The observations of the first gravitational wave by the Laser Interferometer Gravitational-Wave Observatory (LIGO) captured the attention of the world this February, confirming the existence of gravitational waves as well as further confirming Einstein’s theory of general relativity.
The signature of merging blackholes resulted in a flurry of scientific articles reworking theories casting out models and creating new ones. Other observatories probed the sky looking for electromagnetic signatures across all wavelengths, but nothing was seen. Well almost nothing.
The Fermi Gamma-ray Burst Monitor (GBM) claimed to see an event occurring 0.4 seconds after the LIGO event. It was a very week fluctuation in the data lasting only 1 second. SPI Anti-Coincidence Shield on board The INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL), with a similar observing range as GBM, could not confirm the signal even though it should have seen the same event. This lead to an interesting conundrum. If GBM really saw gamma-rays from a black hole merger, many existing theories would have to be changed as it is very difficult to have a scenario where such merger can produce gamma-rays.
Therefore, we teamed up with part of the GBM and INTEGRAL teams to do a thorough investigation of the event. Instead of relying on existing analysis tools, we went back to basic statistics and designed new analysis schemes for the data. We applied the tools to the data from the GBM event and could not find a signal above the normal background in the instrument. When we checked with the normal analysis tools, we could see it. Therefore, data from other weak events that were also seen by INTEGRAL were checked with both sets of tools. This was very strange and we needed to find a way to see which analysis was right.
Luckily, INTEGRAL and GBM see a lot of the same gamma-ray events. For events that both INTEGRAL and GBM agree are real events, we can predict what INTEGRAL would see using parameters from GBM data analysis. Using our method and the standard method, we found again that you get two different predictions for what INTEGRAL would see. Our method always predicted correctly the INTEGRAL signal, but the standard analysis over-predicted the strength in INTEGRAL: just like the presumed gravitational-wave counterpart! Thus, we could explain why GBM results were in conflict with INTEGRAL for the gravitational-wave counterpart and prove that the counterpart was merely a background fluctuation in the GBM data.
There will be a lot of exciting science to come from LIGO! GBM is very likely to see a coincident signal when two neutron stars collide as these events should generate copious gamma-rays. When this day comes, we must have the statistical tools to analyze the data ready so that we get the next event right. Once we know we have a signal, we can explore all the exotic theories.
From a dark matter (DM) hunter’s perspective, this year’s Fermi Symposium was highly anticipated. In the six years since the launch of the Large Area Telescope (LAT), we’ve seen our share of ups and downs. An active community, both in and outside the Fermi Collaboration (FC), works hard to fit dark matter to or explain away every deviation in excess of what we expect from the gamma-ray sky. This year’s gathering got the answer to the latest burning question: do we see dark matter emission from dwarf spheroidal galaxies (dSphs)?
Before we hear the answer, let’s review the mileposts of the LAT DM saga. The first big stir came from the electron/positron spectrum , which featured a weaker version of the bump-like feature already measured by the PAMELA experiment. Theorists rushed to explain this with special flavors of ‘leptophillic’ dark matter, tuned to enhance the local flux while producing nothing other detectors would have seen. After an avalanche of 700+ citations and one even more precise measurement (AMS-02), the excitement died, along with most of those models.
Next we had the (in)famous line feature at 135 GeV. With a far more clear-cut interpretation, I held my breath while this result was seen first in the galactic center (GC), then in nearly every control region, and finally at lower and lower significance  until its appearance at the Symposium reported a mere 0.72 sigma. A dedicated observation from H.E.S.S. II may rescue it, but for now the line looks tenuous at best.
The latest big hint also comes from the galactic center, but in the continuum emission, rather than as a sharp feature. This makes some sense; the galactic center is nearby and massive, so any dark matter signal ought to be strongest there. On the other hand, it is astrophysically complex. So much so that interpretation of the data there seems to have been saved for last (the Fermi Collaboration has yet to weigh in with a publication). Undaunted, several groups (e.g. ) now report that models of known sources do not account for all the emission, leaving what is blandly referred to as the galactic center excess (GCE). Despite a wealth of systematic uncertainty, all parties agree that the galactic center excess is peaked at around 1 GeV and extends fairly symmetrically to about ten degrees from the center of the Milky Way.
