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Gamma-ray transients as seen by the Fermi LAT

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

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Gamma-ray transients as seen by the Fermi LAT

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  1. Gamma-ray transients as seen by the Fermi LAT M. Pshirkov1,2, G. Rubtsov2 1SAI MSU,2INR Quarks-2014, Suzdal’, 07 June 2014

  2. Outlook • Fermi • LAT instrument • Data • Transients • Search (aims, methods,etc.) • Results

  3. 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/

  4. 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

  5. Fermi mission • Two instruments onboard: • GBM (Gamma-ray Burst Monitor): 10 keV – 25 MeV • LAT (Large Area Telescope): 100(20 MeV) – 500 GeV

  6. Fermi LAT • Fermi LAT – pair-conversion telescope From Atwood et al, 2009

  7. 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

  8. 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

  9. 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)

  10. Fermi LAT. Properties I • Energy range: 20 MeV – 500 GeV • FoV: 2.4 sr • Effective area: up to 8000 cm2 (SOURCE class)

  11. Fermi LAT. Properties II • Angular resolution: up to 0.1 degree at >10 GeV

  12. Fermi LAT. Properties III • Energy resolution: better than 10% at 10 GeV

  13. 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

  14. 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

  15. 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)

  16. Transients. Search method • Several steps • 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). • Find ‘physical clusters’ – all photons in triplet/quadruplet are in PSF68% distance • Reality check – could it be a fluctuation?

  17. 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.

  18. 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

  19. Map of multiplets without clear source identification

  20. 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.

  21. 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…

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