gamma-ray transients as seen by the fermi lat m. pshirkov 1,2, g. rubtsov 2 1 sai msu, 2 inr...
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Gamma-ray transients as seen by the Fermi LAT
M. Pshirkov1,2, G. Rubtsov2
1SAI MSU,2INR
Quarks-2014, Suzdal’, 07 June 2014
Outlook
Fermi LAT instrument Data
Transients Search (aims, methods,etc.) Results
Fermi mission
Launched in 11th of June 2008
Two month of on-orbit calibration
All the data since 04 Aug 2008 till yesterday could be found on the Fermi Science Centre website: fermi.gsfc.nasa.gov/ssc/data/
Fermi mission
Orbital parameters
h=565 kme=0.01P=96.5 mini=26.5○
Slowly precessing with a period of T=53.4 days
Fermi mission
Two instruments onboard:
GBM (Gamma-ray Burst Monitor): 10 keV – 25 MeV
LAT (Large Area Telescope): 100(20 MeV) – 500 GeV
Fermi LAT Fermi LAT – pair-conversion telescope
From Atwood et al, 2009
Fermi LAT. Tracker Consists of tracker (TRK), calorimeter (CAL) and anti-coincidence detector (ACD) Tracker – W foils, where conversion takes place + silicon scintillators detecting the direction of e+e- and, thus, the original direction of the gamma-ray Each foil –several % of the RL (3 or 18) (RL ~0.35 cm) Trigger: 3 layers in a row
Fermi LAT. Calorimeter
From Atwood et al, 2009
Calorimeter estimates the energy of the electromagnetic shower produced by the e+e- pair and images the shower profile.
The shape of the shower helps to discriminate between hadronic and leptonic(we are interested in) showers
Fermi LAT. ACD Fermi LAT is operating in very intensive CR background. At 1 GeV there are 100 000 protons and 100 electrons per 1 photon Rejection should be extremely efficient (better than 105) Primary rejection is provided by the ACD—scintillator cover of the experiment effectively (3x10-4) vetoing charged particles Additional rejection is made using analysis of shower profiles (in the calorimeter)
Fermi LAT. Properties I Energy range: 20 MeV – 500 GeV FoV: 2.4 sr
Effective area: up to 8000 cm2 (SOURCE class)
Fermi LAT. Properties II
Angular resolution: up to 0.1 degree at >10 GeV
Fermi LAT. Properties III
Energy resolution: better than 10% at 10 GeV
Fermi LAT. Properties IV
Timing precision: ~μs Dead time: ~26.5 μs Threshold for 5σ detection after 4 years: 2x10-9 ph cm-2 s-1 (E>100 MeV) –better than 1 eV cm-2 s-1
Fermi LAT. Data Different classes are optimized for different goals
More effective background rejection leaves us with a smaller number of bona fide photons—class CLEAN or ULTRACLEAN used, e.g., for DGRB analysis
TRANSIENT class is good for GRB studies where we do have exact spatial and temporal localization
For the most application a balanced SOURCE class is used: in total >3x108 photons with energies >100 MeV
Transients Short time scales: <1000s seconds (in this analysis) Very energetic events -- high fluence and luminosity. Evidence of some truly extreme process.
Model example are GRBs (though LAT is not the most effective experiment for their searches)
Also we could expect flares in blazars, PWN (Crab’s), Solar flares
Something unknown?
Everything is at E>1 GeV (better angular resolution)
Transients. Search method Several steps
I. Pre-selection: finding clusters in photon list. Define distance D between two events:
If it’s smaller than some threshold( say, D0=2),add to j-th cluster corresponding to characteristic time scale τ0 (0.1…100 s).
II. Find ‘physical clusters’ – all photons in triplet/quadruplet are in PSF68% distance
III. Reality check – could it be a fluctuation?
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Transients. Search method II How could we estimate probability in order to avoid false
detections ? Bright sources could occasionally produce several
photons in a row—NOT a transient.
Full MC of the Fermi sky
Refinemenet of simulation parameters allowed to obtain ~5% precision. Number of photons in MC is very close to real one in control patches (10+, all over the sky)
Probability to get this particular multiplet.
Not so easy to tame, yet results are largely negative – we can say that there are no flares from gamma-bright pulsars Vela and Geminga.
Transients. Search method III Another option
We could uncover results at E>100 MeV, previously unused
One could expect that 1GeV+ flare would be accompanied with some excess at lower energies
If it is there – we have a genuine transient
How we quantify number of expected/observed photons?
Following (GR, MP, P. Tinyakov ’12 ) analysis method for GRB searches
find all photons that fall in PSF95% around suspicious direction in selected time interval (-1000…1000s) and during whole mission;
Calculate 2 corresponding exposures Got background estimate
Map of multiplets without clear source identification
Transients. (Very) preliminary results A lot (200+) of detections of genuine transients
Most of them are from known sources (GRBs, blazars in high-state, even solar flares)
7 candidates passed ‘2-sigma test’ at 100 MeV –1000 MeV range.
Gaussianity is not guaranteed(!). In some places we need to revert to Poissonian statistics. In any case Full MC(E>0.1) [underway] would help us to gauge it
Caveats: hard spectrum bursts are handicapped. If dN/dE~E-2 we could have around 30 low energy photons. Only 5-6 in case of dN/dE~E-1.5. Even real bursts from known sources sometimes don’t pass the test. Also low-b transients are harder to confirm because of a stronger background.
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Conclusions We have discovered evidences for existence of new
transients at E>1 GeV energies at 1-100 s timescales
Interesting (astrophysical) part is attempting to identify sources and would be our next step.
Would be quite challenging because of scarcity of number of extra photons and rather poor angular resolution.
Work is in progress…