1. Particle acceleration during the gamma-ray flares of the Crab Nebular
SLAC Accelerator seminar ● SLAC 15 June 2011
Talk about a recent gamma-ray flare last one we saw about 3 weeks ago...
This is the result of the work of a number of people within the collaboration and also by external collaboratios which mainly worked on the chandra data we obtained during this flare I am also going to present you..
R. Buehler for the LAT collaboration and A. Tennant, E. Costa, D. Horns,
C. Ferrigno, A. Lobanov, M. Weisskopf

2. Outline Introduction to pulsars and their nebulae
Introduction to the Crab Nebula
Observations of Crab Nebula gamma-ray flares
Flare implications on particle acceleration

3. Neutron stars Emerge after core collapse Super Novae:
Neutron Fermi pressure balances immense gravitation (Density ~atomic nucleus, escape velocity ~0.3c, radius of ~12km)
Carries most of progenitor stars angular momentum

4. Pulsars Ω μ α First discovered 1967 About 2000 discovered so far
Periods between ms – 10 s
Precise cosmic clocks, have e.g. been used for first evidence for gravitational waves
Strong magnetic fields → Energy outflow electromagnetically dominated Ω μ α

5. Pulsar Wind & Magnetic Field
Near Field (~2RL= c / 2πP)
Far Field (>>2RL) toroidal
“Striped” Pulsar
Wind
Г ~~ 106
Ne+e-~~ 1039 s-1
Force Free MHD
Spitkovsky 2006
Equatorial plane →
Bequator ~ G ,
Electrostatic potential at pulsar surface
due to Lorentz force separating charges: ~

6. Pulsar Wind Nebula
Examples of 54 Chandra observed PWN by Kargaltsev et al.
Pulsar Wind interacts with surrounding material → Pulsar Wind Nebula. About 100 known today. They generally have:
Intense synchrotron radiation from radio to x-ray
Particle cooling observed by shrinking size
Inverse Compton component with TeV emission

7. The Crab Nebula (M1)
Radio VLT
Optical HST
X-rays Chandra

9. The Crab Nebula Remnant from 1054 AD Super Novae at 2 kpc (~6 kly)
synchrotron IC
x-rays
Chandra
~0.3 ly
Hubble 2nd October 2010
10 arcsec ~0.1 pc / 0.3 ly
Atel 2903
2nd October 2010
One introductory slide on the crab, its an historical snr seen in 1054 and this is how it looks like in x-rays -torus, jets, inner ring (pwd is hitting on the ambient medium)
The SED to the right sominated by synchrotron component. It is a very bright and steady source over most wavebands and is therefore used to calibate instrumnets mainly in x-rays and VHE.
However, even before these recent flares there has been evidence for flux variations of the overall flux of the nebula, in the x-rays by a few percent per year as collen will present in more detail tomorrow together with more information on the LAT results also. And in the 90s already based on EGRET and Comptel data Munch et al and o dje et al repoted
Remnant from 1054 AD Super Novae at 2 kpc (~6 kly)
Standard reference in X-rays and VHE
Overall emission expected to be stable within few percent

10. Large Area Telescope on Fermi
Pair creation detector with tracker and calorimeter in space:
Detects gamma-rays between ~20 MeV to 1 TeV
Unprecedented sensitivity (Collection area of ~1 m2)
Angular resolution increases with energy between ~5°–0.3°
Crab Nebula (~0.05°) is a point source

11. Introduction to Crab with Fermi
31 month average
Preliminary
Preliminary
Inverse
Compton
Nebula
Pulsar
Off-peak
Synchrotron
Nebula
So here is how fermi sees the crab. We cannot resolve the nebula and distiguish it from the pulsar in angulas space as the nebula size if a few arc min of the same order as the angular resolution of the lat. But we can distiguish both via timing...
The nebula we see in the transition region
Flux is dominated by pulsar...pulsar and IC is constant so we can measure the synchrotron component as variations above this
Pulsar and nebula on top of each other, use phase to separate

12. Three day Crab synchrotron curve
Abdo et al 2011, Tavani et al 2011, Balbo et al 2011
Synchrotron Nebula
3 day binning
Preliminary
April 2011 flare
← 2007 flare by AGILE
Feb flare
Sept flare
Si here is the flux over time for this componet in three day time bins for the duration of the fermi mission. You can see the average flux above 100 MeV is around this value but there is a persistent variability above and below that. 4 flares have been reported so far from this object: in 2007, 2009, 2010 flare characterised by fast variability and hard spectrum and here the recent flare I am going to focus on now...but before I want to point out persistent varriability, flare judgement call...
Three flares as extremes of persistent variability
Flux increase by ~5 during 2009 and 2010 flares.

