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Purpose

This page aims to pull together definitions for the jargon and technical terms that get used by the Fermi collaboration. A glossary exists in the SLAC Workbook, but as far as we can tell it hasn't been updated in a while. A short glossary also exists in the FSSC Cicerone.

The purpose of this glossary in particular is to provide new Fermi members (especially junior members) with a resource to tackle all the jargon that gets tossed around. Therefore, please avoid using more jargon to explain jargon. The definitions should stand alone as much as possible, and can link to papers / Confluence pages / etc. for more detailed information. The explanations should only provide what is necessary for a basic understanding; details can go in the links.

This glossary focuses on terms that are difficult to just google ("GlastRelease", "theta"). It also includes googleable terms that have specific applications to Fermi (e.g., AGN observations in gamma rays rather than AGN in general).

Note that while this particular page is in the LAT Science Public area of Confluence and is accessible to everyone, many of the links will probably lead to internal (collaboration-only) pages.

0-9

term	definition	links
#FGL	A catalog of LAT sources, which are objects in the sky that are known to emit gamma rays over periods of time. These are often active galaxies, supernova remnants, pulsars, and pulsar wind nebulae. So far, the LAT team has published the 1FGL (which used 11 months of LAT data) and 2FGL (2 years of LAT data). There is also a 0FGL (3 months of LAT data), also known as the Bright LAT source list. The Catalog group is currently working on 3FGL. (see also Catalog)	0FGL page 1FGL catalog (with paper link) 2FGL catalog (with paper link) 3FGL paper
1FHL	A catalog of LAT sources (objects in the sky that are known to emit gamma rays over periods of time) that are only detected above 10 GeV. (In contrast, most standard LAT analysis begins at 100 MeV.) The 1FHL used 3 years of LAT data. (see also Catalog)	1FHL Confluence page
#LAC	Catalogs of LAT detections of active galactic nuclei (AGN).	1LAC Confluence page 2LAC Confluence page
#PC	Catalogs of LAT pulsar detections. 1PC reports on 46 pulsed detections using the first 6 moths of data taken by the LAT. 2PC reports on 117 pulsed detections using 3 years of data taken by the LAT.	1PC Confluence page 2PC Confluence page

A

term	definition	links
ACD	(Anti-Coincidence Detector) The ACD is the collection of scintillating tiles that detect charged particles, and is crucial to background rejection. The ACD covers the tracker (the part of the LAT that converts gamma rays into electron-positron pairs and then tracks these pairs' trajectories). Charged particles will also register as signals in the tracker, so the ACD is used to determine if a signal is actually a charged particle and not a gamma ray. If an ACD tile has a "hit" (detects a charged particle) that lines up with a signal, then that fact is used to "veto" the signal (i.e., declare that it's not a gamma ray).	§2.2.3 LAT Instrument Paper
Acceptance	Defined as the effective area integrated over the solid angle subtended by the field of view (units: m^2 sr). It is a measure of the "effective volume" of the entire field of view.	LAT performance page
AGN (group)	(Active Galactic Nuclei)	AGN group page
ARR	(Autonomous Repoint Request) When the Gamma-ray Burst Monitor (GBM, the secondary instrument on Fermi) detects a gamma-ray burst (GRB) that is especially bright or high-fluence (i.e., emitting many photons over a period of time), it sends a message to the spacecraft to maneuver itself so that the GRB is close to the center of the LAT field of view for 2.5 hours (previously, 5 hours). This allows the LAT to get good observations of the GRB for a longer period of time than it would otherwise be able to obtain.	more info
ASP	(Automated Science Processing) A set of automated science analyses runs on the data when it comes to the ground, to detect sources that are changing rapidly in time. These are often gamma-ray bursts (GRB), which are exploding transient phenomena, or active galactic nuclei (AGN), which can sometimes flare and emit more gamma rays than they normally do for short periods of time.	ASP Data viewer
ATel	(Astronomer's Telegram) Multiwavelength, web-based short-notice publication system for reporting and commenting on new astronomical observations.	ATel page Atel ArXiv paper
ATS	(Absolute Time Sequence) A series of spacecraft and/or instrument commands that are loaded on the Fermi spacecraft.

