A 300-parsec-long jet-inflated bubble around a powerful microquasar in the galaxy NGC 7793

Journal name:
Nature
Volume:
466,
Pages:
209–212
Date published:
DOI:
doi:10.1038/nature09168
Received
Accepted

Black-hole accretion states near or above the Eddington luminosity (the point at which radiation force outwards overcomes gravity) are still poorly known because of the rarity of such sources. Ultraluminous X-ray sources1 are the most luminous class of black hole (LX1040ergs−1) located outside the nuclei of active galaxies. They are likely to be accreting at super-Eddington rates, if they are powered by black holes with masses less than 100 solar masses. They are often associated with shock-ionized nebulae2, 3, though with no evidence of collimated jets. Microquasars with steady jets are much less luminous. Here we report that the large nebula S26 (ref. 4) in the nearby galaxy NGC 7793 is powered by a black hole with a pair of collimated jets. It is similar to the famous Galactic source SS433 (ref. 5), but twice as large and a few times more powerful. We determine a mechanical power of around a few 1040ergs−1. The jets therefore seem 104 times more energetic than the X-ray emission from the core. S26 has the structure of a Fanaroff–Riley type II (FRII-type) active galaxy: X-ray and optical core, X-ray hot spots, radio lobes6 and an optical and X-ray cocoon. It is a microquasar where most of the jet power is dissipated in thermal particles in the lobes rather than relativistic electrons.

At a glance

Figures

  1. Optical/X-ray image of the 300-pc-diameter jet-inflated bubble S26 in the galaxy NGC 7793.
    Figure 1: Optical/X-ray image of the 300-pc-diameter jet-inflated bubble S26 in the galaxy NGC 7793.

    The contours denote the continuum-subtracted Hα emission, which is plotted over a true-colour Chandra/ACIS-S image (ObsId = 3954; principal investigator, T. G. Pannuti). The projected distance between the X-ray hot spots is ~15arcsec, which corresponds to ~290pc at the distance of NGC 7793. The Chandra exposure time was ~50ks. The X-ray colours are as follows: red, 0.3–1keV; green, 1–2keV; blue, 2–8keV. All three-colour images in the X-ray band were lightly smoothed with a 1-arcsec Gaussian core. The continuum-subtracted Hα image was taken with the CTIO 1.5-m telescope in 2001 (exposure time of 600s under seeing conditions with a full-width at half-maximum (FWHM) of 1.2arcsec) for the Spitzer Infrared Nearby Galaxy Survey25 and downloaded through the NASA/IPAC Extragalactic Database. We used matching point-like sources in the full field of NGC 7793 to improve the relative astrometry of the Hα and X-ray images. Note the relatively softer X-ray colour of the hot spots, compared with the blue point-like core; even softer, diffuse X-ray emission is detected over the whole nebula. The intrinsic X-ray luminosity of the southern hot spot is twice as high as the luminosity of the northern hot spot; this is similar to the higher Hα intensity in that region. There is no Hα point-like source or enhanced emission at the position of the X-ray core. The other source of Hα emission a few arc seconds to the east of S26 is an unrelated H ii region. Scaling from the total Hα luminosity of NGC 779325, and assuming a Hα/Hβ Balmer decrement of ~3.0, we determined an Hβ luminosity of ~1.0×1038ergs−1.

  2. Chandra/ACIS-S spectra of the triple X-ray source in S26.
    Figure 2: Chandra/ACIS-S spectra of the triple X-ray source in S26.

    Upper plot: emission from the two hot spots (added together) and from the central source of S26 are shown with red and blue data points, respectively. The black lines show the best-fitting models to the thermal lobe emission and power-law core emission. Lower plot: the ratio between the data and the models. The green line indicates a ratio of 1. The data were extracted from the Chandra archive and analysed with the standard software packages CIAO and XSPEC. We found that the central source has a hard spectrum (photon index, Γ = 1.4±0.5) and an intrinsic 0.3–10-keV luminosity of L0.3–107×1036ergs−1, which is much less than the Eddington limit of a neutron star or a stellar-mass black hole. The emission from the hot spots is significantly softer and is well fitted by a two-component thermal plasma model with kBT1 = keV and kBT2 = keV (contributing roughly the same as the total flux), and no significant absorption above the Galactic line-of-sight column density, NH = 1.2×1020cm-2. We derive intrinsic luminosities of L0.3–101.2×1037ergs−1 and L0.3–100.6×1037ergs−1 for the southern and northern hot spots, respectively. The particle density, nX, of the X-ray-emitting, shocked thermal plasma (assumed to have solar abundance) can be estimated as nX2θ-3/2cm-3, where θ is the diameter of the emitting hot spots in units of arc seconds. A characteristic hot spot size of ~1arcsec is suggested by the marginally resolved Chandra image. This gives a mass for the X-ray-emitting gas of ~500θ-3/2 solar masses, more than can be supplied by a donor star orbiting the black hole. It probably represents a mix between the very hot but dilute jet gas that has passed through the reverse (Mach) shock and the much denser material behind the foreward (bow) shock, which is advancing into the interstellar medium (ISM). Alternatively, a very steep power-law spectrum with Γ = 5.7±1.4 and NH = (4±2)×1021cm-2 cannot presently be excluded, but seems to contradict both the low observed optical reddening4 and the expected slope for a synchrotron component extending from the radio to the X-ray bands. Error bars, 1σ.

