The amazing supernova SNF 20070406-008 (=SN 2007bi)



Highlights:

Updated presentation from the "fireworks 2008" meeting
Nature paper


Context: Supernova explosions; Pair-production SNe

Supernovae involve the explosive destruction of a star. Two distinct physical mechanisms leading to such events are broadly discussed, and are thought to have been identified in nature.

SN 1987A The first involves the gravitational core-collapse of a massive star (M>8 solar masses), following the exhaustion of the nuclear fuel in its core, occurring when silicon is burned to iron, the most stable nucleus. The collapse leads to an implosion which turns into an explosion after iron photodisintegration and neutronization processes occur at nuclear densities, and a rigid proto-neutron star is formed. Such explosions leave behind remnants, which may be neutron stars or black holes. The basic core-collapse scenario is supported by observations of the nearby SN 1987A, including the detection of the neutrino burst signalling to formation of the proto-neutron star.

SN 1987A The second process involves the thermonuclear explosion of a degenerate white dwarf star, accreting material (or merging) with a close companion. Once the temperature in the center of the dwarf rises above a critical threshold, runaway carbon burning begins and the star explodes completely, leaving no remnant. This mechanism is generally assumed to be responsible for the class of events called type Ia SNe.

SN 1987A A third type of explosion has been theoretically predicted many years ago, but is yet to be convincingly observed. The most massive stars, with initial masses above ~100 solar masses or so, develop very heavy cores composed of helium and heavier elements (>40 solar masses, see, e.g., here for a recent short review). Theoretical calculations predict that such heavy cores will undergo a dynamical instability due to e+/e- pair production. This will lead to rapid contraction followed by explosive oxygen burning. In the lower-mass range (cores lighter than 60 solar masses or so), this process can lead to a pulsational instability that will produce several mass ejection episodes that will remove the core from the instability region, eventually forming an iron core and exploding as a core-collapse SN as above. In more massive cores, the instability leads to runaway oxygen burning that totally disrupts the star in a very energetic explosion that leaves no remnant. Identifying explosions of this kind in nature would be a very valuable discovery. Below, we present our discovery of SNF20070406-008 (=SN 2007bi) which appears to provide a clean example of such an explosion.


What will a pair-production supernova look like?

As mentioned above, the "smoking gun" evidence for a pair supernova would be a measurement of a large core (>50 solar masses or so) composed of heavy elements (helium and above). However, this is observationally challenging. Since some models of pair SNe predict a large mass of radioactive Ni 56 is synthesized, observational evidence for very Ni-rich SNe have been sought. It should be noted that some pair SN models predict little or no Ni 56. Still, let us begin by considering Ni-56-rich events.

SN explosions rich in Ni 56 will have the following observational properties: (1) High peak optical luminosity (there is a rather tight Ni 56 - peak optical magnitude correlation, see, e.g., here). (2) Slow decline rate, that should follow the decay rate (0.0098 mag/day) of radioactive Co 56, the product of Ni 56 decay, after several weeks. (3) During the nebular phase, when the ejecta are optically thin, one can measure the Ni 56 content from the strength of nebular smission lines, since these are powered by radioactivity. By the way, nebular spectral modelling can actually constrain the ejected mass in all elements and thus the core mass, making these observational very valuable to constrain pair SNe, whether they make a lot of Ni 56 or not. Still, these observations are difficult (requiring a decent spectrum of the SN at age ~1 year or so). (4) If the supernova went through the pulsational phase, the outer hydrogen envelope may well have been stripped (leaving a Ib/Ic SN). This could also happen due to other mass loss mechanisms in heavier cores. (5) We expect a low-metallicity environment (such as a very small dwarf galaxy), since high metallicity would cause rapid mass loss in line-driven winds, and is assumed to rapidly reduce the mass of event the most massive stars below the pair production regime. Still, since everyone agrees we know little about mass loss, this should be taken as a weak prediction.

