Observation Of X-rays From Birth Of Supernova Leads To All-out Effort To Record Stellar Death

On Jan. 9, 2008, NASA's Swift satellite captured the X-ray burst, the first ever recorded from a normal supernova at the moment of "shock breakout," when the shock wave rebounding from the collapsed core breaks though the star's surface to produce a shower of X-rays.

According to first author Maryam Modjaz, a Miller postdoctoral fellow at the University of California, Berkeley, capturing the breakout flash was a case of serendipity. Swift was already monitoring another supernova in the galaxy when the second one exploded. At the same time, UC Berkeley astronomers and their colleagues were monitoring the first supernova from the ground and caught images of the new one only hours after the explosion.

"Those observations are crucial to understanding the X-ray emission and the progenitor star size," said Modjaz.

"The results are stunning and illuminate one of the last frontiers in stellar death," added co-author Joshua Bloom, UC Berkeley assistant professor of astronomy. "What happens early on speaks directly to the nature of the star and the ways in which massive stars die.There are so few supernovae that have been observed in the first few hours after an explosion."

A paper reporting Swift's discovery of the X-ray outburst will appear in the May 22 issue of Nature and is authored by Alicia M. Soderberg of Princeton University and an international team of astronomers.

Galaxies typically host a supernova only once or twice every 100 years, Modjaz said, making it nearly impossible to record one as it actually explodes. However, Swift and other planned X-ray satellites designed to scan the sky for such X-ray flashes should see hundreds more like this in the future, making all-wavelength, cradle-to-grave analyses of supernovae common, Modjaz added.

She noted that previous X-ray observations of normal supernovae have been at later stages in the explosion, when stellar material collides with the surrounding envelope of gas and dust shed earlier by the star, generating X-rays and other high-energy radiation.

Based on X-ray, optical and near-infrared observations of the supernova, dubbed SN 2008D, Modjaz and her colleagues determined that the stellar explosion was probably from a star, originally more than approximately 30 times the mass of the sun, that must have shed its outer envelope of hydrogen but retained its next-inner layer of helium. Despite its much larger mass, the star had a radius less than or equal to that of the sun, which means it was a specific subtype of Wolf-Rayet star. Wolf-Rayet stars have lost their outer hydrogen layer to strong winds.

All the visible-light data identify SN 2008D as a Type Ib supernova: a core-collapse supernova ignited after nuclear reactions in the star's core convert all the material to iron, at which point nuclear fusion stops. Unable to continue providing the energy and pressure to hold itself up against the inward force of gravity when it reaches a critical mass, the iron core of the star collapses inward, generating a rebound that blows the star to smithereens.

More recent data reveal that the core of the shredded star is not round, but aspherical, Modjaz said, which is important for understanding the geometry and the theories of stellar explosions.

The Swift satellite, launched in 2004 to study gamma-ray bursts, was observing a two-week-old supernova in the spiral galaxy NGC 2770, 90 million light-years away in the constellation Lynx, when SN 2008D went off. Swift recorded 530 seconds of X-ray emissions, including a spectrum.

Once the supernova was announced widely, astronomers at many observatories turned their telescopes on SN 2008D. Modjaz helped organize observations by the Mt. Hopkins telescope in Arizona and the Gemini South telescope in Chile, while UC Berkeley professor of astronomy Alex Filippenko organized observations at the University of California's Lick Observatory, on Mt. Hamilton near San Jose, where his Katzman Automatic Imaging Telescope (KAIT) had independently captured the visible light from the supernova.

In addition to X-ray and ultraviolet recordings captured by Swift and NASA's Chandra X-ray Observatory, Modjaz and collaborators also obtained visible-light spectra and images of the explosion with the telescopes at the Fred L. Whipple Observatory on Mt. Hopkins, the Apache Point Observatory in New Mexico, and Lick Observatory; and also near-infrared spectra through the NASA Infrared Telescope Facility atop Mauna Kea in Hawaii.

Bloom quickly mobilized to observe with Mt. Hopkins' Peters Automated Infrared Imaging Telescope (PAIRITEL) and at the W. M. Keck Observatory on Mauna Kea. Except for periods of bad weather, observations were taken almost nightly for 109 days after the explosion. Modjaz's and Bloom's campus colleagues, Weidong Li and Nat Butler, swiftly analyzed the Swift data as they were beamed down to earth, while UC Berkeley graduate students Ryan Chornock, Ryan Foley and Jeffrey Silverman and Harvard University postdoctoral fellow Stephane Blondin reduced the optical spectra as they were wired down from the mountains.

"The data provided nice confirmation of expected X-ray emissions at breakout," Filippenko said.

Filippenko, who in 1985 discovered one of the first of what came to be called Type Ib core-collapse supernovae, added, "I am really thankful to have the opportunity to study a Type Ib supernova in its infancy. The death of a massive star marks the birth of its explosion, along with the creation and ejection of heavy elements like oxygen and calcium, necessary for life as we know it."

The initial X-ray measurements were different from other X-ray flashes observed by Swift and Chandra and may provide a signature for identifying other Type Ib supernovae, Modjaz and Butler emphasized.

"A purely X-ray-only discovery allows people now to have a signpost in time and place to do all their other follow-up observations," Bloom said

Such "panchromatic" analyses of supernovae will become more common if two planned orbiting X-ray satellites dedicated to all-sky surveys are launched in coming years, Bloom said. One of these, ARGOS-X, is up for review by NASA next month. Another, called EXIST, is in the planning stage.

In addition to those mentioned above, other coauthors of the paper are astronomers D. Perley, D. Kocevski, D. Poznanski, H. Bouy, M. Ganeshalingam, N. Smith, D. Starr and T. N. Steele of UC Berkeley; R. P. Kirshner, M. Hicken, P. Berlind, W. M. Wood-Vasey, C. H. Blake, W. Brown, P. Challis, E. Falco and A. Friedman of the Harvard-Smithsonian Center for Astrophysics; G. S. Stringfellow of the University of Colorado, Boulder; D. Barrado y Navascues of the Laboratorio de Astrofisica Espacial y Fisica Fundamental (LAEFF-INTA) in Apdo, Spain; H. Chen of the University of Chicago; W. H. de Vries of UC Davis and Lawrence Livermore National Laboratory (LLNL); P. Dufour and J. Liebert of the University of Arizona; P. Garnavich of the University of Notre Dame in Indiana; B. Holden, G. Illingworth and J. X. Prochaska of the University of California Observatories, Lick Observatory, UC Santa Cruz; G. H. Marion of the University of Texas at Austin; S. S. Olivier of LLNL; A. Stockton of the University of Hawaii; and G. G. Williams of the University of Arizona's Steward Observatory.

The work was supported in part by the Miller Institute for Basic Research in Science, the National Science Foundation, the TABASGO Foundation, NASA and a Department of Energy SciDAC grant.