13. First let me quickly recall what is in the paper..
So here is what happened

14. 2009 and 2010 flares spectrum
2010 flare
2009 flare
Photon index:
2.7 ± 0.2
One of the most important point we make in the paper is that during the second flare synchrotron emission is detected to >1GeV. This is very hard to do in diffusive show acceleration and point towards an other mechanism of the electrons responsible for the emission..
4.3 ± 0.3
3.69 ± 0.11
Second flare has hard synchrotron spectrum and extends >1 GeV

15. Short term variability during 2010 flare
Pulsar + Nebula
>100 MeV
Shortly after our paper a folow up paper by balbo et al came out claiming that there is actually variability down to lower time scales than we claimed in the paper.
So here is what they did...
So I received many question whether if this is real or not, so I did the same analysis and it turns out its true. The fit on calibration sources in the same time bins and they are compatible with constant, so this are not systematic effects...
Nepomuk has actually even gone further and did a lightcurve in 6h which show significant variation.
So this tells us that the upper limiton the emission region is 16 times smaller than what we wrote in the paper (0.1 arc sec)
Balbo et al. September 2010 flare composed of 3 rapid (~12h) flares

16. Nothing unusual during the flares in the timing residual
Pulsed emission
preliminary
by P. Ray
Nothing unusual during the flares in the timing residual

17. The 2011 outburst 12-18 April 2011 August 2008 - April 2011
Crab
Geminga
So coming for the April flare here is a visualisation of the brightness of this flare, here you have the average where the emission here is dominated by the pulsar, and here are the six days around the peak of the april flare. You cann see the nebula completelly outshining the pulsar...
> 100 MeV
1º smoothed
During the flare, the Crab was the brightest source in the gamma-ray sky

18. 2011 flare in ~20 min. binning Preliminary
Synchrotron Nebula
Preliminary
~20 min time bins
average
Thanks t Pass 7 selection and the ToO we can zoom in even further into the main part of the flare, now in 20 min time binning.
You can see again these two main outburts show a second bump here and again this very fast variability on timescales of an ~hour.
Synchrotron nebula increased by factor ~30 during
Fast variability ( ~1h)

19. Nebula 2011 flare spectrum Preliminary Preliminary
≈ 1% of spin
down power
Preliminary
Preliminary
April 2011 flare
Pulsar
Average nebula
Shown here is the average spectrum of the flare and this is a fairly messi plot so let me explain... …
..and here is the average spectrum during the six days I showed you nefroe and you can see it is very interestin. While the pasts were..here clear curvature now we see the peak in the SED emission. Exponential cutoff. Pulsar like but there is no pulsation in the flare signal.
New component with “pulsar like” spectrum but no sign of pulsation in flare photons

20. Multi-wavelength correlations?
After 2009 and 2010 flares simple idea:
10 arcsec
Chandra
Find correlation between gamma-ray flares
and X-ray, Optical or Radio
Angular resolution allows to pin down
gamma-ray emission site
Monitoring Programs
(Chandra program led by Martin Weisskopf)
So coming to the MW observation. After the September 2010 flare we put several MW observations in place. Chandra led by Martin Weiskopf with AGILE and Fermi col members. Mnthly + ToO
The idea fairly simple correlation with structure. Go to image inner ring...only here
Inner ring
The following results produce by Allyn Tennant..

21. Chandra after the 2010 flare
28th Sept. 1010
~mCrab
6 days after September 2010 flare, followed by ~monthly images

22. Chandra during the 2011 flare
So here is what we saw..excelent coverage, two more after not shown
Knot 1 first excitement
..faded, no correlations, also not outside of this region shown here, so my first thought when I saw this, the emission must be coming from the part here
Xyars cannot, but the optical we can we got Keck observations right at the peak..
Preliminary
Optical
Two more observations on 21th – 28th April 2011

