The Unusual Supernova Remnant Surrounding the Ultraluminous X-Ray Source IC 342 X-1
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ING Newsletter No. 7, December 2003

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The Unusual Supernova Remnant Surrounding the Ultraluminous X-Ray Source IC 342 X-1

T. P. Roberts1, M. R. Goad2, M. J. Ward1, R. S. Warwick1

1: University of Leicester. 2: University of Southampton.

WHT/INTEGRAL observations have shown a large-diameter (110 pc) supernova remnant to encircle the position of the Ultraluminous X-ray Source (ULX) IC 342 X-1 (Roberts et al., 2003). We infer a remarkable initial energy input to the SNR, at least 2–3 times greater than the canonical value for an ‘ordinary’ SNR of 1051 erg. In addition, two regions on the inside of the SNR shell are bright in [OIII] λ5007 emission, possibly as the result of photoionization by the ULX. If this is the case, the morphology of the nebulosity implies that the X-ray emission of the ULX is anisotropic. The presence of the ULX, likely to be a black hole X-ray binary, within an unusually energetic SNR suggests that we may be observing the aftermath of a gamma-ray burst.


Ultraluminous X-ray sources are the most luminous point-like extra-nuclear X-ray sources located coincident with nearby galaxies, displaying X-ray luminosities in excess of 1039erg s–1. Whilst some ULXs are known to be associated with recent supernovae, the majority appear to show the characteristics of accreting black holes (Makishima et al., 2000). However, at their observed X-ray luminosities they match, or in many cases greatly exceed, the Eddington limit for accretion onto a stellar-mass (~10M ) black hole. ULXs may therefore provide observational evidence for accretion onto a new, 102–105 M   intermediate- mass class of black hole (e.g. Colbert & Mushotzky, 1999). Alternatively, they could constitute the extreme end of the accreting stellar-mass black hole population, with their high apparent luminosities a result of either truly super-Eddington X-ray emission (Begelman, 2002), or an anisotropic radiation pattern (e.g. King et al., 2001).  

One method of investigating the nature of ULXs is through detailed multi-wavelength follow-up observations. We have undertaken one such programme using the integral field unit INTEGRAL on the William Herschel Telescope to obtain optical spectro-imaging data, through 189 fibres over a 16.5×12.3 arcsecond2 field-of-view, of the immediate environment of fourteen nearby ULXs. A crucial element of this programme is that we use sub-arcsecond X-ray astrometric data from NASA’s Chandra X-ray observatory to locate the ULXs, which dramatically reduces the confusion problems inherent to older, less accurate X-ray positions. This programme has already borne fruit with the detection of the first stellar optical counterpart to an ULX; a young stellar cluster coincident with NGC 5204 X-1 (Roberts et al., 2001; Goad et al., 2002), which suggests that this ULX may be an extremely luminous high-mass X-ray binary. Here, we outline the results of an observation of the environment of a second ULX, IC 342 X-1, which reveals a very different optical counterpart.

The IC 342 X-1 Nebula

The first observation, on February 1st 2001, highlighted a shell-like emission-line nebula in the immediate environment of IC 342 X-1 (Figure 1; this nebula, and its SNR-like line ratios, was also detected by Pakull & Mirioni (2003a), who christen it the “tooth” nebula due to its distinctive morphology). A high [SII]/Ha emission-line ratio of ~1.1 is seen over the extent of the nebula, a classic indicator that the nebula is a supernova remnant (SNR). Our new Chandra position clearly locates the ULX in the central regions of the nebula, raising the intriguing possibility that the two may be physically related.

Figure 1
Figure 1. Narrow-band INTEGRAL images of the environment of IC342 X-1 in the 5300–5500Å continuum band (left), continuum-subtracted Ha+[NII] (centre), and continuum-subtracted [SII] (right). The circle represents the uncertainty in the ULX position relative to the INTEGRAL data, and each panel is 16.5×12.3 arcsecond2 in size. [ JPEG | TIFF ]

By utilising both the imaging and spectroscopic measurements provided by INTEGRAL, and assuming the SNR is in the pressure-driven snowplough phase (c.f. Cioffi, McKee & Bertschinger, 1988), we were able to place the constraints on the SNR properties shown in Table 1. The SNR appears unusually large, with a projected diameter of at least 110 pc. For comparison, Matonick & Fesen (1997) argue that a typical single SNR with an initial energy E51=1 should not remain visible once it has expanded beyond a diameter of 100pc. The unusual size of this SNR can be attributed to an extraordinary initial energy (assuming a single explosion) of at least 2×E51.

Radius (for d = 3.9 Mpc): Rneb = 55pc
Shell velocity: Vs < 180 km s-1
Age: tneb > 92,000 yr
Initial energy (×1051 erg): E51 > 2
Ambient ISM density: n0 > 0.12 cm-3
Electron density: Ne < 40 cm-3
Electron temperature: Te < 3×104 K
Table 1. The properties of the SNR.
[OIII] Nebulosity

A second remarkable feature of this nebula is demonstrated in Figure 2. The morphology of its [OIII] emission is distinctly different to the other emission-lines, appearing to sit in two patches on the inside of the larger nebula. Importantly, the [OIII] recombination time for the inner edge of the nebula is far less than its age, implying that a process other than the supernova blast wave must have energised the [OIII] emission. One possibility is that the excitation originates in the high-energy emission of the ULX. Unfortunately our observation is not sensitive to the HeII λ4686 line, the classic signature of an X-ray Ionised Nebula (XIN; Pakull & Angebault, 1986). However, calculations show that the ULX can produce a photoionizing flux sufficient to excite at least the inner regions of the SNR shell. If the excitation is due to the ULX, then its morphology strongly suggests that the X-ray emission of the ULX is anisotropic, consistent with the beamed X-ray binary models of King et al. (2001).

Figure 2
Figure 2. The unusual ionization structure of the IC342 X-1 SNR. The three colours show 5300–5500Å continuum emission (green), continuum-subtracted Ha +[NII] emission (red), and continuum-subtracted [OIII] (blue, highlighted by contours). The uncertainty in the position of IC342 X-1 is again shown by the circle. [ JPEG | TIFF ]

A Hypernova Remnant?

The location of a probable black hole X-ray binary within an unusually energetic supernova remnant appears to satisfy the conditions for a hypernova remnant, i.e., the aftermath of a gamma-ray burst in which a massive star has collapsed to a black hole triggering a very energetic supernova explosion. If so, this observation provides direct evidence that gamma-ray bursts do occur when black holes are formed.  

However, there are other possible origins for the nebula. It might be the result of multiple supernovae occurring in a relatively short space of time (~105 yr). This would require a population of young stars within the nebula, which future deep optical continuum observations would detect if present. A second alternative origin could be in jets originating in the ULX. A Galactic analogue of such a system is the W50 nebula, thought to be inflated by the relativistic jets of the microquasar SS433 (Dubner et al., 1998), which has similar energetic requirements to the IC342 X-1 nebula. Finally, it is possible that the entire nebula could be X-ray ionised. Pakull & Mirioni (2003b) suggest that XIN should contain an extended warm low-ionisation region with strong characteristic lines such as [SII], which would mimic a SNR spectrum.


We thank the WHT/INTEGRAL team for their assistance in the planning and implementation of our programme, and for the use of their data reduction routines. TPR is grateful to PPARC for financial support in the form of a PDRA at the University of Leicester.¤


Email contact: Tim Roberts (

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