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 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 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 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…