Plausible conventional production mechanisms for the galactic center excess include pulsars and cosmic rays, but it can also be neatly accounted for by the existence of a weakly interacting massive particle (WIMP) of about 30 GeV which self-annihilates into b quarks. Without completely ruling out alternatives, this possible dark matter explanation remains just that — possible. What made this dark matter model so exciting was that it also happened to fit a slight excess (1.4 sigma) seen in the dwarf spheroidal galaxies (dSphs) orbiting the Milky Way . As dSphs represent an independent data set, and a nearly background-free one at that, the coincidence was indeed tantalizing. One well-known physicist called the galactic center excess “the most compelling signal we’ve had for dark matter particles – ever.” 
The dSph excess was so low that, if it was not just a fluctuation of the background, we would have to wait many years to confirm the dark matter explanation of the galactic center excess. Fortunately, the LAT’s excellent ground team just gave us a big push. A pair-conversion telescope, Fermi relies on extensively calibrated classification algorithms to reconstruct incoming gamma rays from their electronic signatures. These routines have been periodically overhauled throughout the mission as knowledge of the instrument continues to improve. The latest overhaul, known as “Pass 8,” marks the biggest advance yet, boosting the instrument’s effective area while lowering its point spread function like a pair of glasses. Upgrade in hand, the time was right to look again at the dSphs.
Dark matter searches in dwarf spheroidal galaxies have been a specialty of the Oskar Klein Centre. C. Farnier and M. Llena-Garde have both played lead roles in Collaboration papers on the subject (, ), and J. Conrad introduced the statistical technique now used to combine information from multiple targets . For the latest publication, G. Martinez derived the dSph mass distributions by a nested Bayesian analysis of their hosted stellar populations . Building on this success, yours truly, as part of the FC, took a peek at the Pass 8 data.
What we found was a whole lot of nothing. The significance of the GCE model dropped drastically, along with all other WIMP annihilation masses and channels. Dropped so far, in fact, that we can now set limits which exclude the annihilation cross section WIMPs need to make up all dark matter out to masses of 100 GeV (see Figure 1). These are now the best limits in the world below 1 TeV, and represent a big bite out of the parameter space left to the indirect dark matter detection field’s favorite class of models. While these constraints do not conclusively rule out the dark matter interpretation of the galactic center excess, they lend no support. “Tension” is the colloquial term.
So at this year’s Fermi Symposium, though debate still raged over the galactic center, I had to report that dSphs had pulled their support from the dark matter interpretation. Like all of us, I was disappointed to find we still have no answer to one of the greatest physics questions of the day, but as I said, dark matter hunters are used to highs and lows. Could gamma rays from annihilating dark matter still be buried in the LAT data? Of course, but there are not many coming from dSphs.
The Fermi satellite was launched in 2008 and since then it has continuously monitored the sky at gamma-ray energies above 100 MeV. Most of the sources detected at these energies are blazars, Active Galactic Nuclei in which the accretion onto a supermassive black hole also leads to the launching of two opposite relativistic jets. If a jet is pointing close to our line of sight we will see intense high energy emission due to strong Doppler boosting.
Fermi has so far detected well over 1000 blazars. One of these is B0218+357, which is known from optical and radio observations to be gravitationally lensed by a foreground spiral galaxy. The lens forms two closely spaced images of the blazar. The Fermi Large Area Telescope (LAT) can not spatially separate the two images so it can only measure the sum of both. However,
with timing analysis it is still possible to separate the signal of the individual components. This is because the path length from the blazar to us is different for the two images so we measure all blazar variability twice, with some time separation. Continue reading The first gravitational lens seen in Gamma-rays→
About once a day, a gamma-ray burst is detected. When this happens, e-mails get sent around and scientists scramble to detect whatever few photons might have been sent our way. But sometimes things are different…
On April 27th this year, an e-mail alert was sent around signifying the detection of yet another GRB. Yet this event was like no other. Rather than fighting to catch photons, there were suddenly too many to detect! The main emission episode was so bright that the GBM instrument on Fermi became saturated. And not only that – the GeV emission lasted for more than a day! Continue reading GRB130427A – a challenge to our models→
The Fermi satellite has given us a completely new view of the extreme events in our Universe. And it keeps getting better. Just as we were testing a new form of data analysis, Fermi captured a record breaking gamma-ray burst and delivered results that are difficult to explain with most popular models.
Gamma-ray bursts are the biggest explosions observed in the Universe, and are among the most distant sources that can be seen. The emission we see is probably sent out when a black hole is born. In this catastrophic event matter is shot out almost at light speed in two narrow jets, and if the jet happens to point towards us we see a bright flash of radiation.
