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.
Background
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 10
39erg 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, 10
2–10
5 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 arcsecond
2 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. 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. 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
(~10
5 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.
Acknowledgments
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.
¤
References:
- Begelman M. C., 2002, ApJ, 568, L97. [ First citation in text | ADS
]
- Cioffi D. F., McKee C. F., Bertschinger E., 1988, ApJ, 334,
252. [ First citation in text | ADS
]
- Colbert E. J. M., Mushotzky R. F., 1999, ApJ, 519, 89. [
First citation in text | ADS
]
- Dubner G. M., Holdaway M., Goss W. M., Mirabel I. F., 1998, AJ,
116, 1842. [ First citation in text
| ADS
]
- Goad M. R., Roberts T. P., Knigge C., Lira P., 2002, MNRAS,
335, L67. [ First citation in text
| ADS
]
- King A., Davies M. B., Ward M. J., Fabbiano G., Elvis M., 2001, ApJ,
552, L109. [ First citation in text
| ADS
]
- Makishima K. et al., 2000, ApJ, 535, 632. [ First citation in text | ADS
]
- Matonick D. M., Fesen R. A., 1997, ApJS, 112, 49. [
First citation in text | ADS
]
- Pakull M. W., Angebault L. P., 1986, Nature, 322, 511. [
First citation in text | ADS
]
- Pakull M. W., Mirioni L., 2003a, in Proceedings of the Symposium “New
Visions of the Universe in the XMM-Newton and Chandra era”, ed. F. Jansen,
ESA SP-488 [astro-ph/0202488]. [ First citation
in text | astro-ph
]
- Pakull M. W., Mirioni L., 2003b, RMxAC, 15, 197. [
First citation in text | ADS
]
- Roberts T. P., Goad M. R., Ward M. J., Warwick R. S., O’Brien P. T.,
Lira P., Hands A. D. P., 2001, MNRAS, 325, L7. [ First citation in text | ADS
]
- Roberts T. P., Goad M. R., Ward M. J., Warwick R. S., 2003, MNRAS,
342, 709. [ First citation in text
| ADS
]
Email contact: Tim
Roberts (
tro@star.le.ac.uk)