With the discovery of very bright SNe (e.g., SN 2006gy and SN 2005ap, the interest in Ni-rich SNe increased. Such events certainly match (1) above (high peak magnitude), and some (e.g., SN 2006gy) decay rather slowly as in (2) above. However, other explanation are possible, for example, a failed GRB engine can produce a bright, but rapidly decaying event like SN 2005ap, while interaction with circumstellar material can account for events which are both bright and and slowly decaying like SN 2006gy (see, e.g., here and here). Obtaining a nebular spectrum turns out to be a key observation needed to robustly establish a pair production explosion as the source of a very luminous SN.


Discovery and study of SNF20070406-008 (SN 2007bi)

On April 6, 2007, the SN factory team discovered a new bright point source.

SN 1987A

This turned out to be a type Ic SN in a very faint host galaxy, detected in the SDSS.

SN 1987A
The only previous analog that could be identified was SN 1999as, the most luminous SN Ic known till then.

SN 1987A

We have then triggered a follow-up campaign as part of the PTF "dry run" campaign. Photometry was obtained using the Palomar 60" and 200" telescopes, while archival data was retrieved from the SN factory and Catalina all-sky surveys. The resulting light curve look like this:
SN 1987A

Note, that SNF 20070406-008 fits well with expectations from pair SNe listed above, namely, it has a very luminous peak (as bright as the most luminous SN Ic known, though still about 2 magnitudes less than the most luminous SNe known) and a slow decay, consistent with the expected rate due to Co 56 decay. So, a good candidate for a pair SN. However, to get the smoking gun evidence, we had to wait 16 months to get a nebular spectrum at Keck.


The nebular spectrum

On August 3, 2008, we observed SNF20070406-008 with the LRIS spectrograph mounted on the 10m Keck telescope. The SN was 480 days old at this time, and its spectrum has finally turned nebular. Reduction of the spectrum, followed by careful host galaxy contamination removal, showed very prominent emission lines, fitting our expectations from a very Ni-rich event. Comparing to one of the most Ni-rich events known previously, SN 1998bw, we see that the event produced ~8 times more Ni 56 than SN 1998bw, which leads to an estimate of ~4 solar masses of Ni, by far the most Ni 56-rich event yet to be studied in this manner. Strong emission lines of iron further support a very large initial mass of Ni 56 (decaying into Co 56, then iron).

SN 1987A



Modelling and summary

With the data in hand, we now turn to quantitative comparison with models of pair SNe. First, we analyzed the nebular spectrum using the models of Mazzali and collaborators:

SN 1987A

Considering the signal to noise of the spectrum adecent fit is obtained, from which we can derive the mass and composition of the ejecta, and compare them to specific models from the literature from Heger and Woosley (2002):

SN 1987A

The two main points to note are: (1) The Ni mass we measure (3.7 solar masses), implies a core mass of 95-100 solar masses (i.e., an initial mass of ~200 solar masses). (2) Our measured total core mass (46 solar masses) requires a pair explosion in any case. To fit the Ni 56 mass, we need to further assume that the core was further stripped of most of its outer C/O/Ne/Mg prior to exploding.

We have also calculated the light curves expected from these models, (Kasen et al.), and find that the data are well fit by an explosion of a 100 solar mass core:

SN 1987A

In summary, during the PTF "dry run", we have discovered SNF20070406-008, a supernova explosion that provides a very good fit to quantitative predictions of theoretical models of pair SNe, to the degree that this interpretation seems hard to avoid. Numerous additional such discoveries are expected during the PTF itself and similar contemporary wide-field surveys.



Collaborators:

A. Gal-Yam (Weizmann Institute)
S. Kulkarni and the PTF team at Caltech
P. Nugent and the SN facotry team at LBL
J. Bloom, A. Filippenko and members of the UCB SN team
P. Mazzali (INAF/MPA) and members of the ESO team
D. Kasen (UCSC)
M. Sullivan (Oxford)
A. Drake and members of the CSS team (Caltech)
D. Young (QUB)



Constructed: December 2008, by: Avishay Gal-Yam , E-Mail: email