il cielo dei raggi gamma - dipartimento di fisica ... · luca baldini (unipi and infn) pisa, april...

Il cielo dei Raggi Gamma(o meglio: alcune sue sfaccettature, con un occhio particolare a Pisa)

FISICA 2013–2020, Congressino di Dipartimento, Pisa, 17 aprile 2013

Luca Baldini (Universita di Pisa e INFN-Sezione di Pisa)

[email protected]

Gamma-ray astronomy: motivationsNote: gamma-ray is a generic term covering ∼ 1/2 of the EM spectrum

I S. Hayakawa, Prog. Theor. Phys., 8 (1952), pp. 571–572I Gamma rays produced by decay of neutral pions produced by

cosmic-ray (CR) interactions with the interstellar medium (ISM).

I P. Morrison, Il Nuovo Cimento, 7 (1958), pp. 858–865I Production mechanisms: synchrotron radiation, bremsstrahlung,π0 decay, radioactive decays, e+ e− annihilation.

I Possible sources: the active sun1, the Crab Nebula2, anomalousextragalactic sources (M 873), Galactic diffuse emission4, isotropicdiffuse emission5.

I (Sstrangely enough, dark matter is not mentioned at this time.)I Straight from the abstract: “. . . γ-rays arise rather directly in nuclear

or high-energy processes, and yet travel in straight line.”

1ApJ, 745 (2012), pp. 1442ApJ, 674 (2008), pp. 1037; ApJ, 708 (2010), pp. 1254–1267 and many others3ApJ, 707 (2009), pp. 55; Science, 325 (2009), pp. 444–4484ApJ, 750 (2012), pp. 35PRL, 104 (2010), pp. 101101

Luca Baldini (Unipi and INFN) Pisa, April 17, 2013 2 / 28

Gamma-ray astronomers have two big problemsCredits: T. Weeks, Fermi Summer School 2012

Wavelength [m]

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I Problem #1: no optics (i.e., collection area = detector area).I Gamma rays cannot be focused nor reflected (yet).I Solution: make the detectors bigger.

I Problem #2: the Earth’s atmosphere is equivalent to ∼ 1 m of lead.I Or ∼ 28 X0: no primaries above 10 eV reach the Earth’s surface.I Solution #1: go up in space (space-based γ-ray astronomy);I Solution #2: look at secondaries (ground-based γ-ray astronomy).

Note: this talk does not cover at all the Compton regime and the extensiveair showers (EAS) detectors.


Space-based gamma-ray astronomyDetection principle (in the pair-production regime)

Photon Energy

1 Mb

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- experimental σtot

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γ ray

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Tracker/converter

Calorimeter

Anti-coincidence shield

Tracking plane

Conversion plane

I Pair production is the dominant interactionprocess for photons above ∼ 10 MeV;

I e+e− pair provides the information aboutthe γ-ray direction/energy;

I e+e− pair provides a clear signature forbackground rejection (charged particlesoutnumber gamma rays by 103–104).


Space-based gamma-ray astronomyHistorical overview

— First generation: counters

I OSO-III (1968)I First full-sky survey, 621 photons above 50 MeV.

— Second generation: spark chambers

I SAS-2 (1972–1973)I Detection of diffuse emission and a few point sources.

I COS-B (1975–1982)I Comparable to SAS-2 in performance, much longer duration.

I EGRET (1991–2000)I First detailed full sky survey, hundreds of point sources discovered.

— Third generation: silicon-strip detectors

I AGILE (2007–?)I Comparable to EGRET in terms of effective area, limited to

∼ 20 GeV by its mini-calorimeter.

I Fermi/GLAST (2008–?)I ×50 EGRET in acceptance.


The Fermi Large Area Telescope

Large Area telescope

I Numerology: 1.8 × 1.8 m2 footprint, 3000 kg weight, 650 W power, ∼ 1.2 Mb/s bandwidth.

I Observing 20% of the sky at any time, or the entire sky for 30 minutes every 3 hours.

1.8 m


The LAT silicon tracker

The LAT silicon-strip tracker

I 12k sensors, 90 m2 of silicon active area, 1M readout channels, built and tested right here!

I After ∼ 5 years in space: 99.9% average hit efficiency ∼ 500 (∼ 5 × 10−4) strips masked.


Ground-based gamma-ray astronomyDetection principle

I Simple technique, simple detectors(but only in principle!).

I The atmosphere is part of thedetector.

I Good: free and limitless.I Bad: no control over its conditions,

source of variable background light.

I Physics of EM showers wellunderstood.

I Detailed and realistic Monte Carlosimulations possible.

I Fundamental difference in the showertopology between EM and hadronicinteractions.

I Key to background rejection.


Ground-based gamma-ray astronomyHistorical overview

— First generation: various materials (including a trash bin)I Crimea, Dublin, Whipple, Narrabri. . . (1960–1985)

I No imaging capabilities, weak or no background discrimination.

