#sec:obs}
====================

All the data were obtained on the SAO 6m telescope, Russia. Two spectrographs were used: panoramic MultiPupil Fiber Spectrograph MPFS [@MPFSdesc (Afanasiev , 2001)] and SCORPIO focal reducer [@scorpio (Afanasiev & Moiseev, 2005)] in long-slit mode. The details of data reduction processes and analysis technique will be presented in [@mf16_main]. Panoramic spectroscopy has the advantage of providing unbiased flux estimates. However, SCORPIO results have much higher signal-to-noise ratio and reveal rich emission-line spectrum of [\[Fe [iii]{}\]]{}. We also confirm the estimates of the total nebula emission-line luminosities by [@bfs]. $H\beta$ line luminosity obtained from our MPFS data is $L(H\beta) = (7.2\pm0.2)\times10^{37}erg\,s^{-1}$.

Using line ratios for the integral spectrum we estimate the mean parameters of emitting gas as: $n_e \simeq 500\pm 100 \,cm^{-3}$, $T_e \simeq (1.9\pm0.2) \times 10^4 K$. Interstellar absorption is estimated as $A_V \sim 1{^{\rm m}\!\!\!.\,}3$, close to the Galactic value ($A_V^{Gal} = 1{^{\rm m}\!\!\!.\,}14$, according to [@schlegel_abs])

We confirm the estimate of the expansion rate obtained by [@dunne], coming to the conclusion that the expansion velocity is $V_S \lesssim 200$[$km\,s^{-1}\,$]{}. In this case the total emission-line luminosity can be estimated using for example the equations be [@DoSutI]:

$$\begin{array}{l}
F_{H\beta} =   7.44 \times 10^{-6} \left(  \frac{V_s}{100 km\, s^{-1}} \right)^{2.41} 
\times \left( \frac{n_2}{cm^{-3}}\right)
+ \\
\qquad{} 
9.86 \times 10^{-6} \left(  \frac{V_s}{100 km \,s^{-1}} \right)^{2.28}
\times \left( \frac{n_1}{cm^{-3}}\right) \, erg\, cm^{-2} s^{-1}
\end{array}$$

Here $V_S$ is the shock velocity and $n_1$ the pre-shock hydrogen density. If the surface area is known, one can obtain the total luminosity in $H\beta$ from here. For $V_S = 200km/s$ and $n_1 = 1cm^{-3}$ it appears to be $L(H\beta) \simeq 1.6 \times 10^{36}$[ergs s$^{-1}$]{}, that is too low compared to the observed value. So we suggest an additional source of power providing most of the energy of the optical nebula.

Photoionization Modelling
=========================

We have computed a grid of CLOUDY96.01 [@cloudy98 (Ferland , 1998)] photoionization models in order to fit MF16 spectrum avoiding shock waves. We have fixed X-ray spectrum known from [*Chandra*]{} observations [@RoCo (Roberts & Colbert, 2003)], assuming all the plasma is situated at 10pc from the central point source, and introduced a blackbody source with the temperature changing from $10^3$ to $10^6$K and integral flux densities from 0.01 to 100 $erg\,cm^{-2}\,s^{-1}$.

The best fit parameters are $\lg T(K) = 5.15\pm 0.05, F = 0.6\pm 0.1 erg\,cm^{-2}\,s^{-1}$, that suggests quite a luminous ultraviolet source: $L_{UV} =
(7.5\pm0.5) \times 10^{39} erg\,s^{-1}$.The UV source is more than 100 times brighter then what can be predicted by extrapolating the thermal component of the best-fit model for X-ray data [@RoCo (Roberts & Colbert, 2003)].

Ultraluminous UV sources?
=========================

At least for one source we have indications that its X-ray spectrum extends in the EUV region. It is interesting to analyse the implications in the frameworks of two most popular hypotheses explaining the ULX phenomenon.

For the standard disk of [@ss73] the inner temperature scales as:

$$T_{in} \simeq 1~keV\,  \left(\frac{M}{M_\odot}\right)^{-1/4}  \left(\frac{\dot{M}}{\dot{M}_{cr}}\right)^{1/4}$$

In Fig. \[fig:seds\] we present the reconstructed Spectral Energy Distribution of NGC6946 ULX-1 including optical identification by [@bfs] and the best-fit blackbody from our model. For comparison, a set of MCD SEDs for IMBHs accreting at 1% of critical rate is shown. To explain the high EUV luminosity and roughly flat SED in the EUV region, a rather high IMBH mass is needed, $M \gtrsim 10^4 $[$M_\odot$]{}.

For supercritical disk this relation breaks [@poutanen (Poutanen , 2006)], and the outcoming radiation becomes much softer, except for the X-rays escaping along the disk axis [@superkarpov (Fabrika , 2007)]. Most part of the luminosity is supposed to be reprocessed into EUV and UV quanta, creating the nearly-flat SED of NGC6946 ULX1. In optical/UV range contribution of the donor star may become significant.

