Mar 5th: Using Light Echoes to Observe Historical Transient Events

By on March 5, 2015 in

Podcaster: Cosmic

Apogee PodcastTitle: The Apogee Podcast –   Using Light Echoes to Observe Historical Transient Events

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Description:    In this Apogee Podcast, Cosmic discusses the detection and use of Light Echoes to observe and measure transient events (like supernovae) that took place in the past.

Bio: Cosmic is a self- and crowd-funded independent research astronomer and space musician.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by — no one. We still need sponsors for many days in 2015, so please consider sponsoring a day or two. Just click on the “Donate” button on the lower left side of this webpage, or contact us at


Hello!  This is Cosmic, and welcome to the Apogee podcast!  The format of this podcast is myself describing, discussing, and critiquing an article of my choice from an astronomical journal.  Sometimes I expand on sub-topics discussed in the article, especially if I find them interesting or in need of a more detailed explanation.  These podcasts will take place at or near the date of the apogee which is when, along its orbit around the Earth, the Moon is furthest away.

If you have any articles to suggest for future podcasts, I would be happy to take a look at them.  I can be reached at cosmiclettuce AT gmail DOT com.

The music you hear in the background are my own compositions.  These and many others can be downloaded from my soundcloud channel ‘cosmiclettuce’.  I hope you enjoy listening to space music as much as I do.

Also make sure to watch my daily podcast ‘Jupiter Today’ for detailed information and perspectives on events happening at and in the environment of Jupiter.

The apogee for this podcast will take place today, 5 March 2015, at 07:36 UTC. The lunar distance at that time will be 406385 km, which is 231 km further away than last apogee on 6 February, and 374 km further away than the next apogee on 1 April 2015.  Today the moon is as far away as it’s going to be until the apogee on 14 September.

The vernal equinox takes place this year at 22:45 UTC on the 20th of March 2015.  The equinoxes and solstices are my adopted holidays.  So here’s wishing you all a wonderful equinox.

Ok, on with the paper!

This apogee, I’m going to talk about a fairly new method of observing and measuring transient events like supernovae that took place in the past.  The paper that captured my interest is entitled “Light Echoes of Ancient Transients with the Blanco CTIO 4m Telescpe” by Rest, Sinnott, and others from a conference of the Astronomical Society of the Pacific in 2014.

I can’t describe it any better than this quote from the paper: “Light from an astronomical source may reach an observer directly or after being scattered by interstellar dust.  In the latter case, the arrival of any variations from the source object will be delayed by the longer path length (relative to the direct path) – resulting in what is referred to as a light echo.”

The first thing to do is not confuse light echoes with the light from ejected material or shock waves.  These are very different sources of light.  The light from light echoes come from the source itself, reflected off of material between us and the event.  The light from a shock wave comes from the material (presumably being compressed and heated) through which the wave is moving.

Light echoes were first seen and identified as such around Nova Persei in 1901. In 1940, the suggestion that light echoes could be used to learn more about historical events was made.  Serendipitous observations of light echoes took place, but not until 2005 were light echoes unambiguously associated with known supernova remnants in the Large Magellanic Cloud.

The advent of large survey telescopes coupled with large, sensitive CCD arrays (capable of seeing to a visual magnitude of 23 and fainter) have allowed astronomers to detect these faint light echoes by using a very powerful technique called difference imaging.  This is when an image of some part of the sky is taken, and then sometime later an image of the same part of the sky is taken again.  Subtracting the former from the latter will reveal any differences between the two in high contrast.  This is how most transient events, variable stars, and new minor planets and comets are discovered.  But instead of creating a difference image from two images taken hours or days apart, using difference imaging to detect light echoes requires images taken months, years, or decades apart.  Why?  Because they are very far away.  The light echoes from SN1987a, for example, are about 163,000 light years away. So even though light travels very fast (about 300,000 km/s) it takes a while for us to be able to detect any change in the location of these light echoes because the apparent motion is very small.

It’s through the use of difference imaging that light echoes are detected and located.  Astronomers can then use spectrometers to take spectra of these regions to see what the echoes look like.  A spectrum is a rainbow of colors created by light either being refracted though a prism or diffracted with a grating.  Fine structures within a spectrum allow astronomers to determine important pieces of information about the object being observed — primarily their chemical makeup, temperature, and motion.

So what do these light echoes look like?  It might be easiest to answer that by thinking about what a sound echo sounds like.  A sound echo is a copy of the original sound, modified in some way by the material that the sound is reflecting off of.  Echoes are always fainter than the originals because no surface is perfectly reflective — some of the energy gets absorbed into the material and some energy gets scattered — and the sound goes in all directions not just towards the listener.  But regardless of how faint the echo might be, the structure of the sound stays the same. For example, if you yell into a canyon “Today is the Apogee!” then what you’ll hear in the echo is exactly the same thing.  The quality of the echo is usually good enough for you to even be able recognize your own voice.  Sound echoes also happen after the original event.  How much time after depends on the distance between the source of the sound and the reflecting material, and the speed at which the sound is travelling.  So now imagine that you’re with a friend hiking in a canyon.  Your friend is ahead of you by some distance.  He or she turns around and yells something at you.  You’ll first hear his or her voice coming directly at you, and then you’ll hear their voice echoed off of the walls of the canyon.

All of the same goes for light echoes.

When astronomers looked at the light echoes associated with SN1987a, they found that the spectra and light curves looked nearly identical to the supernova itself!  A light curve is typically acquired by measuring the brightness of an object over time.  Supernovae, for example, have light curves that suddenly brighten and then slowly dim.  The details of how these light curves and spectra change over time allow astronomers to categories the supernovae into groups and subgroups.

Positive results from the work with light echoes of SN1987a gave astronomers confidence to apply this technique to light echoes from other events which no direct observations exist — either because they happened too far in the past, or because we just weren’t looking at the time.  This has now been done for a number of historical supernova events.  Looking at the light curves and spectra taken from the light echoes, astronomers have been able to determine the type of supernova they were, and make a decent guess as to when (how long ago) the event took place.  Not only that, but the apparent motion of these light echoes point directly back to the origin of the signal (the location of the event itself).  Looking at Figure 1 of Paper 2, it’s obvious where the event took place — right in the center of circles.

Light echoes usually aren’t this obvious, but estimates as to the location of source events can still be done fairly accurately (see Figure 2 of Paper 2).  Light echoes also allow astronomers to “view” these events at different angles. No supernova event is symmetrical as evidenced by the observations of material and shock waves emanating from the event.  But when we actually observe a supernova event, we are only seeing it from our particular “line-of-sight” point of view (usually called “on-axis”).  But what does the supernova look like off-axis?  With light echoes, we can answer this question and in so doing get a more complete understanding of the event.

For the present, the observations and measurements of light echoes are limited to events that take place within a couple hundred thousand light years of us. The distances to other galaxies are much too far to allow for the detection of light echoes with difference imaging.  Higher spacial resolution (either by using ground-based adaptive optics or space-based telescopes) will be required to detect light echoes from events in other galaxies.  This has yet to be done. However, there is evidence that light echoes do play a role in “contaminating” light curves and spectra from extra-galactic supernova events (see Paper 6).

I look forward to seeing new discoveries of historical transient events being made using this technique.  Our understanding of transient events like supernovae will be hugely increased once we’re able to observe light echoes in other galaxies — primarily because there are simply more of them.  I also sense that this technique could be applied to observing other phenomena like star formation.  Future telescopes like LSST and JWST will enable all of this to happen.

So until next apogee … I bid you Peace.

End of podcast:

365 Days of Astronomy

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