— Second generation: Imaging Atmospheric Cherenkov Telescopes(IACTs)

I Whipple, Crimea, CAT, HEGRA, Durham. . . (1985–2003)I A source, at last: the Whipple 10 m reflector, with a 37-pixel

camera, detects the Crab at 9σ6, ApJ, 342 (1989), pp. 379–395.I In 1992 Whipple detects the first extra-Galactic source (Mkr 421).

— Third generation: small arrays of large IACTsI MAGIC-2, HESS-5, CANGAROO-III, VERITAS-4 (2003–present)

I Stereo-operation enhances the background rejection capabilities.

6. . . Prof. A.E. Chudakov suggested that the acceptable standard of credibility for anew VHE γ-ray source should be 5σ. [. . . ] At that stage we had managed toaccumulate a 3σ result. . .Years later [. . . ] I reminded him of his 5σ credibility criterion [. . . ] and proudlyannounced that [. . . ] we now had a 9σ signal from the Crab. He paused for a momentand then said drolly: “I think I should have said 10σ.” —T. Weeks (9910394v1)


The MAGIC telescope. . .

MAGIC

I Operating at Canary Island La Palma, 2200 m above the sea level.

I Stereoscopic system composed of two 17 m-diameter telescopes, record-breaking low-energy threshold.

The secret contribution to thesuccess of the experiment!


. . . But first came CLUE!

A. Menzione, Italian Phys. Soc. Conf. Proc., 8 (1987), pp. 352


Space- vs. ground-based gamma-ray astronomy

Parameter Space-based Ground-based

Effective area ∼ 1 m2 ∼ 105 m2

Field of view ∼ 2.5 sr ∼ 2.5 × 10−2 sr

Angular resolution 0.1–5◦, depending on E ∼ 0.1◦ at all E

Duty cycle ∼ 85% ∼ 10%

Energy range ∼ 20 MeV–300 GeV ∼ 50 GeV–50 TeV

I Operative summary: space-based and ground-based instrumentlargely complementary.

I Space-based instruments: act as all-sky monitors (source catalogs,transients, study of the diffuse emission).

I Ground-based instruments: pointing, best for source studies(especially extended sources).

I Moreover, they are sensitive in different energy ranges, withdemonstrated overlap.


Dissecting the gamma-ray sky

I The Fermi γ-ray skyI Rate map (exposure corrected) of γ-candidates above 200 MeV

collected by Fermi during the first year of data taking.



=

I Fermi: Galactic diffuse emission (accounts for 80–90% of γ rays).I The cosmic-ray/gamma-ray connection (Hayakawa, Morrison).I Also: direct CR measurements and imaging of the acceleration sites.



= +

I Fermi: resolved point sources (a.k.a. the source catalog).I 2FGL: 24 months of data, ∼ 36 M events in 100 MeV–100 GeV.I 1873 sources (575 of which are unassociated).



= + +

I Fermi: isotropic diffuse (a.k.a. extragalactic diffuse emission)I Unresolved sources, residual CRs and truly diffuse emission.I This would be very hard (impossible?) for IACTs.


The TeV source cataloghttp://tevcat.uchicago.edu/

143 sources,20 discovered by MAGIC

(Fermi skymap overlaid)

I About an order of magnitude less than the 2FGL.I Remember: IACTs are pointing instruments, while Fermi looks at the

entire sky ∼ continuously.


http://tevcat.uchicago.edu/

Zooming in. . .

MAGIC 0.3–1 TeV

MAGIC > 1 TeV

Fermi 2–10 GeV

I IACTs definitely have betterimaging capabilities forextended sources:

I better PSF;I larger effective area.


And now a spectrum

I Combined Fermi-MAGIC spectrum of the Crab Nebula.I Note the overlap between ∼ 50 and 300 GeV.I Remarkable agreement between two completely different techniques.


Looking back to just a few years ago. . .

I TeV gamma rays from the Galactic Center, ApJ, 619 (2005),pp. 306–313:

I Significant discrepancies between different IACTs: miscalibration ofdetectors or time variability?

I Spectral gap between EGRET and IACTs.

I Crucial to have good and simultaneous coverage, possibly fromdifferent instruments.


More on point-sources: gamma-ray pulsars

I Second Fermi-LAT pulsar catalog about to be submitted:I based on three years of data, 117 pulsars above 100 MeV;I unprecedented sample: clarify emission mechanisms, allows for

population studies.I Fermi public list of 125 discovered gamma-ray pulsars, already.

I Only 7 gamma-ray pulsars were known before Fermi.


IACTs see pulsation, too!

I MAGIC detects pulsation from the Crabpulsar above 25 GeV:

I Science, 322 (2008), pp. 1221–1224I Relatively high cutoff energy in the

phase-averaged spectrum.

I VERITAS detects pulsation above 100GeV:

I Science, 334, 2011 (69), pp. :I “The detection cannot be explained on

the basis of present pulsar models.”

I MAGIC confirms the VHE pulsation:I A&A, 540 (2012), pp. A69


Transient sources: Gamma-Ray Bursts

I The Fermi low-energy instrument (GBM) detects ∼ 250 GRBs/yearI ∼ 1/2 in the LAT field of view, ∼ 10% detected by the LAT.

