GRBs: A New Standard Candle?

Astronomers might be on the brink of developing a new rung on the cosmic distance ladder.

An artist's conception of a gamma-ray burst.  European Southern Observatory

An artist's conception of a gamma-ray burst.
European Southern Observatory

Astronomy is a discipline pursued at unimaginable distances. And yet actually measuring the distance to a nearby exoplanet, or to a galaxy shining at us from the dark depths of the cosmos, seems almost futile.

One of the simplest methods is to use standard candles — objects with a known intrinsic brightness — and infer their distances based on how bright they appear to be when seen from Earth.

Astronomers have used Type 1a supernovae (SNe) as standard candles to great success. These explosions are the death cry of a dense white dwarf once it has collected too much additional matter. But we’re constantly on the search for new standard candles that could be an independent rung on the cosmic distance ladder: a tool for measuring distances to farther and farther galaxies.

Now, two new papers submitted to separate journals have independently found that gamma-ray burst supernovae just might be “standardizable.” And although both have yet to go through the rigorous peer-review process, outside experts are commending their work as solid starting points.

Gamma-ray Bursts as Standard Candles?

Roughly once a day the sky is lit up by a mysterious flash of energy. These events — gamma-ray bursts (or GRBs for short) — are among the most explosive and energetic events in the universe, sending out as much energy in a fraction of a second as our Sun will give off during its entire lifespan. Long GRBs (lasting more than 2 seconds) come from massive stars going supernova.

“When these stars undergo core collapse, they form a ‘central engine,’ which is expected to either be a rapidly rotating black hole that is surrounded by an accretion disk, or a neutron star with an exceptionally large magnetic field,” says Zach Cano (University of Iceland), who authored one of the two studies. “When the core collapses, the central engine creates a bipolar jet that pierces through the star, and at a large distance from the star, creates a burst of gamma rays, and later an afterglow.”

Type 1a SNe are easily used as standard candles because the amount of light we receive over time follows a specific pattern. The plots of this emission, known as light curves, have a characteristic shape, allowing astronomers to determine the explosion’s intrinsic brightness based on this shape alone.

But at first glance, the supernovae that create GRBs have irregular light curves.

So Cano and a second, independent team comprising Xue Li and Jens Hjorth (both from University of Copenhagen, Denmark) simultaneously dug a little deeper. Both teams looked at separate sets of eight GRB-SNe events in order to search for any consistency across the light curves.

They used two different approaches. Cano assumed that all GRB-SNe follow the behavior of the prototypical GRB-SN 98bw (a step that astronomers have assured me is valid). Li and Hjorth instead directly used the light curves of the GRB-SNe and looked for a correlation among them.

At the end of the day Cano found that the supernova’s luminosity correlated surprisingly well with the light curve’s width, while Li and Hjorth found that the luminosity correlated surprisingly well with the light curve’s decline rate.

“Before the two teams ... [submitted] their papers, it was not clear whether GRB-SNe could be standard candles,” says expert Steve Schulze (Pontifical Catholic University, Santiago, Chile). “The papers by Cano and Li and Hjorth provide compelling evidence that GRB-SNe are standardizable.” It is even more promising that two independent teams unknowingly supported each other’s results so well.

There are multiple advantages to using GRB-SNe as standard candles, and both teams are excited to move forward.

“It appears that GRB-SNe may be as good standard candles as Type 1a SNe,” says Hjorth. He explains that a major advantage is GRBs’ high redshift range. Astronomers have detected GRBs at redshifts as high as 8, when the universe was only 0.6 billion years old. Using GRBs to measure distances in the early universe would better enable astronomers to understand the universe’s mysterious expansion over time.

But the next step is actually using GRBs as standard candles. Cano thinks he has found a way to do this, but he’s not going public just yet. "Building upon this result, initial (unpublished) results indicate that GRB-SNe can be used in the same fashion as SNe Ia to constrain cosmological models,” says Cano. “The initial results also show that the universe is comprised of mostly dark energy, and with a Hubble constant between 60 to 70 km/s/Mpc," which matches calculations by members of ESA's Planck mission. "The results are preliminary, but the results are exceedingly encouraging."

Of course it’s important to stress that this is still a very young method with plenty of hurdles to jump through. Both teams will have to look at much larger data sets before the community will agree this approach works.

References:

Z. Cano. “Gamma-ray Burst Supernovae as Standardizable Candles.” Posted to arXiv.org on July 9, 2014.

Xue Li and Jens Hjorth. “Light Curve Properties of Supernovae Associated With Gamma-ray Bursts.” Posted to arXiv.org on July 13, 2014.


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