S. Digel
KIPAC/SLAC National Accelerator Laboratory
On behalf of the Fermi LAT Collaboration
• Fermi mission and the Large Area Telescope
• Post-launch analysis updates at 3 years – Event classification
– All-sky and (almost) all the time
• Recent results*
– 2FGL Catalog and source populations – Update on pulsars
– Transient sources: Crab Nebula, PSR B1259-63, GRB 110731A
• Summary
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Most LAT team publications are not cited in this talk; complete listings and links are here: https://www-glast.stanford.edu/cgi-bin/pubpub
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* Minimizing overlap with other LAT team presentations: AGN (Fegan), Dark Matter (Cuoco, Garde, Zimmer), Diffuse (Johannesson), EGB
(Ackermann), Positrons (Sgro’), SNR (Funk)
• Two instruments: Large Area Telescope & Gamma-ray Burst Monitor*
• Launched (as GLAST) by NASA in June 2008
• Routine science operations of Fermi began on August 4, 2008
• Gamma-ray data are publicly available since August 2009
Support for the LAT is from NASA and DOE in the US with key international contributions from France, Italy, Japan, and Sweden
• The LAT is imaging, calorimetric (energy resolution ~10% in GeV range), has a huge FOV and frequent sky coverage
• Pair conversion tracker has 83 m2 silicon strips (228 µm pitch)
• Trigger rate is 2-5 kHz; actual rate of celestial gamma rays after cuts is ~2-3 Hz
• We don’t see the Sun and Moon shadows; we see the sun and moon in emission (cosmic-ray interactions on the limb + for the sun, IC scattering of the solar radiation field)
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Energy Range
Ang. Res.
(68% radius)
Eff. Area (cm2)
FOV (sr)
Sky Coverage Fermi LAT 20 MeV −
>300 GeV
5° – 0.1°
(100MeV – >10GeV)
~8000 2.4 8x/day
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Sun >100 MeV Moon
>100 MeV
• The high-level results rest on the event reconstruction and
classification
• The event analysis relates directly to the Instrument Response
Functions (effective area, PSF, energy dispersion)
• This week the first new event classification since launch (Pass 7) was released through the Fermi Science Support Center*
– The corresponding Instrument Response Functions
incorporate an in-flight PSF in the higher-level event classes
• LAT Low Energy (LLE) results are also starting to appear – Getting to 20 MeV for bright transient sources
– Distribution of data and response matrices for specific
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Acceptance PSF 68% Cont. Radius
Pa ss 7 Pa ss 6
For these representative event classes, the tradeoff is somewhat broader PSF in exchange for greater and more uniform acceptance (for comparable residual background)
• Second Fermi Gamma-ray LAT Catalog (2FGL) – Succeeding the 1FGL
– Detection, classification
• Coming very soon 2LAC (Second LAT AGN Catalog), Second LAT Pulsar Catalog, LAT GRB Catalog, and ‘10 GeV’ (3-year) Catalog
– Population studies, joint GeV-TeV science
• The combination of quality and quantity of the LAT data is unprecedented for high-energy gamma-ray astronomy
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>1 GeV log scaling
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>1 GeV log scaling
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• 24 months of data 100 MeV to 100 GeV, 47.7 Ms live time
• 35.7 M events over the whole sky (Pass 7 Source class γ-rays)
• New in-flight instrument response functions (taking into account pile-up effects) – Pass7V6_Source
• Fairly uniform exposure (range factor ~1.5, ~5 x10-7 km2 sr yr)
• 2 years vs. 11 months
• Pass 7 vs. Pass 6
Some events were also removed from the previous data set.
Pass 7 events not in Pass 6 (11 months of 1FGL)
• Improved model for diffuse Galactic emission
– Angular resolution, gridding, fitting with preliminary 2-yr source list, localized regions of enhanced CR density
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Casandjian on behalf of the LAT Collaboration (Fermi Symposium 2011)
• Integrated, iterated source detection and position optimization
• Source characterization (model fitting) was via a binned likelihood analysis vs. unbinned for 1FGL
– Binned is computationally favorable with more data and produces somewhat more conservative source
significances
• Non-power-law spectra (important for pulsars and bright blazars)
• Spatially extended
These are resolved in LAT data and for 2FGL modeled
with their published (or soon to be)
characteristics
NASA
Centaurus A
sources
• Source coordinates and error ellipses at 95% confidence – Detection based on integrated data (not on flares)
– Precise localization
• Source significance (threshold significance just over 4 σ)
• Association with external catalogs
• Average flux in 5 energy bands 0.1 – 0.3 – 1 – 3 – 10 – 100 GeV, plus spectral fit parameters, curvature index
• Flux per month, variability index
• Quality flags: sensitivity to diffuse model, confusion, error ellipse not well defined, etc.
