Where in the universe is SCP 06F6?

A tale written in carbon, calcium, magnesium and nickel.

Figure 1, Barbary et al. 2008. Hubble Space Telescope images from as late as May 2006 still showed emission from the source SCP 06F6, first detected that February. Its light curve was unprecedentedly long, it showed no discernible host galaxy, and even the distance to the source was completely unknown. In a sky full of strange things, SCP 06F6 might be one of the strangest. Image credit: NASA/ESA/Hubble Space Telescope. Public domain.

Astronomers are used to seeing something in the sky and not knowing with any certainty what they’re looking at. It can take decades to figure out just what that point of light is — decades spent improving models, making follow-up observations, and redoing data analysis. Most of the time, though, at the very least astronomers are able to figure out where the thing is in space and how far away it is. If you know an object’s distance and its flux, you know its luminosity — and that can be extremely important.

But what if you don’t know how far away something is? Typically, distance measurements on cosmological scales are done by looking at a source’s spectra, which are redshifted according to Hubble’s law. You can imagine a situation where your data has no recognizable spectral features — no self-consistent absorption complexes, recombination lines, molecular transitions, nothing. That would be extraordinarily annoying for astronomers.

Figure 3, Kirshner 2004. A plot of Type Ia supernova measurements illustrates Hubble’s law: Distant objects recede from us at velocities proportional to how far away they are. Knowing a source’s recessional velocity therefore can tell us its distance.

Over a decade ago, scientists had to deal with exactly that scenario, and that’s what this post is about — an event known as SCP 06F6. First seen in February 2006, it initially appeared to be a supernova, but it displayed a puzzling, extended light curve that lasted for three months. Spectroscopy from multiple instruments produced no conclusive redshift measurement, and to this date four separate figures have been proposed. Astronomers knew virtually nothing about what exactly it was. We still don’t have a clear picture of the mechanism behind the transient, but let’s at least try to figure out one thing about it: how far away it is.

That should be easy, right? Well, you’d think so. And you would be completely wrong.

No galaxy, no redshift, no luck

SCP 06F6 was discovered (Barbary et al. 2008) during the Hubble Space Telescope Cluster Supernova Survey, which aims to find high-redshift supernovae by scanning massive galaxy clusters (see Dawson et al. 2009 for more information). In early 2006, an optical transient appeared near the cluster CL 1432.5+3332.8, which lies at a redshift z=1.112. No host galaxy was definitely observed; if the source lay at the same redshift as the cluster, it would have to be 290 kpc away from the cluster center.

Figure 2, Barbary et al. 2008. The transient wasn’t so transient, taking about 100 days to reach its peak luminosity and about the same amount of time to fade away. An ordinary supernova should fade from view after about a month or two — indicating that SCP 06F6 was anything but ordinary.

What was surprising about the transient was its longevity. It took approximately three months to reach its maximum brightness and roughly the same amount of time to fade away. The source was thought to be a supernova of some sort, but the timescales involved were several times too long — it should take weeks for a supernova to reach its peak, not months. Nonetheless, the Subaru Telescope on Mauna Kea was still able to image the transient well into June of 2006.

The other peculiar thing about the object was its spectrum. The group collected optical and infrared spectra with Subaru, the Very Large Telescope, and the Keck Observatory, focusing on the 4000–10000 Å range. Although they were able to pick out several major absorption and emission features, the spectra was different than any known supernova spectrum. This is, of course, an enormous problem, given how much spectroscopy can tell us about an object — composition, distance, temperature, and more.

Barbary et al. considered two possibilities. The first was that the transient came from within the Milky Way. Under this interpretation, the group was able to identify Hβ and Hγ absorption. However, there were no signs of the corresponding Hα features you would expect to see, and important features at 5360 Å and 6330 Å were left unexplained.

Figure 3, Barbary et al. 2008. Spectra from the Subaru Telescope, the Keck Observatory, and the Very Large Telescope did an excellent job of being extraordinarily confusing. Pay attention to the absorption feature at 5800 Å, shown in detail at bottom right. It will be a key player on our story. The mystery lies throughout the region from 4000 Å to 6400 Å — if you could figure out what’s behind those line complexes, you’d solve a big piece of the puzzle.

What if — as you might have guessed — the signal was extragalactic? An absorption complex at 5800 Å seemed to closely match a known magnesium doublet. Assuming the cluster redshift, (z=1.112), the corresponding luminosity of the event would be similar to some of the brightest known supernovae, meaning the extragalactic interpretation was not out of the question. The group did place a hard upper limit of z=2.7, based on a lack of Lyman absorption upwards of 4500 Å. Normally, the longest wavelength in the Lyman series is 1216 Å, so the transient lay at redshifts higher than z=2.7, there should be Lyman absorption upwards of 4500 Å — but none was observed.

Is carbon the culprit?

An ultraviolet image of the carbon star CW Leonis, taken by the Galaxy Evolution Explorer (GALEX). CW Leonis is at the center of the blue circle in the middle of the picture, a bow shock formed as it travels through the interstellar medium. SCP 06F6 might be a supernova with a progenitor similar to a much more massive version of CW Leonis, which once weighed 3–5 times the mass of the Sun but has ejected so much mass that it now weighs in at 0.7–0.9 solar masses. Image credit: NASA/GALEX, public domain.

