Too Far to Be Seen

Sometimes it is pretty exciting when you look for something and don't see it. Last April there was a gamma-ray burst (GRB 090423) detected by NASA's SWIFT satellite. One of the first ground-based telescopes to look for the visible light afterglow was the automated 60-inch telescope at Palomar. The 60-inch was imaging the source within three minutes of the satellite's detection of the GRB. The result? The 60-inch didn't see it.


You might not expect that I would devote any time or space on this blog for talking about something that we didn't see. But that non-detection (unlike LCROSS) was pretty exciting.

As Brad Cenko said in his report:

The lack of an optical afterglow, together with the fact that the X-ray column density is consistent with the Galactic value (Krimm et al., GCN 9198), make GRB 090423 an interesting candidate high-redshift event. We encourage observations at longer wavelengths to search for a NIR counterpart.

Translation: This object should be bright, but it wasn't seen in visible light. That means it could be an extremely distant event. So distant, that the expansion of the universe shifted its light completely out of the optical and into the infrared. That was indeed the case.

Telescopes observing the object in the near infrared and radio wavelengths did indeed see the optical afterglow of the event and it is the most distant object ever observed.

How far is it? Just over 13 billion light-years from Earth. GRB 090423 occurred 630 million years after the Big Bang, when the universe was only four percent of its present age of 13.7 billion years. Explosions like this give us a glimpse into the early universe and confirm the idea that massive stars, like the one that blew up creating the gamma-ray burst, existed even back then.

Supernova Remnant W49B

(Credit: Caltech/SSC/J.Rho and T. Jarrett)

This is from a couple of years ago but I felt like posting an astrophoto today, so this is what you get. The image above shows a supernova remnant known as W49B.

It is a composite image taken by the orbiting Chandra X-ray Observatory and the 200-inch Hale Telescope at Palomar Observatory. It is a false-color combination of X-ray (blue) and near-infrared (red and green) light. The image is 5.7 arcmin on each side.

It is images such as this one that really highlight how ground-based and orbiting telescopes work hand in hand for astronomers as they attempt to unravel the secrets of the universe.

Here's what the Chandra website has to say about the image:

supernova remnant W49B reveals a barrel-shaped nebula consisting of bright infrared rings around a glowing bar of intense X-radiation along the axis.The X-rays in the bar are produced by 15 million degree Celsius gas that is rich in iron and nickel ions. At the ends of the barrel, the X-ray emission flares out to make a hot cap. The X-ray cap is surrounded by a flattened cloud of hydrogen molecules detected in the infrared. These features indicate that jets of hot gas produced in the supernova have encountered a large, dense cloud of gas and dust.

The following sequence of events has been suggested to account for the X-ray and infrared data: A massive star formed from a dense cloud of dust and gas, shone brightly for a few million years while spinning off rings of gas and pushing them away to form a nearly empty cavity around the star. The star then exhausted its nuclear fuel and its core collapsed to form a black hole. Much of the gas around the black hole was pulled into it, but some, including material rich in iron and nickel was flung away in oppositely directed jets of gas traveling near the speed of light. When the jet hit the dense cloud surrounding the star, it flared out and drove a shock wave into the cloud.

An observer aligned with one these jets would have seen a gamma-ray burst, a blinding flash in which the concentrated power equals that of ten quadrillion Suns for a minute or so. The view perpendicular to the jets would be a less astonishing, although nonetheless spectacular supernova explosion. For W49B, the jet is tilted out of the plane of the sky by about 20 degrees, but the remains of the jet are visible as a hot X-ray emitting bar of gas.

W49B is about 35 thousand light years away, whereas the nearest known gamma-ray burst to Earth is several million light years away - most are billions of light years distant. If confirmed, the discovery of a relatively nearby remnant of a gamma-ray burst would give scientists an excellent opportunity to study the aftermath of one of nature's most violent explosions.

The infrared data was observed with the 200-inch Hale Telescope's Wide-field Infrared Camera (WIRC). The individual frames, in very high resolution) from Palomar and Chandra can be found here.

Lifting the Veil on Dark Gamma-Ray Bursts

Credit: Aurore Simonnet/Sonoma State University, NASA Education & Public Outreach

Gamma-ray bursts are some of the most energetic explosions in the universe. They come in two varieties: long and short. The short bursts are thought to be the result of the collision and merger of objects like neutron stars and black holes. The long burst (those that last longer than 2 seconds!) are the result of the collapse and explosion of a massive star.

While the burst of actual gamma-rays is short they are often accompanied by a flash of visible light that can last minutes to hours. Astronomers use a variety of telescopes to track down this "afterglow". Gamma-ray bursts (GRBs) are first identified by NASA's SWIFT or other orbiting satellite (gamma-rays do not penetrate our atmosphere). Rapid follow-up observations are often made from ground-based telescopes like the Palomar 60-inch telescope. Through March 2008, the 60-inch telescope conducted follow-up observations of 29 GRBs discovered by Swift. 14 of those bursts were classified as "dark" - meaning there was little or no visible light observed.

GRBs are usually bright enough to be observed from billions of light years, essentially from about as far away as can be observed. Why were these GRBs dark? Astronomers like Brad Cenko and others (formerly with Caltech and now with UC Berkeley) have been checking into the mystery by looking following up on the dark GRBs identified at Palomar with the twin 10-meter Keck telescopes in Hawai'i.

Massive stars live fast and die young. It makes sense that such stars would lie in or near the nebulae from which they were born (like the artwork above). The findings on dark GRBs, announced at the AAS meeting this week, suggest that these GRBs are dark because the massive stars that formed them were hidden in vast, dusty nebulae. These nebulae are dense enough that they absorb much of the light from the explosions that the GRBs.

Read the full story here.

A Shot in the Dark

Earlier this week astronomer's announced that they had found yet another mysterious gamma-ray burst. NASA's Swift satellite detected GRB 070125 last January in the constellation of Gemini and sent its alert to ground-based astronomers.

Palomar's 60-inch telescope was one of the first to respond. The 60-inch, and indeed the entire observatory, is linked into the High Performance Wireless Research and Education Network (HPWREN). HPWREN enabled the message from Swift to be received, allowing the automated 60-inch telescope to quickly measure the fast-fading visible-light afterglow of the GRB. Data was then sent away from Palomar. Measurements prompted observations with the giant 8-meter Gemini North telescope and the 10-meter Keck I telescope, both located in Hawaii. Astronomers were able to determine the distance to the GRB - 9.4 billion light-years distant.


Astronomers are interested in things like the peak brightness of the burst, how fast it fades, its distance and what type of galaxy it is located in. Measurements indicated that this burst was likely produced by the collapse and explosion of a massive star. These stars "live fast and die young" and are expected to be found in a galaxy where new stars are being produced, yet deep images from Keck failed to find any signs of a galaxy. This means there shouldn't have been that type of star where the burst was seen.

So where did this burst come from? Maybe a faint tidal tail, produced as galaxies collide, is lurking too faint for even Keck to see. Maybe our understanding of this type of GRB is flawed. Deep searches with the Hubble Space Telescope hope to answer the question soon. Stay tuned.