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Astronomy 100


        Answers to Frequently Asked Questions

The following is a quick response to some of the questions I have been asked:
How much energy does a hypernova explosion release?

A hypernova explosion typically has a mechanical energy output of ~ 10^53 ergs, or about a factor of 100 greater than a supernova. If a hypernova releases much of the energy in a few to a few thousands of seconds, a timescale of  a typical Gamma-ray burst, the energy output rate is then about 10^16-10^19 orders of magnitude greater than that of the Sun.

What is the energy source of a hypernova explosion?

No one is really sure about the explosion mechanism yet. But most likely the explosion is due to the collapse of a massive star into a black hole. Thus the source is likely due to the release of the star's binding energy in the form of gravitational potential, rotation, and/or magnetic field.
How did you recognize the remnants?

Actually we were not searching for hypernova remnants. We were just looking for optical or radio counterparts of X-ray sources discovered in our deep X-ray imaging of the galaxy M101. Some of these sources are easily identified as foreground stars in our Galaxy; others are background
quasar-like objects. Only about half of the sources seen in the image are associated with M101. Many of them are believed to be X-ray binaries, as known in our Galaxy. However, two sources are found to coincide positionally
with two shell-like optical nebulae. Both have distinct optical and/or radio spectral signatures of supernova remnants, and are among the most unusual supernova remnant candidates known.

    While the probability for a chance coincidence is small, the most reasonable explanation is that the X-ray sources are associated with the remnants. Such an association, however, means that the energy involved must
be huge. The thermal energy inferred from the X-ray emission alone significantly exceeds the canonical mechanical energy release of a supernova. A detailed calculation shows the explosion energy of NGC5471B is, at least, an order of magnitude greater than the supernova
energy. For MF83, the required energy is a factor of 10 greater. We can also rule out the possibility that the remnants are produced by multiple supernovae. Constrained by the size and age of the nebulae, such concerted
explosions could only occur in young and massive star clusters. We find no evidence for such clusters within the nebulae. Therefore, the remnants most likely due to hypernova explosions.
When did the hypernovae explore?

The age of the hypernova remnant NGC5471B is about 30 thousand years, while the age of MF83 is about 1 million years. However, since the galaxy M101 is 25 million light year away, the explosions actually occurred much earlier. But compared with GRBs, which are even farther away,
the hypernovae that are responsible for the remnants were relatively recent events.
What is the beaming effect of a GRB and why is it important?

The beaming effect is a measure of how focused the energy release from
a GRB is. If an explosion confines its energy release into one direction,
as a flashlight does, the total energy output may then be substantially
smaller than that inferred from an isotropic release (radiating
equally in all directions). The beaming effect is the single most
important source of uncertainties in inferring the total energy release
of a GRB from the radiation flux observed by astronomers.
What impact does a hypernova explosion, or a GRB, have on its environment?

The blast wave from a hypernova explosion can sweep up its surrounding
interstellar gas to form a dense shell, that may enclose a tenuous, very hot
(million degree) plasma. Such a remnant can last tens of million years. But
similar remnants may also be produced by other mechanisms, such as multiple
supernova explosions. Thus it is normally difficult to be sure about the true
origin of such remnants, except for relatively young ones such as the two
discovered in M101.
What impact may a hypernova have on us?

If such an explosion occurs near the earth, it could be really
devastating. But fortunately, the chance for this to happen is very small.
The estimated rate of hypernovae in the entire Milky Way is less than
about 1 per million years.
Could a hypernova be seen by naked eyes?

If it occurs in our galaxy, it may well be seen. But it would be
difficult to see at the distance to the galaxy M101.
Do you think other extragalactic supernova remnants may really be
hypernova remnants? Are you looking for more?

Most of them are probably not. There are a few candidates for hypernova
remnants. But none of them has been as well studied at this point to directly
infer their explosion energies.
What was different about a star that causes a hypernova, not supernova, explosion?

A supernova from the collapse of a massive star (so-called type II supernova) is
believed to form a neutron star. A hypernova is believed to form a black hole
and is presumably from a very massive star. But no one is yet sure about the
exact mechanism of a hypernova.

You mention in the press release that it's unusual to see x-ray
emissions from these remnants at the distance of M101 (25 million light years). Do we see supernova remnants in many other galaxies? Or only the "closest" ones?

Only in the closest ones. There are, however, a few very young supernova remnants in galaxies that are at distances comparable to M101 and are X-ray
bright. But these remnants are typically due to the strong interactions between supernova eject with dense circumstellar materials, something like what is going to occur in the SN1987A remnant in a couple of years. Such remnant typically cannot be resolved even with HST.

Daniel Wang Astronomy 100