Pulsars are some of the most exotic objects detected by the Fermi satellite. They are a special kind of neutron star, ultra-dense remnants of very massive stars whose lives end in supernovae. They are about 10 km in radius and have the mass of ~1.5 Suns. They rotate very quickly -- they spin once every 0.001-100 seconds! -- and have very strong magnetic fields. They emit over a wide range of energies, including radio emission and the gamma rays seen by the LAT. Pulsars are tilted (like the Earth); their rotation and magnetic axes are offset, and so their radiation appears to pulse as it crosses our line of sight. About 25% of pulsars detected by Fermi are "radio quiet" and seen only in gamma rays.
Cake by Sylvia Zhu, David Green and Judy Racusin. Pulsar Technical Consultant was Megan DeCesar
The 2013 AAS Rossi prize was awarded to Alice Harding (GSFC) and Roger Romani (Stanford) for their work on gamma-ray pulsar modeling. So when it came time to enter the "Science as Food" competition in the annual Goddard poster party, it seemed only fitting to create a tribute to pulsars -- in cake form.
For our pulsar cake, the "neutron star" was a hemispherical vanilla cake with Swiss buttercream frosting. The cake was very dense, but not as dense as a real neutron star (a teaspoon of neutron star would weigh more than a mountain!). The surface of a real neutron star sometimes cracks under the strains of rapid rotation and large magnetic field. To show this cracking, the cake surface was covered in thin pieces of poured sugar (similar to Jolly Ranchers) and sugar crystals.
The radio emission sweeps around like a lighthouse beam as a pulsar rotates, and comes out from the magnetic poles. The radio beam was represented by a cone of styrofoam covered in fondant, and was the only nonedible part of the cake. The magnetic field lines also start from the magnetic poles; ours were made of pulled sugar, like candy canes. In a real pulsar, the closed field lines would not have been so tight and close to the neutron star (but real pulsars don't have to worry about fitting everything onto a cake stand).
The light cylinder is defined by the radius at which the magnetic field lines would have to travel at the speed of light in order to co-rotate with the neutron star. The gamma rays that Fermi observes originate in the outer magnetosphere near the light cylinder; we represented these regions with purple cotton candy. (This turned out to be an unfortunate choice given the surprising warmth and humidity of the day.) The entire setup was placed on a rotating cake stand; we were able to safely spin our pulsar at a speed of a few rotations per second.
After the judging process (we won the contest, although -- in the interest of full disclosure -- there was only one other entry) we removed the field lines and sliced up the neutron star. The cake pieces were all eaten within a few minutes. And although our pulsar cake was only a cartoonish representation of a pulsar, it was certainly much more delicious than a real pulsar would have been.
The gamma-ray burst monitor (GBM) instrument on Fermi detected its 1000th gamma-ray burst today! This figure from Valerie Connaughton shows the location on the sky of these 1000 cosmic explosions.
The 1000th burst was detected at 21:03 UT on September 21. It lasted for around 3 seconds, and consisted of a single large pulse of gamma-rays. It was automatically detected on board the observatory by the GBM and an alert was sent to the ground, that was then relayed to a worldwide team of astronomers in less than 15 seconds.
Originally, predictions indicated that we would need to wait for around 5 years before getting to the 1000th burst. However, due to excellent search routines implemented by the team of scientists who developed GBM, the rate of GRB detections has been significantly higher.
GRBs allow Fermi to see farther than any other class of object itdetects and each GRB is a probe of the oldest and most violentexplosions in the Universe. Every new one helps us better understand these interesting events.
What does the universe look like at high energies? Thanks to the FermiLarge Area Telescope (LAT), we can extend our sense of sight to “see”the universe in gamma rays. But humans not only have a sense of sight,we also have a sense of sound. If we could listen to the high-energyuniverse, what would we hear? What does the universe sound like?
A gamma-ray burst, the most energetic explosions in the universe, converted to music. Made by Sylvia Zhu (music) and Judy Racusin (animation)
Every photon has its own energy and frequency; the higher the energy, the higher the frequency. Some photons have just the right frequencies for us to see them as different colors, while others — such as the gamma rays studied by the Fermi LAT — are much too energetic to be seen with our eyes. Sound waves have frequencies too, and similarly, we can hear some of them as musical notes. So what happens if we convert high-energy photons into musical notes?
