Supernova Remnant Cassiopeia A: Decoding Light to Uncover the Mystery of its Birth
May 29, 2008
We have all dreamed of using a time machine to go back in history. What’s exciting is that astronomers at Subaru went back in time to observe light that originally arrived at Earth more than 300 years ago. They didn’t use a time machine, but something similarly interesting called a “light echo”.
Cassiopeia A (Cas A), a supernova remnant located 11,000 light years away, is the second youngest known remnant in the Milky Way Galaxy. Although very well studied, Cas A has kept its origins hidden through the veils of time – until now. A group of international astronomers from the Subaru Telescope, Max Planck Institute of Astronomy (MPIA), and University of Arizona’s Seward Observatory came together to finally determine the history behind Cas A.
In 2005, Dr. Oliver Krause of MPIA used the Spitzer Space Telescope to discover that infrared emissions around Cas A were highly variable and quickly moving outwards as the light from the supernova explosion traveled further from its origin. It was found that these so-called infrared echoes could be used to home in on faint visible-light echoes as light from the supernova scattered off dust. These light emissions were referred to simply as a “light echo” (see Figure 1). The light echo, just like a sound echo, is a phenomenon where the light wave is reflected or re-emitted by the material located away from its origin, and arrives at the observer with a delay. The light echo of Cas A was the light of the initial supernova outburst that arrived on Earth 300 years ago as it swept past the planet (see Note 1). Dr. Krause revealed that the light echo would hold the secrets to the origin of Cas A (see Science, Volume 308, 10 June 2005).
From these early results, Drs. Tomonori Usuda and Takashi Hattori from Subaru teamed up with Drs. Krause, Birkmann and Goto from MPIA and Drs. Rieke and Misselt from Steward Observatory to spot the light echo from Cas A and decode it by spectroscopic observation, which, in turn, would reveal the secret to the supernova’s birth. An initial obstacle was that they had no idea when, where, and how bright, the next light echo would happen, until it actually happened. And, unfortunately, most of a light echo is usually too faint for spectroscopic observations. Moreover, a light echo fades away in mere weeks of time. What the astronomers needed was an exceptionally bright echo, and, of course, a telescope as powerful as Subaru to accomplish their mission.
The team started regular patrols in 2006 of a potential light echo region in Cas A using Spitzer in the infrared wavelength (see Figure 2) and wide field monitoring observation by the Calar Alto 2.2 meter telescope in Spain. Waiting for a light echo, it was not until the autumn of 2007 when fateful news arrived at Subaru that a candidate echo, perhaps bright enough for spectroscopic observation, had been located. On the night of 9 October 2007, the Faint Object Camera And Spectrograph (FOCAS) at Subaru confirmed the viability of the light echo (see Figure 3 & 4). The faint light echo was two magnitudes fainter than the original expectation (R=23.5 mag). After five hours of exposure, the instrument resolved an image with clear spectral lines, the fingerprints of a very young supernova (see Figures 5). This was the initial signal sent from Cas A over 300 years ago.
From that point, the astronomers compared the data to the spectral database of various supernovae, and found that the spectra of the Cas A light echo was almost identical with that of supernova SN1993J (see Note 2 and Figure 5). They surmised that the progenitor of Cas A must have been the same as SN1993J: a red super giant with more than 10 solar masses which had experienced a similar type of explosive outburst, a Type IIb (see Note 3). These findings are significant because more than twenty years after Cas A was first recognized as a supernova remnant, long and excited debates over the origin of Cas A have taken place. Finally, the debates may come to the end.
A question that continually followed this study on a parallel path concerned why the initial supernova explosion of Cas A was not more widely witnessed in the 17th century. Our group of astronomers agreed that three supernovae known-to-date occurred in the previous millennium: SN1054, SN1572, and SN1604. Their remnants earned their own names: Crab Nebula, Tycho's, and Kepler's supernovae, respectively. These supernovae outbursts are so extravagant that they were observable by the naked eye, and left detailed description in historical literature, such as Teika Fujiwara's diary 'Meigetsu-ki', and observing logs by renowned astronomers at that time, Tycho Brahe and Johannes Kepler.
Coming back to Cas A, the question remained why were there almost no records of the event in historical literature at the time of the outburst. Based on the current expansion rate of millions of miles an hour, the event should have been seen around 1680. Our astronomers surmised that it’s possible that because Type IIb supernovae fade away relatively quickly, poor weather of several days, that somehow overlapped with the peak intensity of the light curve (around magnitude 3.2), would have been enough to let the supernova event escape from history. Nonetheless, there was one astronomer who may have seen Cas A near its peak. An Astronomer Royal, the founder of the Royal Greenwich Observatory, John Flamsteed reported a dim magnitude 6 star in the direction of Cas A in 1680. The 6-mag star, however, disappeared, and the star was removed from the star charts by astronomers in later generations. At a relatively faint brightness of 6th magnitude, the light of the Cas A supernova event would have been extinguished relatively quickly by the dust grains in the foreground of the expanding supernova remnant along with the characteristically fast decay of light from a Type IIb outburst. With these findings, the answer to our historical puzzle seems to be finally answered.
The results from this study appear in the 30 May 2008 issue of the journal Science.
Note 1: Casioppeia A is located a distance of 11,000 light years from Earth. The supernova explosion occurred approximately 11,000 and 300+ years ago. Time relevant to this study is from when the light from Cas A supernova first arrived at Earth around 1680.
Note 3: The Type IIb supernovae originate from the core-collapse of massive stars that have lost most of their hydrogen envelopes prior to exploding and mainly consist of a bare helium core at the time of collapse. Type IIb supernovae fade away relatively quicker than other Type II supernovae which have their hydrogen envelopes.
Note 4: Team members
O. Krause (Max-Planck-Institut for Astronomy (MPIA)) , S. M. Birkmann (MPIA) , Tomonori Usuda (Subaru Telescope, NAOJ), Takashi Hattori (Subaru Telescope, NAOJ), Miwa Goto (MPIA), G. H. Rieke (Steward Observatory) , K. A. Misselt (Steward Observatory)
Figure4: Optical image of the echo region. It is a color composite of I-band (red), R-band (green), and V-band (blue) by FOCAS at Subaru Telescope. The faint white features in the middle of the image are the light echoes.
Reference Figure: Wide Field Image of the Cassiopeia A Supernova Remnant. It is a color composite of Mid-Infrared by Spitzer Space Telescope (red), Visible by Hubble Space Telescope (green), and X-ray by Chandra X-Ray Observatory (blue).