Transcript Gamma-ray transients as seen by the Fermi-LAT

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 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
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:
D   2 ik  (ti  tk ) 2 /  0
2
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?
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.
  ( N obs (0.1  1.0)  N bckg (0.1  1.0)) / N bckg (0.1  1.0)

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…