  3. The stellar and high-excitation nebular content of S26.
    Figure 3: The stellar and high-excitation nebular content of S26.

    a, Optical-continuum greyscale image of S26, taken with the FORS1 instrument on the ESO Very Large Telescope (VLT) in 2002 and downloaded from the public archive. The narrowband filter used for this image was centred at 5,105Å, with a FWHM of 61Å (exposure time of 1,600s under seeing conditions with a FWHM of 1.0–1.5arcsec). The size and orientation of the image are as in Fig. 1. The positions of the X-ray core and hot spots have been overplotted as red circles of radius 0.7arcsec. Note the relative brightness of the optical counterpart to the X-ray core; we estimate B23mag, corresponding to MB-5mag. b, Continuum-subtracted greyscale FORS1 image in the He ii 4,686Å emission (narrowband filter centred at 4,684Å, with a FWHM of 65Å; exposure time and seeing conditions were the same as for the image in a). The image was smoothed with a 0.6-arcsec Gaussian core, to highlight the extended nebular line emission. The observed flux ratio between He ii 4,686Å emission and the H ii Balmer lines suggests shock ionization with vshock275kms−1. Note also the point-like He ii 4,686Å emission from the core, for which we estimate an equivalent width of ~30Å. The green lines show the position and width of the slit used to acquire the spectrum shown in Fig. 4.

  4. Spectrum of Doppler-broadened emission lines of S26 indicating an expansion velocity of 250[thinsp]km[thinsp]s-1.
    Figure 4: Spectrum of Doppler-broadened emission lines of S26 indicating an expansion velocity of 250kms−1.

    Part of a medium-resolution (FWHM, ~0.7Å) long-slit ESO VLT FORS2 spectrum (taken in October 2009) covering the wavelength range between 3,600 and 7,200Å, with the slit running across the eastern body of S26 as shown in Fig. 3. The exposure times for the red and blue settings were 2,200s under good seeing conditions (FWHM, 0.7–0.8arcsec). The reduced spectra were analysed using ESO-MIDAS routines. The dispersion is along the horizontal axis with increasing wavelengths towards the right. In the vertical (spatial) direction, one pixel corresponds to ~0.2arcsec; the total extent of S26 is ~10arcsec along the slit. Narrow, constant-intensity, vertical emission lines are due to spectroscopically unresolved atmospheric O and OH night glow; the most prominent emission lines from S26 are, from left to right, [N ii] 6,548Å, Hα and [N ii] 6,584Å. The ‘bulged’ appearance, mostly visible in the intense Hα emission, reflects the remarkable kinematics of the emitting material in S26. The small extent in wavelength at the lower and upper nebular boundaries reflects the small velocities of the emitting gas along our line of sight (that is, mostly perpendicular motion). The central part of the line covers the whole range of radial velocities across the nebula, which we assign to expansive motion from zero up to 250kms−1 around a central velocity that is close to the apparent radial velocity in that part of NGC 7793. The density of the ISM (n0.7cm-3; see text) is high enough that the cooling time behind the shock, ~200v1004.4/Znyr (ref. 26), where v100 = vshock/(100kms−1) and Z is the metallicity relative to the solar value, is smaller than the age of the bubble. Therefore, the cooling/recombination zone behind the shock is largely complete, that is, the shock is largely radiative27, in agreement with the presence of relatively strong low-ionisation or neutral species. One such diagnostic species is [O i], which emits the 6,300Å line4 with Iλ6,300/I0.63.

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Affiliations

  1. University of Strasbourg, CNRS UMR 7550, Observatoire Astronomique, 11 rue de l’Université, F67000 Strasbourg, France

    • Manfred W. Pakull &
    • Christian Motch
  2. MSSL, University College London, Holmbury St Mary, Surrey RH5 6NT, UK

    • Roberto Soria

Contributions

M.W.P. designed the study and analysed the optical spectroscopic observations. R.S. contributed the X-ray spectral analysis and the relative astrometric calibrations of the multiband datasets. C.M. carried out the reductions and analysis of the optical imaging data. All authors discussed the results and made substantial contributions to the manuscript.

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