A look back on Fermi in 2011

If you pay attention to astro-ph, you may notice that along with the full-length papers, there are often shorter write-ups too. These conference proceedings are not typically refereed and can be hard to find outside of the preprint server, depending on your institute’s subscription policy, but offer a concise “snapshot” of current research without going into all the gory details that accompany journal articles.

Some conferences no longer seek to have their proceedings collected and published in a physical book, preferring to post them online, and the annual Fermi Symposium is one of those. For the past couple of years, scientists working on projects relevant to the Fermi Gamma-ray Space Telescope have gathered to share their results and then, later, post them to the arXiv. While proceedings from the 2011 conference have been slowly trickling down the astro-ph stream, it was only yesterday that I really took notice, and I thought it would be a good opportunity to step back and take a look at the Fermi mission more generally.

To recap, the Fermi Gamma-ray Space Telescope (the telescope formerly known as GLAST) is an international space observatory which is run by NASA, DoE, and institutions in France, Germany, Japan, Italy, and Sweden. Using two instruments, the Large Area Telescope (LAT) and the Gamma-ray Burst Monitor  (GBM), Fermi surveys the sky in the range from 10 keV to 300 GeV.

According to the mission website, some of the main science objectives are to

  1. Explore the most extreme environments in the universe,
  2. Investigate the nature of dark matter,
  3. Explain how supermassive black holes create relativistic jets, and
  4. Offer a new way to study gamma-ray bursts.
There have already been a few astrobites about Fermi, so I’ll just focus on Objectives 2 and 3 below.

Extragalactic Relativistic Jets

Title: Gamma-ray Band and Multi-wavelength Variability of Blazars with the Fermi Large Area Space Telescope
Author: Stefano Ciprini
Author’s Institution: ASI Science Data Center

When you look at the gamma-ray sky, the brightest extragalactic sources are blazars, galaxies where the jet created by an actively accreting black hole is pointed directly at us and dominates the observed spectral energy distribution. The material in the jets interacts strongly with the surrounding magnetic field, resulting in a classic synchrotron emission spectrum as well as a high-energy inverse-Compton component created when the synchrotron photons are scattered off the relativistically moving electrons that created them. It is this high-energy component that Fermi observes, but because the two components are physically related, multiwavelength campaigns are quite common (especially those involving gamma-ray and radio or gamma-ray and submillimeter observations).

Blazars have long been known to exhibit variability on a variety of timescales, and studies of flares have been conducted at wavelengths covering the synchrotron and inverse-Compton components over the years. The ubiquity of the phenomenon has led to studies of blazar variability being a central research interest in the blazar community, but progress has been slow and sometimes controversial. In the conference proceedings posted on Wednesday, Stefano Ciprini summarizes some of the results in the context of Fermi, noting that roughly two-thirds of the blazars in the Fermi catalog exhibit some kind of variability, which is typically irregular and aperiodic at all timescales.

Blazar variability with Fermi
Figure 1: Blazar variability as probed by Fermi. The left panel shows the distribution of blazars based on their luminosity and variability, with the most luminous sources (typically FSRQs) also being the most variable. The right panel shows a histogram of the structure function slopes for both the BL Lacs and FSRQs, with very little difference between the two. This supports the idea that BL Lacs and FSRQs share a similar underlying physical mechanism for variability.

Figure 1 shows some of the basic points highlighted by the proceedings, including the relationship between gamma-ray luminosity and the degree of inherent variability and the results of structure function analysis for different populations. The difference between BL Lac-type objects (BLLs) and flat-spectrum radio quasars (FSRQs) is primarily one of luminosity, but FSRQs also exhibit an optical component that is attributed to the host-galaxy. The first-order structure function is one of the common ways people investigate blazar variability, and is a method based on “running-variance” computations and related to the auto-correlation function and the power spectrum. The structure function power index is related to the nature of the variability (e.g., shot noise, flicker noise, white noise, etc.).

If this is a topic you think is interesting, there’s plenty to look at, including the proceedings discussed here. A quick search on ADS reveals hundreds of abstracts, including a paper on gamma-ray light curves and a couple of arXiv submissions of more recent work.

Dark Matter

Title: Search for Unknown Dark Matter Satellites of the Milky Way
Authors: Alex Drlica-Wagner et al.
First Author’s Institution: Stanford University

DM particle annihilation

I once read a joke that went something like this: “Dark matter is the Rome of astronomy; all observations lead to dark matter.” Of course, this is a natural consequence of the fact that we don’t really understand what dark matter is, despite actively trying to understand it for a very long time! One of the ideas about dark matter is that it may exist in the form of weakly interacting massive particles (WIMPs), which could self-annihilate and produce an observable signal (see the illustration to the right). Essentially, a WIMP and an anti-WIMP destroy one another and produce energy and particles according to the conservation of energy, momentum, and quantum numbers; among the potential products of this reaction are gamma-rays, which could then be observed by Fermi. But where would we observe these gamma-rays?

Cosmological simulations predict a large number of smaller dark matter satellites around galaxies like the Milky Way, which because of the lack of luminous matter are unobservable by traditional techniques. However, if the dark matter particles are WIMPs, gamma-rays produced via self-annihilation may be detectable. The gamma-ray luminosity of a dark matter satellite is related to the square of the density of dark matter particles along the line of site and the interaction cross-section and, therefore, is extended on a scale related to the size and distance of the satellite. The spectrum of the gamma-ray emission will also be slightly different from the power-law which is typically expected in cases of, say, inverse-Compton from a blazar.

In proceedings posted to astro-ph last month, Alex Drlica-Wagner et al. describe the search for dark matter satellites around the Milky Way by looking for exactly these two things in the LAT catalog: extended emission and a hard gamma-ray spectrum. To narrow things down, they only look at gamma-ray sources which have not been associated with counterparts at other wavelengths and are are high Galactic latitudes. After combing through the 385 unassociated sources and source candidates, they come up with only two that pass the spatial extension cut, 1FGL J1302.3-3255 and 1FGL J2325.8-4043. Only one of these has a spectrum which is consistent with dark-matter annihilation, but Drlica-Wagner et al. rule out both, citing the subsequent association of 1FGL J1302.3-3255 with a millisecond pulsar and the high probability of 1FGL J2325.8-4043 being unresolved emission from two point sources (AGN, in this case) and, therefore, not actually spatially extended. The null detection allows the team to place an upper limit on the interaction cross-section, which they report as 1.95 x 10-24 cm3 s-1 for a 100 GeV WIMP annihilating through the baryon/anti-baryon channel.

More details about their candidate selection criteria and the method used to place an upper limit on the cross-section can be found in the proceedings and the full paper, which has recently been submitted to ApJ.

About Allison Strom

I am a graduate student at Caltech, working with Chuck Steidel on issues related to gas and galaxies at high redshift. My undergrad years were spent at the University of Arizona, where I worked with Jill Bechtold and Buell Jannuzi on quasar absorption spectra, and I also took a year "off" before coming to grad school to study abroad at the University of Cambridge.

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1 Comment

  1. Thanks Allison for recapping the Fermi mission! One of my old projects has briefly used Fermi data to correlate with radio detection on nearby spheroidal dwarf galaxies in order to study dark matter. Beside that, I have not worked much on gamma astronomy. It’s cool that you pointed out the astro-ph stream of archiving the conference papers.

    Can’t wait to read more articles from you and others 🙂



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