Elena Moretti is the first of the about 300 applicants who was selected to become an Oskar Klein Fellow this year. She comes from a little country-side town, called Cartura, on the south of Padua in Italy, where she graduated in physics in 2006. She got her PhD in Trieste where she worked with the AGILE and Fermi experiments on GRBs. She developed a method that was used to calculate the flux upper limits on the GRB emission that was used in both experiments. In 2010 she moved to Stockholm working as a postdoc at the KTH. We ask her to tell us more about herself and the work she will be doing at the Oskar Klein Centre.
Congratulations Elena! You have been offered an Oskar Klein Fellowship. How does it feel? It feels good! It gives me the opportunity to develop my newborn interest in the polarimetry field. Wen I came here 2 years ago I was working only in the high energy astrophysics field with the 2 gamma-ray experiments Fermi and AGILE. After one year a new interest was tickling me: PoGOLite. I started to work on it as a “side job” on my spare time….well I guess that would change soon. Continue reading Interview with a new Oskar Klein Fellow→
Maja Llena Garde is a PhD student in the Cosmology, Astroparticle Physics and String Theory group at the Oskar Klein Centre. She is involved in the Fermi-LAT collaboration together with her supervisor Jan Conrad. The Large Area Telescope (LAT) is a space based imaging high-energy gamma-ray telescope launched in orbit in June 2008.
Their recent paper on Dark Matter has attracted some attention, thus we asked Maja to tell us more about it.
It has been shown that about 25% of our universe consists of dark matter, i.e. an invisible type of matter that neither emit nor reflect electromagnetic radiation. Physicists around the world try to figure out what this mysterious matter is.
One way to search for dark matter is by looking at gamma rays. If the dark matter consists of weakly interacting massive particles (WIMPs), then they can self-annihilate or decay into standard model particles, and this process will give rise to a gamma-ray signal. There are many places to search for this gamma-ray signal, and one example is dwarf spheroidal galaxies. These are satellite galaxies to the Milky Way and are known to have a large dark matter content. They are free from other gamma-ray sources, quite near-by and many of them are situated far away from the galactic plane so the galactic foreground is low. These properties make them excellent targets for dark matter searches. But the gamma-ray signal is expected to be very low. Continue reading One step closer to the mysterious dark matter→
The results presented at the III Fermi symposium in Rome reflected, in particular, what a magnificent instrument the Fermi LAT is for observing active galactic nuclei and pulsars. The 2 source catalogue 2FGL was presented and will soon be released with 1888 sources. Much attention was given to the blazar 3C454.3 which has been monitored since the launch and has undergone a series of very bright outbursts. The multiwavelength analysis by Stefan Larsson revealed a far more complex behaviour than expected in the simple picture we had of AGN jets before the launch of Fermi. The discovery of spectral breaks at GeV energies was nicely interpreted by the former Stockholm astronomer Juri Poutanen and collaborators as a result of gamma-ray absorption via photon-photon pair production on He II Lyman recombination continuum and lines within the broad-line region.
It was also made clear that all models we have for description of the high energy emission around pulsars are, more or less, wrong. Fermi has told us for certain that the emission is from high altitudes in the outer magnetosphere; Fermi has killed the polar cap model and the classical TPC, while the other models are in need of modifications. Continue reading The Fermi symposium 2011: AGNs, pulsars and gamma ray bursts→
The Fermi Symposium of 2011 in Rome has now reached its last day and we have heard many interesting talks, ranging all the way from dark matter to various astrophysical sources and observations. The OKC has been very well represented with participants both from the Department of Physics and the Department of Astronomy at Stockholm University and by the KTH group.
Science-wise, and from my personal dark matter oriented perspective, I think one of the most interesting talks was Maja Llena Garde’s talk on placing limits on dark matter models from dwarf galaxies using Fermi data in a stacked likelihood analysis. The limits are really approaching the vanilla WIMP expectations. Continue reading The Fermi Symposium 2011: a dark matter perspective→
Zhaoyu Yang is one of the OKC postdocs, working at both Fermi and Atlas experiments. It so happens that Zhaoyu also shares the office with me at the Elementary Particle Physics group, on the fourth floor, which is why it came natural to me to start by getting to know her better. With this interview we start a series featuring people working at OKC. Continue reading Interview with Zhaoyu→