1. The variable gamma-ray sky
Observing astrophysical accelerators in real time
Rolf Bühler • 2nd June 2017 • Ginzburg Centennial • Moscow

2. Counts
>800 MeV

3. Counts
~1 GeV
~100 GeV
Selig et al. A&A 2015

4. Std. deviation
>800 MeV

5. Max. variation
>800 MeV

6. Particle accelerationUV
X-rays
Su el al. Nature Physics
From Steven J. Schwartz.
Astrophysical Plasmas
The most studied acceleration mechanisms are shock acceleration and magnetic reconnection

7. Shock acceleration
Sironi et al. SSRv
Upstream
𝑑𝑁 𝑑γ ∝ γ − 𝑠 γ
Sironi and Spitkovsky ApJ
Theoretically studies since the 70's, more recently with 2D/3D Particle In Cell simulations

8. Magnetic reconnection
Sironi and Spitkovsky ApJ B
X-point
Magnetic islands
Kagan et al. SSRv
Particles are accelerated predominantly at X-points and by scattering on magnetic islands
sγ = 1.5
sγ = 2.0
sγ = 3.0
sγ = 4.0

9. Fermi All-sky Variability Analysis
Photometric
Analysis
100 – 800 MeV r95 ≈ 0.2°
0.8 – 2000 GeV r95 ≈ 0.1°
Likelihood
Analysis
Applied all-sky in weekly time bins in two energy bins over 7.4 years flares detected above 6σ.

10. The red crosses represent the
x – sources
circles are flares
The red crosses represent the
2FAV sources. The flares used to construct the 2FAV are also shown: yellow circles are
flares with the best determined position from the TS maps. Flares with worse TS map
positioning are in orange, if they have been assigned to a cluster, or cyan otherwise. Flares
with only FAVA positions are in magenta if they have been assigned to a cluster, green
if they constitute a separate cluster. For all the flares, the radius of the drawn circles is
equal to r 95 .

11. 2FAV catalog 518 sources Matteo Giomi 77
Abdollahi et al., sub. ApJS, arXiv:

12. See Maxim Lyutikov’s talk
Source classes
AGNs are the dominant source class
GRBs detected on a week time scale
Galactic flaring sources are binaries (HMXB, Novae, pulsars)
Crab nebula is still the only flaring Pulsar Wind Nebula
See Maxim Lyutikov’s talk

14. Another Crab nebula? Sensitivity in the Galaxy F ≈ 2 10-7 ph cm-2 s-1
Preliminary
MSPs
Radio loud
Radio quiet
Sensitivity in the Galaxy
F ≈ ph cm-2 s-1
Crab flare with
F ≈ ph cm-2 s-1 would be detected in a large fraction of our Galaxy (≈10 kpc).
Possibly PWN N157B in the Large Magelanic cloud
Saito et al. AIP Proc. 2017

15. Spectral properties
“Harder when brighter” for FSRQs and BL Lacs. FSRQs softer. Photon index always harder than 1.5
Isotropic Thompson

16. Spectra Crab and HMXB typically softer than for other sources
Spectral properties
Spectra Crab and HMXB typically softer than for other sources

17. Summary & Outlook
Cosmic accelerators energize particles rapidly in many different source classes
The second catalog of flaring sources was recently released by the LAT collaboration:
AGNs dominate the variable sky:
FSRQS have softer flaring spectra than BL LACs
In no case is the index of the flare photon spectrum significantly harder than 1.5
In the Galaxy binaries are the dominant population. The Crab remains the only PWN for which flares are observed.
FAVA is run in real time (typical 12h processing delay) and publicly available...

18. FAVA online https://fermi.gsfc.nasa.gov/ssc/data/access/lat/FAVA/ Dan
Kocevski

19. Backup slides

20. Transition of the binary PSR J1023+0038
Pulsar detected
in radio only
(P = 1.7ms, 4.8hr eclipsing binary system)
Gamma-ray flux and X-ray flux increase as radio pulsations disappear and double peaked optical lines appear
This is interpreted as transition from a millisecond pulsar to a Low Mass X-ray Binary
This phenomenology was already detected
in the case of two redback systems, PSR J and PSR J1227−4853,
for which we present here a dedicated stud
Stappers et al. ApJ

21. State transition of PSR J1021+4026
Pulsar detected in
gamma rays only
(P = 0.26s, τ = 77 kyr)
Gamma-ray flux decreases as spin-down frequency rate increases
Allafort et al. ApJL

22. Fermi Gamma-ray Space Telescope
Large Area Telescope (LAT):
FoV: 2.4 sr FoV (20% sky)
Energy range: 20 MeV to 2 TeV
Angular resolution: 1GeV
Dead time: 15 % (SAA) + 9% (trigger)
gamma e+ e-
Gamma-ray Burst Monitor (GBM)
FoV: Complete unocculted sky
Energy range: 8 KeV- 40 MeV
Positional accuracy: 4o-8o