B

term	definition	links
BA	(burst advocate) BAs are LAT team members who keep an eye on gamma-ray burst (GRB) activity. The GRB group has automatic software to check if the LAT detected statistically significant emission from GRBs. The BA's job is to check the output from the software and alert the rest of the group. BAs are on duty for a week at a time.	BA Confluence page
back	The back half of the LAT is the 4 layers of thicker tungsten (which converts the photons into electron-positron pairs) that are at the bottom of the LAT. Photons that convert in this region tend to not be as well reconstructed as photons that convert in the front. It is possible to only use the front or back events on their own in an analyses, but generally, the fact that events are either front- or back-converting is automatically accounted for in the analysis software and instrument response functions.	Cicerone
back-converting	Back-converting events are photons that are converted into electron-positron pairs in the bottom portion of the LAT (see back).
backsplash		Cicerone
batch / batch farm	SLACs workload management platform, and job scheduler. Uses the LSF (load sharing facility) product. Allows queuing and scheduling of batch jobs on the non-interactive slac computers. Basically, the batch farm is useful if you have code that takes a while to run / needs to run on a lot of data.	LSF at SLAC presentation from 2012 Info on using the farm
BGO	(bismuth germanate detector) The Gamma-ray Burst Monitor (GBM), the second instrument on Fermi, is made of 2 BGO detectors (and 12 NaI ones). The BGOs detect photons between 200 keV and 40 MeV. They are positioned on opposite sides of the satellite.
blazar
BL Lac
BLR
boresight	The boresight is the line of sight pointing out from the center of the LAT that is perpendicular to the surface of the LAT. (Essentially, it's "straight up" from the center of the LAT.)

C

term	definition	links
C&A (group)	(Calibration and Analysis)	C&A group page
CAL	(Calorimeter) The calorimeter is the part of the LAT that records stops each electron-positron pair and records its energy, after the tracker has converted the photons into these pairs. The main purpose of the calorimeter is to record the energy, but the CAL is also able to roughly track the trajectories. Physically, it is located at the bottom of the LAT. It is made of cesium iodide (CsI) crystals.	§2.2.2 LAT Instrument Paper
cat. / Cat.	(category) Papers written by the LAT collaboration are assigned a category out of I, II, and III.	category explanation
catalog	A catalog is a paper that contains information on all of a certain type of object that the LAT has observed. Catalogs can be more general ("all objects in the sky that emit gamma rays") or more specific ("all pulsars").	LAT catalogs
Catalog (group)		Catalog group page
clean (event class)
CR	(cosmic ray) Cosmic rays are charged, high energy particles from outer space. Mostly consisting of protons, some electrons and heavier elements, they range from a few MeV to EeV. Cosmic rays are the main source of background that the LAT detects.
CTB	(Classification Tree Bill/Best)

D

term	definition	link
dec	(declination)
diffuse (group)		Diffuse group page
diffuse (event class)	In previous versions of the LAT data, the diffuse event class was the event class with the most stringent cuts on background. However, the "diffuse" name is no longer used.
DM (group)	(Dark Matter)	DM group link
DQM	(Data Quality Monitor) Also known as "datamon." A DQM is the LAT member whose duty it is to monitor the spacecraft's automatic plots and diagnostics to make sure all is well. If all is not well, the DQM's task is to alert the experts, who can take a closer look. DQM shifts last for one week at a time.	DQM information page
ds9	(Deep Space 9 (seriously)) ds9 is a piece of software that is used for data visualization and imaging. For instance, someone analyzing Fermi data might use ds9 to look at a counts map (a map of where the photons that Fermi observed came from).	http://ds9.si.edu/site/Home.html
DRP
dSph	(dwarf spheroidal galaxy)

E

term	definition	link
Earth limb / Earth's limb	The Earth is a bright emitter of gamma rays, due to cosmic rays interacting with the atmosphere. When the Earth is in the LAT field of view, the part of the Earth that is visible is known as the Earth limb. Almost all LAT analysis needs to minimize the contribution from the Earth limb. (see zenith angle cut)
EBL	(Extragalactic Background Light)
effective area	The number of photons detected divided by the source flux. The LAT effective area is a function of photon energy and inclination angle.	FSSC page LAT performance page
EPO (group)	(Education and Public Outreach)	EPO group link
event	An "event" is a catch-all name for "things that the LAT detects" (although sometimes it is used specifically to mean photons). In practice, these are either photons or cosmic rays.
event class	Events that are detected by the LAT are sorted into different classes based on how confident we are that the event is a photon.	Cicerone
Evo	Video conferencing software that has now been superseded by SeeVogh. see also "SeeVogh"
exposure

