Fermi

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
30 eggs
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.