![NGC6946 ULX1 SED reconstruction. Optical source $d$ [@bfs (Blair , 2000)] is shown by an asterisk, and the upward arrow above indicates the unabsorbed optical luminosity: it is the lower estimate because only Galactic absorption was taken into account, $A_V =
1{^{\rm m}\!\!\!.\,}14$ according to [@schlegel_abs]. Dashed line represents the best-fit blackbody from our CLOUDY fitting. Thin solid lines are MCD models for accreting IMBHs with infinite outer disk radii. Mass accretion rate was set everywhere to $0.01 \dot{M}_{cr}$. []{data-label="fig:seds"}](abolmasov_fig1.eps){width="\textwidth"}

In [@mf16_main] we make estimates for the detectability of ULXs with GALEX, coming to the conclusion that at least some of them (the sources with lower Galactic absorption) may be bright enough targets even for low-resolution spectroscopy.

Conclusions
===========

We conclude that MF16 is most likely a dense shell illuminated from inside. This can be a certain stage of the evolution of a ULXN, when the central source is bright and the shell itself rather compact. We suggest that ULXs must be luminous EUV sources as well in some cases, and may be also luminous UV sources.

This work was supported by the RFBR grants NN 05-02-19710, 04-02-16349, 06-02-16855.

Abolmasov, P., Fabrika, S., Sholukhova, O. & Afanasiev, V. 2005 in[*Science Perspectives for 3D Spectroscopy* ]{}, ed. M. Kissler-Patig, M. M. Roth. & J. R. Walsh (Springer Berlin / Heidelberg)

Abolmasov, P., Fabrika, S., Sholukhova, O. 2007 [*in preparation*]{}

Afanasiev V.L., Dodonov S.N., Moiseev A.V., 2001, in [ *Stellar dynamics: from classic to modern*]{}, eds. Osipkov L.P., Nikiforov I.I., Saint Petersburg, 103

Afanasiev, V., Moiseev, A., 2005 Astronomy Letters, 31, 194

Begelman, M. C. 2002, [*ApJ*]{}, 568, L97

Blair, W. P., Fesen, R. A. 1994 [*ApJ*]{}, 424, L10371.8

Blair, W. P., Fesen, R. A., Schlegel, E. M. 2001 [*The Astronomical Journal*]{}, 121, 1497

Colbert, E. J. M., Miller, E. C. 2005 in [*The Tenth Marcel Grossmann Meeting*]{}. Eds.: Mário Novello, Santiago Perez Bergliaffa, Remo Ruffini. Singapore: World Scientific Publishing.Part A, p. 530

Dopita, M. A., Sutherland, R. S. 1996 [*ApJSS*]{}, 102, 161

Dunne, B. C., Gruendl, R. A., Chu, Y.-H. 2000, [*AJ*]{}, 119, 1172

van Dyk, S. D., Sramek, R. A., Weiler, K. W. 1994, apj, 425, 77

Fabian, A. C., Terlevich, R. 1996 [*MNRAS*]{}, 280, L5

Fabrika, S., Mescheryakov, A., 2001, In [*Galaxies and their Constituents at the Highest Angular Resolutions* ]{}, Proceedings of IAU Symposium N205, R. T. Schilizzi (Ed.), p. 268, astro-ph/0103070

Fabrika, S. [ *Supercritical disk and jets of SS433* ]{} 2004, *ARAA*, vol. 12

Fabrika, S., Abolmasov, P., Sholukhova, O. 2005, in [*Science Perspectives for 3D Spectroscopy* ]{}, eds. Kissler-Patig, M., Roth., M. M. & Walsh, J. R.

Fabrika, S., Karpov, S., Abolmasov, P. 2007 [*in preparation*]{}

Ferland, G. J. Korista, K.T. Verner, D.A. Ferguson, J.W. Kingdon, J.B. Verner, E.M. 1998, [*PASP*]{}, 110, 761

King, A. R., Davies, M. B., Ward, M. J., Fabbiano, G., Elvis, M. 2001, å, 552, 109

Lehmann, I., Becker, T., Fabrika, S., Roth, M., Miyaji, T., Afanasiev, V., Sholukhova, O., Sánchez, S., Greiner, J., Hasinger, G., Constantini, E., Surkov., A, Burenkov, A. 2005 å, 431, 847

Liu, J.-F., Bregman, N. 2005 [*ApJSS*]{}, 157, 59L

Matonick, D. M., Fesen, R. A., 1997 [*ApJSS*]{}, 112, 49

Osterbrock, D. E. “Astrophysics of Gaseous Nebulae” 1974, San Francisco, eds. W. H. Freeman and Company

Pakull, M. W., Mirioni, L. 2003 RevMexAA (Serie de Conferencias), 15, 197

Poutanen, J., Fabrika, S., Butkevich, A., Abolmasov, P. 2006 [*in press*]{}

Roberts, T. P., Colbert, E. J. M. 2003 [*MNRAS*]{}, 341, 49

Schlegel, D. J., Finkbeiner, P. F., Davis, M. 1998, [*ApJ*]{}, 500, 525

Shakura, N. I., Sunyaev, R. A. 1973, å, 24, 337

Swartz, A. D., Ghosh, K. K., Tennant, A. F., Wu, K., 2004 [*ApJSS*]{}, 154, 519