F

term	definition	link
FA	(flare advocate)
FITS / fits / .fits / fit (file type)	(Flexible Image Transport System) FITS files are often used in astronomy, and are made of one or more HDUs (Header and Data Unit). Each HDU has a header with general information about the file, the instrument, etc., as well as a table or image. Public LAT data is always (?) distributed as FITS files.	FITS Support Office
FltOps	(Flight Operations)
FOT	(Flight Operations Team) The FOT is the staff at the Mission Operations Center (at NASA Goddard), which controls the spacecraft. The FOT is responsible for, among other things, inputting special maneuvers such as TOOs.
FOV / FoV	(field of view)
front	The front half of the LAT is the 12 layers of thinner tungsten (which converts the photons into electron-positron pairs) that are at the top of the LAT. Photons that convert in this region tend to better reconstructed than photons that convert in the back (or bottom). It is possible to only use the front or back events on their own in an analyses, but generally, the fact that events are either front- or back-converting is automatically accounted for in the analysis software and instrument response functions.
front-converting	Front-converting events are photons that are converted into electron-positron pairs in the top portion of the LAT (see front).
FSRQ
FSSC	(Fermi Science Support Center)	Fermi Science Support Center
FSW	(Flight Software)
FT1 (file)	FT1 files are the LAT public data files with information on each individual photon.
FT2 (file)	FT2 files are the LAT public data files with information on the spacecraft (where it is, where it's pointing, etc.).
FTOOLS	A package of tools to manipulate FITS files.	FTOOLS main page
fv	(Fits Viewer) fv is a program to view FITS files.	fv main page

G

term	definition	link
GALPROP
GBM	(Gamma-ray Burst Monitor) The GBM is the other instrument on Fermi. It is made of 12 NaI (sodium iodide) and 2 BGO (bismuth germanate) detectors. The GBM detects emission from gamma-ray bursts, solar flares, terrestrial gamma-ray flashes ...
GCN	(Gamma-ray Coordinates Network)
GEANT
ghosts	Ghosts are events where two particles enter the LAT in a small enough time frame that the LAT considers the interactions a single event. A typical example would be a gamma ray and proton entering the LAT near simultaneously. This caused a fraction of gamma-ray events to be vetoed. This problem was first addressed in Pass7 and substantial improvements have been made in Pass8 to recover gamma-ray data from these ghost events.
GI	(Guest Investigator)
GLAST	(Gamma-ray Large Area Space Telescope) GLAST is the name that Fermi was referred to before it was launched. It is tradition to give NASA missions a different name after launch, as a good-luck superstition.
GlastRelease
GLEAM
GRB (group)	(Gamma-Ray Burst) The LAT has detected high-energy emission (>100 MeV) from ~80 GRBs since launch. In some cases, it detects the prompt emission (the short "burst" of photons lasting anywhere from less than a second to hundreds of seconds), and in the majority of cases it detects the afterglow (the long lasting, slowly fading emission). The Gamma-ray Burst Monitor has detected keV-MeV emission from over 1000 GRBs.	GRB group page
gt_____	The standard set of software used to analyze LAT data is called the Science Tools. Each individual tool has a specific function. The names all start with "gt", for "GLAST", which was the original pre-launch name of Fermi.	Science Tools references

H

term	definition	link
HEASARC	(High Energy Astrophysics Science Archive Research Center)	HEASARC NASA page

I

term	definition	link
IRF	(Instrument Response Function)	Cicerone
ISOC	(Instrument Science Operations Center)

J

term	definition	link
JIRA

K

L

term	definition	link
L1Proc	(Level 1 Processing)
LAT	(Large Area Telescope) The LAT is the main instrument on Fermi. The parts of the LAT are the Tracker, Calorimeter, and ACD.
light curve	A light curve is generally a plot of the photon flux versus time. For pulsars, a light curve is usually a plot of the photon flux versus the pulsar phase.
likelihood
livetime
LLE	(LAT Low Energy) LLE is a loose event selection that starts at 30 MeV (while the standard LAT event classes start at 100 MeV). The LLE is background-dominated, so it is only appropriate for short, transient events such as gamma-ray bursts or solar flares.
LMC	(Large Magellanic Cloud)
LPA

M

term	definition	link
MC	(Monte Carlo)
merit / merit file
MET	(Mission Elapsed Time) Many missions record times in MET, which is the number of seconds since January 1, 2001.	A tool for converting from MET to dates
MIP	(minimum ionizing particle)
MOC	(Mission Operations Center)
MSP	(millisecond pulsar)
multimessenger	Multimessenger studies include data from instruments that detect things other than light. For instance, one might combine LAT data (photons) with neutrinos, cosmic rays, gravitational waves, etc.
MW	(multiwavelength) Multiwavelength studies are conducted across different wavelengths and energies. For instance, one might combine LAT data (MeV-GeV) with optical, radio, IR, TeV, etc. data.