Gamma-ray bursts (GRBs) are some of the most powerful explosions in the universe. GRB 080916C was a particularly energetic burst that occurred in September of 2008. The brightest part of it lasted less than a minute, during which the LAT detected hundreds of gamma rays from the extremely-distant explosion; when we converted the data to music, we slowed the rates down by a factor of five times to hear the individual gamma rays better.
In translating the gamma-ray measurements into musical notes we assigned the photons to be “played” by different instruments (harp, cello, or piano) based on the probabilities that they came from the burst. This particular conversion is a fairly simple one; We built this on work done by other members of the LAT team (Luca Baldini and Alex Drlica-Wagner) who explored converting our data into music in different ways.
In the beginning of the song, before the burst starts, the harp plucks out a few lonely notes. After about half a minute, the piano joins in on top of the harp background, and the notes begin to pile on more and more rapidly. The cello enters the scene as the burst begins in earnest.
We created an accompanying animation to help see what is happening. The top panel shows each individual gamma-ray. The colors refer to low (red), medium (blue) and high (green) quality gamma-rays (played by harp, cello and piano respectively). The energy of the gamma-ray is on the y-axis (higher energy gamma-rays are towards the top of the plot) and the arrival time of the gamma-rays are on the x-axis (later arriving gamma-rays are further to the right). The vertical white line tells you where the music is currently playing. The bottom panel shows the number of gamma-rays (which is the number of notes played) in each time slice.
By converting gamma rays into musical notes, we have a new way of representing the data and listening to the universe.
The cake features a (hand drawn) Fermi gamma-ray skymap, showing the bright bandproduced by diffuse emission from the disk of our Galaxy, the Fermibubbles (in black) – huge lobes of gamma-rays extending above and below theGalactic disk, and many point sources of gamma-rays (active galaxies,pulsars and much more).
The Fermi observatory, sculpted here from fondant, shows the Large AreaTelescope (grey box) and a 3-d representation of the NaI (black/yellow)and BGO (orange) detectors of the gamma-ray burst monitor. Combinedthese instruments provide observations over an extraordinarily largeswath of the electromagnetic spectrum (from 8keV to over 300 GeV).
A pen is included to show the scale – this was a monstrous cake! The 70 or soof us at the launch anniversary celebration only got through half thecake, despite being a delicious combination of chocolate and vanilla. This is fortunate for our waistlines given the followingingredient list:
7 lbs flour
9 lbs sugar
6 lbs butter
3 lbs marshmallow
1 lb corn starch
8 cups of buttermilk
You can now keep up to date with Fermi activities via a new iphone/ipad app developed by my Italian colleagues. This is available from iTunes (search for “fermi” to find it). Some screenshots are shown below:
The timing of this release is not accidental – the world-wide community of Fermi-users are meeting in Rome next week for the 3rd Fermi Symposium (the first was in Palo Alto, California in 2007 and the second was in Washington, DC in 2009). Over 400 scientists are meeting to discuss the implications of the exciting observations we have made with Fermi over the past 3 years, and to announce new discoveries. These meetings occur roughly every 18 months, and are a major highlight in the Fermi scientific calendar. Keep your eye out for announcements of new Fermi results next week!
A solar eclipse occurs when the moon comes between the Sun and the Earth and thus casts a shadow on Earth. The shadow can be quite large – as you can see from the excellent image in the January 2, APOD.
There will be a partial solar eclipse tomorrow (January 4). Fermi orbits the Earth every 96 minutes: for two of those orbits tomorrow Fermi will pass through the shadow of the eclipse. This won’t cause any problems – each orbit, we pass through the nighttime and thus dark side of the Earth. Passing through the eclipse means that we will spend a little more time recharging the spacecrafts battery from the solar panels than we would ordinarily need.
The Fermi flight operations team closely monitors the performance of the observatory, they need to know if we will pass through an eclipse so that we won’t interpret the change in battery charging performance as a potential problem on the spacecraft.
It’s neat to think that a observatory designed to detect gamma-rays from the Universe can notice more classical local phenomena on Earth.
Last March, I spent an afternoon talking about Fermi to the children at Aziza’s place in Phnom Penh, Cambodia. Aziza’s place is a home and learning center for impoverished children. I was in Cambodia to visit my sister. She lived in an apartment next door to Aziza’s place and and had come to know the people at Aziza’s place. She suggested that I might like to visit and talk with the children about Astronomy, NASA and Fermi. It was a remarkable experience. The children had several astronomy lessons and activities in anticipation of my visit and were extremely enthusiastic and friendly. The discussion started with Fermi and astronomy and rapidly expanded to include rockets, spaceflight and the nature of the moon.
