Presentation on theme: "Gamma-ray transients as seen by the Fermi LAT M. Pshirkov 1,2, G. Rubtsov 2 1 SAI MSU, 2 INR Quarks-2014, Suzdal’, 07 June 2014."— Presentation transcript:
Gamma-ray transients as seen by the Fermi LAT M. Pshirkov 1,2, G. Rubtsov 2 1 SAI MSU, 2 INR 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 km e=0.01 P=96.5 min i=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 10 5 ) 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 cm 2 (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 >3x10 8 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, D 0 =2), add to j-th cluster corresponding to characteristic time scale τ 0 (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?
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.
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…