N

term	definition	link
nadir	The nadir is the line going from Fermi pointing directly to the center of the Earth.
nadir observations	When Fermi conducts nadir observations, it reorients itself so that it is essentially upside down. During normal operations, Fermi is looking at the sky, but during nadir observations it observes photons from the Earth.
NaI	(sodium iodide) The Gamma-ray Burst Monitor (GBM), the second instrument on Fermi, is made of 12 NaI detectors (and 2 BGO ones). The NaIs detect photons between ~8 keV and 1 MeV. They are positioned around the spacecraft to view the entire sky at all times. The GBM team uses the amount of signal in each NaI detector to calculate localizations.

O

P

term	definition	link
Pass #, version #
Pgwave
phi
pipeline
pointed observations		more info
pole pointing
PSF	(point-spread function)
PSR	(pulsar)	Pulsar group page
PWN	(pulsar wind nebula)

Q

R

term	definition	link
RA	(right ascension)
ROI	(region of interest) The ROI is the region of space that is being analyzed. A typical ROI for an individual source (such as a pulsar or a gamma-ray burst) is 10 or 12 degrees, although the exact size can differ and depends on the many factors.
ROOT, .root file	ROOT is an object oriented software package built on C++ by particle physicists at CERN to analyze large amounts of data. The Merit files are root files and require ROOT to read the data from said file.	http://root.cern.ch/drupal/

S

term	definition	link
SAA	(South Atlantic Anomaly) The SAA is a region of the sky over the southern part of the Atlantic ocean that contains a lot of charged particle activity. Both the LAT and the GBM detect charged particles as part of the background, so when the spacecraft enters the SAA, the detectors are turned off (data is not stored). The LAT and GBM have slightly different definitions of the borders of the SAA.
Science Tools
SeeVogh	SeeVogh is the video conferencing software that is used by the LAT collaboration.	SeeVogh Research Network
selection
senior review
SMC	(Small Magellanic Cloud)
SNR	(supernova remnant)	SNR group page
source (event class)
survey mode
star trackers

T

term	definition	link
TDRSS	(Tracking and Data Relay Satellite System)
TGFs	(terrestrial gamma-ray flashes)
theta
ToO	(Target of Opportunity)	more info
transient
transient (event class)
TRK	(Tracker) The Tracker is a part of the LAT whose purpose is twofold: 1) To convert gamma rays into electron-positron pairs, and 2) To track these pairs of charged particles through the instrument in order to figure out where the original gamma ray came from. The Tracker is made of alternating layers of tungsten foils (to convert the gamma rays) and silicon strip detectors (to track the pairs), arranged in 64 columns (8 by 8).	§2.2.1 LAT Instrument Paper
Trunc64
TS	(test statistic)

U

term	definition	link
ultraclean (event class)

V

W

term	definition	link
walkthrough
WAM	(Weekly Analysis Meeting) An online collaboration-wide meeting held every Friday at 8 am PST / 11 am EST / 5 pm CST. The WAMs are held over SeeVogh, which can be downloaded at this website. Topics discussed during WAMs include collaboration-wide items of interest (such as newly released Fermi data), detections of GRBs (gamma-ray bursts) and flaring AGN (active galactic nuclei), and walkthroughs (short summaries) of upcoming papers within the LAT collaboration.	WAM agenda page
white paper

X

Y

Z

term	definition	link
zenith	The point on the sky that is opposite to the Earth for the Fermi spacecraft.	(diagram from Wikipedia) (imagine the person is Fermi)
zenith angle cut	The zenith angle cut defines the maximum zenith angle for all the photons that will be used in an analysis. The zenith angle cut is used to cut down on the emission from the Earth limb (the part of the Earth that is in the LAT field of view).