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…