Today has been an exciting day. This morning we noticed that one of the many active galaxies in the Fermi sky had become extremely bright in gamma-rays. We decided to interrupt our usual mode of continuously scanning the entire sky, and instead repointed the spacecraft to stare right at this galaxy for the next few days. This will allow us to track very carefully how it behaves during these bright outbursts. This is the first time that we have ever chosen to repoint the spacecraft to look at an interesting flaring object. We call these kinds of unplanned observations “Targets of Opportunity”.
This object, known as 3C 454.3, has been exciting to watch throughout the mission. Shortly after launch in 2008, when we first turned on the telescope, we noticed an unexpected bright object in the newly observed gamma-ray sky. This bright source was the topic of our very first Fermi scientific communication – an “Astronomers Telegram” to communicate to other astronomers that this was an object to watch. It then dimmed somewhat and behaved more quietly for the next year. In December 2009 it flared up again, becoming the brightest persistent source ever seen in high energy gamma-rays (gamma-ray bursts are momentarily brighter, but they are over in a few minutes).
We think that 3C 454.3 may now be getting as bright as it was in December 2009, or even brighter!
The location of this flaring active galaxy is very fortunate. It is about 38 deg away from a star known as V407 Cyg. Last month, we discovered a bright gamma-ray flare from the direction of this star while it was undergoing a huge optical outburst. Seeing gamma-rays from this kind of stellar outburst was unexpected – this is the kind of surprise that we love to find. We have been monitoring the star very closely with Fermi and many other telescopes to try to figure out what is going on (this could be the topic of a whole blog entry all by itself). While 38 degrees seems like a big distance (it is around 80 times larger than the size of the moon), the Large Area Telescope on Fermi sees a huge fraction of the sky at once. We can easily point at V407 Cyg and 3C 454.3 at the same time. I can’t wait to see what we find from these observations!
More than 100 billion events have been detected by the Large Area Telescope on Fermi after 616 days in orbit. That is a lot of events.
What is an event?
The Large Area Telescope detects gamma-rays, one at a time, at a rate of a few per second. We measure where each one comes from and use this information to build up a deeper and deeper picture of the gamma-ray sky. However for each gamma-ray, we detect over one thousand cosmic-rays – charged particles moving at close to the speed of light. We call each cosmic-ray or gamma-ray detected by the LAT an event.
We don’t send the data from all events down to the ground. We are primarily interested in the gamma-rays so we run software onboard the Large Area Telescope to identify and delete events that are clearly cosmic-rays. This reduces the rate of events from 4000 per second to around 400 per second. This allows us to make better use of the precious data transfer connection between the observatory and the ground. Once we get the data down to the ground, we use a large computer farm (>1000 machines) to process the data carefully and pick out the real gamma rays, which arrive a few per second.
Lots and lots of gamma rays!
Compared with previous gamma-ray telescopes the LAT detects gamma-rays at a far greater rate. The plot shows the number of gamma-rays detected by four previous gamma-ray telescopes over their operating lifetime compared to the number of gamma-rays detected by LAT in a single year. There are so many gamma-rays detected by LAT that you can barely see that any are detected by the previous missions (OSO-3, SAS-2, COS-B and EGRET). The enhanced ability of LAT to detect gamma-rays so efficiently is one of the reasons that Fermi is such a ground-breaking mission.
One year ago today, Fermi started sky survey observations after completing observatory and instrument commissioning ahead of schedule.
What a year!
The previous entry described our first light results. Since then, we have discovered new populations of pulsars in our Galaxy. We have observed a extraordinary gamma-ray burst, which was the most powerful explosion in the Universe ever seen. We have explored the properties of the diffuse gamma-ray radiation, which permeates the Milky Way. We see the the Sun, the Moon and the Earth shining in high energy gamma-rays. Our continuous monitoring of the high-energy gamma-ray sky has uncovered numerous outbursts powered by supermassive black holes at the center of distant galaxies.
Comings and goings…
We were sad to see Steve Ritz leave NASA in July. As project scientist he has overseen the development of all aspects of the GLAST/Fermi mission from before launch and guided the transition into the smoothly operating mission that we have now. I am daunted to be filling such big shoes. Happily, Liz Hays has has joined Dave Thompson, and Neil Gehrels as Fermi Deputy project scientists so we remain at full strength.
We are looking forward to continuing this blog to share mission highlights, science results and interesting operations tidbits.