23. Multi-wavelength correlations?
No correlation found with the nebula, how about the pulsar?
Keck observations
of Hai Fu analyzed
by Roger Romani
Pulsar
(Thanks J. Graham, C. Max)
Knot 1
PI A. Melatos
The observations were taken by and analysed by Roger ..He cmpared this figure to 2002 observations with Gemeni and he found no significant flux increase from the pulsar or the knot I. Knot one is
Additionally simulateneous EVLA observations taken thanks and showed no strong flux increse in radio...
..so basically we see no correlation at any wavelength, even so we had some of the most sensitive instruments pointed at the Crab during its brightest outbursts ever seen...this reminded me of Faust when he gets depressed about having studied sciences and arts all his live and still not finding basic answers and he says...
Also no correlations, measured in optical and radio

24. And here, poor fool! with all my lore I stand, no wiser than before..
Of course faust was thinking of this regarding the question of existens, we are much more modest, we would just like to know where the gamma-ray emission of the crab comes from and how it is produced...
..At least for the first question we where hoping to find this directly but this has not given the expected results...
…in some sense this is so unexpected that it makes this even more interesting and it has made us actually wiser than before in many respects...
And here, poor fool! with all my lore I stand, no wiser than before..

25. Not true.. Basic conclusions:
New “pulsar like” spectral component with an energy ~1% of the pulsar spin- down power released at ~500 MeV
Very hard spectrum before the spectral break → electron energy distribution close to mono-energetic
For magnetic field strength of Bperp <105 G electron energies E >TeV, accelerated on hour time scales
Rapid variability implies synchrotron emission from a compact emission region < ly
No pulsations → Flare originates from outside the light cylinder
We can already draw some basic conclusions..
...New component, which has a “pulsar like “ spectrum and peaks at
The absence of correlation indicated hard spectrum
We see significant emission above 1 GeV likelly notr DSA . I had no time to go into this but please see the paper if you are interested. So we think this is likelly directly connected to the EM field of the pulsar, either magnetic reconnection or acceleration int the electrostatic field of the pulsar.
The emission region must be very compact.
All of this points hint to the fact that the emission comes from very close to the pulsar and is directly emitted in the pulsar wind. But it needs to be in a very hard spectrum to not produce measurable optical emission as we saw. So this is very much work in progress, experimantally to undestand the data dn theoretically to interpret it. We have an incredible dataset so there should be much more to come...
Emission likely from the Pulsar Wind region between
the light cylinder and the wind termination

26. How are particles accelerated?
Standard astronomers recipe of Diffusive Shock Acceleration?
Thought to be at work at termination of pulsar wind. Severely challenged by the flares.
Synchrotron emission above 1 GeV
ε 𝑝𝑒𝑎𝑘 ∝ 𝐸 2 𝐵200𝑀𝑒𝑉
𝑙𝑎𝑟𝑚𝑜𝑟𝑟𝑎𝑑𝑖𝑢𝑠 𝑐𝑜𝑜𝑙𝑖𝑛𝑔𝑙𝑒𝑛𝑔𝑡ℎ ∝ 𝐸 𝐵 −1 𝐸 −1 𝐵 −2 ∝ 𝐸 2 𝐵
𝑙𝑎𝑟𝑚𝑜𝑟𝑟𝑎𝑑𝑖𝑢𝑠 𝑐𝑜𝑜𝑙𝑖𝑛𝑔𝑙𝑒𝑛𝑔𝑡ℎ ≈2× 10 −2 ε 𝑝𝑒𝑎𝑘 4

27. Alternatives?
Acceleration directly related to electromagnetic field from the pulsar:
Magnetic reconnection of the Striped Wind? (At the termination of the pulsar Wind? Close the light cylinder?)
Acceleration in the Electrostatic Field of the pulsar?
Just beginning to explore these possibilities, they will likely revise our understanding of pulsars and their connection to pulsar winds and their particle acceleration..

28. Alternatives? ..lets ask the experts?
Acceleration directly related to electromagnetic field from the pulsar:
Magnetic reconnection of the Striped Wind? (At the termination of the pulsar Wind? Close the light cylinder?)
Acceleration in the Electrostatic Field of the pulsar?
Just beginning to explore these possibilities, they will likely revise our understanding of pulsars and their connection to pulsar winds and their particle acceleration..
..lets ask the experts?

29. Backup slides

30. x-rays Chandra ~0.3 ly 10 arcsec ~0.1 pc / 0.3 ly
Hubble 2nd October 2010