A couple years later, another group (Gänsicke et al. 2009) noticed that the spectrum of SCP 06F6 looked a lot like the spectrum of a high-redshift carbon star. Carbon stars are red giants and supergiants with unusually high carbon abundances in their atmospheres. In particular, Gänsicke et al. thought that several of the absorption features — including the strong one near 5800 Å — strongly resembled Swan bands.

Swan bands are characteristic signals of a carbon star. They arise from particular vibrational bands of diatomic carbon, meaning that the energy of the absorbed photon goes into the molecule’s vibrational modes — rather than, as we might be used to, changes in the energy state of an electron. If the 5800 Å complex was indeed a Swan band, it meant that the transient lay at a more modest redshift: z=0.143. This translates to about 600 Mpc, placing SCP 06F6 much closer than the galaxy cluster being targeted by the original Hubble survey.

Figure 1, Gänsicke et al. 2009. The spectra of SCP 06F6 (black) and the carbon star SDSS J001836.23–110138.5 (grey) look nothing alike (top) — until you account for redshift (bottom), at which point they look quite similar at wavelengths longer than 5000 Å. At the top of the second plot is the carbon star spectrum (light grey line) with some broadening and smoothing added to simulate an outflow expanding at 4000 km/s.

Equipped with x-ray observations from Chandra and XMM-Newton, the authors proposed that the culprit could be either a supernova with a carbon star progenitor or the tidal disruption of a carbon star by a black hole. Each would cause slight changes to the spectrum because of Doppler broadening of the spectral lines due to the expanding outer layers of the star. Gänsicke et al. did emphasize that the main outstanding problem was the lack of a host galaxy; while Barbary et al. had located a potential faint object near the transient, its redshift could not be determined.

Calcium enters the mix

Figure 1, Chatzopoulos et al. 2009. An annotated spectrum of SCP 06F6 taken from Subaru/VLT data. There are some tantalizing fits to calcium-II H&K lines, assuming z=0.57, but there are still differences between the transient and the Type II supernova model used, as well as the Type Ia supernova example at top (SN 1992A).

That same year, a group of astronomers at the University of Texas at Austin (Chatzopoulos et al. 2009) proposed a different redshift: z=0.57. Such a redshift would mean that the broad 5800 Å feature could correspond to the calcium-II doublet known as the H and K Fraunhofer lines. This redshift also provided a good fit with continuum emission at wavelengths above 4000 Å, but notably diverged from Type II supernova template spectra near 5300 Å. The team was careful to not read too much into the similarities, saying

“The comparison in Figure 1 does not suggest that SCP06F6 is either a Type Ia or a Type II, but that SCP06F6 might have Ca II and iron-peak absorption in the rest-frame UV moving with velocities typical of supernovae.”

Assuming that the transient was indeed a supernova-like event, various spectral models should be able to reconstruct the explosion’s properties. A number of different figures can be determined, among them the mass of the ejecta and the ejecta composition. The mass of nickel in the ejecta is an important quantity, as radioactive decays from nickel and related elements (primarily cobalt-56) lead to emission in the weeks after the explosion.

Figure 3, Chatzopoulos et al. 2009. Upper bounds on the transient’s redshift can be determined from fitting ejecta models to the data, but the precise limits depend on the opacity of the system. z=1.1 appears to be a safe bet for a Type II core collapse supernova.

When the group plugged redshifts above z=1.1 into their model, they found nickel masses greater than the total ejecta mass, giving a new claimed upper limit to the redshift of SCP 06F6. Redshifts above z=1.1 also appeared to predict that more energy would be radiated than the ejecta’s kinetic energy — another impossible result. On the other hand, the z=0.57 model seemed to lead to realistic luminosity estimates.

Four of a kind

After Chatzopoulos et al. proposed that calcium could be powering the mysterious absorption feature, yet another group weighed in, with another proposed redshift — and, this time, lots of new data. Quimby et al. 2011 had observations from the Palomar Transient Factory (PTF) for three new supernova candidates, located at redshifts of z=0.501, z=0.258, and z=0.349. What was exciting was that the spectra for those three transients featured the same magnesium-II doublet which Barbary et al. 2008 had identified with the 5800 Å feature! If the complex seen in SCP 06F6 was indeed due to magnesium, Quimby et al.’s models would place the transient at z=1.189 — close to the redshifts originally proposed, but still beyond the targeted galaxy cluster.

Figure 2, Quimby et al. 2011. Spectra of SCP 06F6, SN 2005ap, and several similar PTF transients, with corrections made for cosmological redshift. Are these all representatives of a new class of supernova, the pair-instability supernova? Perhaps.

The group drew comparisons between the PTF observations, SCP 06F6, and a highly luminous Type Ic supernova called SN 2005ap. All five showed similar spectra and very low upper limits on the brightness of any putative host galaxies. They appear to be potential pair-instability supernovae, where an increase in electron-positron pair production in an extremely massive star’s core leads to a drop in radiation pressure, instability, and eventually the collapse of the entire star.

I’m inclined to be a little more cautious than Quimby et al.; given the sheer number of possible models, it may take a while for them to all be ruled out — even now, ten years after the last paper I’ve discussed. The three PTF events do make an excellent case for the magnesium-II interpretation, and the pair-instability model seems to do a good job of explaining the peculiar light curve. But there are even more proposed explanations than I’ve been able to cover here (see Soker et al. 2010 for a few additional ideas). More and more exotic supernovae are discovered every year, and while SCP 06F6 may have been one of the first of its kind seen on Earth, that number is still growing.