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• Identifications: correlated MW variability (AGNs, pulsars, HMBs, and the Nova), or measurement of angular extent
• The Source association is like calibrated cross correlation
between catalogs – the goal being quantitative probabilities of association and to control the false association rate
– As for 1FGL we adopted P = 0.8 threshold
– The primary associations were based on position
– For AGNs two additional association procedures were
adopted that factor in radio hardness and X-ray properties to reduce the effective density of counterparts, increasing the confidence of some associations
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[Here will show a version with the same symbol for each source and no legend]
1873 sources (~4 σ significance threshold)
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Red: Identified (127) Blue: Associated (1043)
2FGL Catalog is available from the Fermi Science Support Center http://fermi.gsfc.nasa.gov/ssc
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Preliminary
CTA1 Pulsar (exponentially cutoff power law)
A faint blazar (power law)
4C 21.35 (log parabola)
Some examples:
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PKS 1510-089 An unassociated 2FGL source
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Yellow bands indicate times when the Sun was within 2.5°
One-month time scales (>100 MeV flux), 458 sources found to be variable with high confidence
• Confusion (even at high latitudes)
• Probably missing a few dozen otherwise detectable sources at latitudes |b| > 10°
Distance to nearest neighbor 2FGL source (deg)
Density of 2FGL sources
• Sources in regions with suspected errors in the diffuse emission model
(162 red) as well as sources with properties very
sensitive to the diffuse emission model, or
implausibly curved
spectrum, or poor location determination, or possibly affected by the Sun (315 blue) are flagged
• Pay attention to these flags
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• The LAT is continuing to find pulsars, in coordination with the Pulsar Timing Consortium (Smith et al. 2008) and the Pulsar Search Consortium
– Radio (primarily) and X-ray timing
• 88 pulsars so far
Preliminary (and out of date)
– Many millisecond pulsars have been found in LAT
unassociated sources
– Many radio quiet pulsars have been found in blind
searches of LAT data
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Feb 2009 Sept 2010
Spectrum of Crab Nebula During Flares
N.B. April 2011 flare is not included above
• Detection of flares of the Crab
Nebula also was reported by AGILE (Tavani et al. 2011)
• The measurement of >1 GeV
synchrotron gamma-rays implies electrons > 1 PeV are present
– The loss time scales are
extremely short (cooling length ~ Larmor radius) as are
acceleration time scales – A potential interpretation is
enormous accelerating potential from the interaction of the pulsar wind with the nebula
• Pulsar/Be star binary – a well-studied system with a 3.4-year period
• Last December was the first periastron passage during the Fermi mission
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Circumstellar disk
(Chernyakova et al. 2006)
PSR B1259-63
• As also reported by Tam et al. (2011) the gamma-ray flux brightened around the time of periastron passage, while the pulsar was in the circumstellar disk
• During the second disk passage the system unexpectedly flared in gamma rays (only). The luminosity during this flare is nearly equal to the spin-down power of the pulsar – relativistic beaming
Flux (>100 MeV) Spectral Index
28 TITLE: GCN CIRCULAR NUMBER: 12218 SUBJECT: Fermi-LAT Detection of GRB110731A DATE:
11/07/31 18:13:26 GMT FROM: Julie McEnery at NASA/GSFC <julie.e.mcenery@nasa.gov>
Johan Bregeon, Julie McEnery and Masanori Ohno report on behalf of the Fermi Large Area Telescope (LAT) collaboration. Based on an on-ground analysis, the Large Area Telescope (LAT), on-board the Fermi Gamma-ray Space Telescope, detected high energy emission above 100 MeV, with greater than 10 sigma significance, from the Swift
detected burst GRB 110731A (Oates et al, GCN 12215). The preliminary LAT location of the ground automated analysis is (Ra,Dec) = (280.39,-28.53)+/-0.20 d (68% cont.) which is consistent with the Swift localization and within 0.1d from the optical counterpart position. Further analysis is ongoing. The Fermi LAT point of contact for this burst is Johan Bregeon (johan.bregeon@pi.infn.it). The Fermi LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.!
Also seen and well localized by Swift (Oates et al. in XRT and UVOT, 0.5”
position). A redshift of 2.83 was reported by Tanvir et al. from Gemini-North observations
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• LAT analysis at the event classification level is continuing to advance along with higher-level LAT science, and the latest advances are now available as Pass 7
• The all-sky coverage and great advance in sensitivity of the LAT for GeV γ-ray astronomy make catalogs of LAT sources of great utility for population discovery and characterization
– The 2FGL catalog is available now, and associated catalogs are coming soon
• The regularity of the sky coverage of the LAT is an excellent complement to the variability of the gamma-ray sky and the transience of cosmic particle acceleration
– And is being put to good use for multiwavelength science
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The Large Area Telescope (LAT) on the $Fermi$ mission began routine science operations almost exactly three years ago, on August 4, 2008, and has operated nearly flawlessly and stably. It has surveyed the sky in the 20 MeV to >300 GeV energy range with unprecedented depth and resolution and frequency of
coverage. The rate of scientific return from the LAT continues to be high. I will present an overview of recent work, including general LAT catalogs and studies of source populations, investigations of transient sources as varied as the Sun, the Crab Nebula, and gamma-ray bursts, results on cosmic-ray production in the Milky Way, studies of diffuse Galactic and extragalactic gamma-ray emission, and limits on WIMP dark matter.
• Populations
• Spectra
• Globular clusters suspected to be powered by MSPs – 8 globular cluster associations published
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Blue = radio quiet LAT Pulsar Catalog
Vela
~500k γ-rays/yr 17 17
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