I First Fermi-LAT GRB catalog just submitted (arXiv:1303.2908v1):I Based on three years of data, 35 GRBs detected above 20 MeV.

I Modern IACTs equipped to react to real-time alerts and repoint.I But no positive detection, so far.


GRBs and tests of Lorentz invariance

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I GRBs make for perfect objects fortime-delay experiments:

I Test for deviation of the standarddispersion relation in the form:

v(E) = c

[1 ±

(E

mQG ,n

)n]I Need large ∆E and large distances.

I GRB 090510: a 31 GeV photondetected 0.829 s after the trigger fromz ∼ 0.9:

I Nature, 462 (2009), pp. 331I mQG ,1 larger than the Plank mass,

i.e. rules out models with n = 1.

I IACTs rule for n = 2!I Physically more interesting if CPT is

not violated.


More transient sources: the Sunhttp://apod.nasa.gov/apod/astropix.html


Even more transient sources: two surprises!


Indirect searches for Dark Matter

Galactic center

Good statistics but sourceconfusion and diffuse backgroundSatellites

Low background and goodsource id, but low statistics

Milky Way halo

Large statistics but diffusebackground

Galaxy clusters

Low background but low statistics

Spectral lines

No astrophysicaluncertainties, good source idbut small branching ratio

Extra-galactic γ radiation

Large statistics, butastrophysics and galacticdiffuse background

And electrons/positrons!

Complementary to γs


A gamma-ray line in the diffuse emission?

I C. Weniger, JCAP, 1208 (2012), pp. 007 and many, many othersI Yes: Fermi-LAT data are public. . .

I The LAT collaboration has a line-search paper on preparation:I Significance slightly lower with updated instrument calibration and

better energy dispersion model;I feature seems to be narrower than the energy resolution;I (smaller) feature at the same E in the Earth limb control sample.

I Too early to draw any definitive conclusion.I But substantial chances to answer the question in the mid term.I (Substantial rework of the event-level analysis in progress.)


Looking at the future: the clear part

I The Cherenkov Telescope Array (CTA):I Large array of telescopes (three different mirror sizes, layout and site

choice to be frozen).

I Exciting prospectives for studying γ-ray sources above 10 GeV:I Large flexibility in optimizing the observation strategy;I all-sky survey not unfeasible.

I Would still definitely benefit from a large-FOV space observatoryoperating at the same time.

I e.g., GRB alerts, flaring sources, multi-wavelength coverage.I (And also: how about the study of the diffuse emission?)


Looking at the future: the foggy part

I Proposing a new space mission? Need to do an order of magnitudebetter than the previous one.

I Fermi already saturates the space available on the typical launcherfor a mid-size mission (hard to scale ×10).

I New concepts being explored but no clear plans for the near future.I A relatively low-energy detector with no converters?I Or rather a high-energy7 instrument with emphasis on energy

resolution (CALET, Gamma-400, DAMPE, HERD)?

I Maybe even another Fermi would be a useful thing to have?I Surely it would for CTA.I And the gamma-ray sky is highly variable (i.e., always new).I In passing: Fermi is about to finish its prime (5-years) phase and

missions extensions are decided every other year—the currentbaseline is to operate through 2016.

7But remember: there is only about 1 celestial gamma-ray per week above 1 TeVtraversing an instrument as big as the Fermi LAT!


Conclusions

I This is a very exciting time for gamma-ray astronomy.I Huge leap over the last decade, both theoretically and

observationally.I Gamma-ray astronomy: no more an “order-of-magnitude” type of

science.

I Pisa is at the forefront of all this:I with a multi-faceted, well-rounded contribution.

I Much more has to come on the horizon of the next ∼ 5 years.I Fermi will continue collecting data and will go through a major

upgrade of the event reconstruction.I Several IACTs (MAGIC, H.E.S.S., VERITAS) taking data.

I Beyond that. . .I The path is more or less clear for the ground-based community;I not that much for the space-based instruments.


Spare slides

Luca Baldini (Unipi and INFN) Pisa, April 17, 2013 Spare slides

LAT IRFs

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The Fermi Large Area Telescope

Large Area telescope

I Numerology: 1.8 × 1.8 m2 footprint, 3000 kg weight, 650 W power, ∼ 1.2 Mb/s bandwidth.

I Large field of view (∼ 2.4 sr): observing 20% of the sky at any time, or the entire sky for 30 minutesevery 3 hours.

I 85% duty cycle (dictated by the SAA), long observation time (10 years goal).

“It uses less power than a toaster and we talk to it over a telephone line.” (Bill Atwood)

Silicon Tracker

I Silicon strip detectors, Wconversion foils; 1.5 radiationlengths on-axis.

I 10k sensors, 73 m2 of siliconactive area, 1M readoutchannels.

I High-precision tracking, shortinstrumental dead time.

Anti-Coincidence Detector

I Segmented (89 tiles) tominimize self-veto at highenergy.

I 0.9997 average efficiency(8 fiber ribbons coveringgaps between tiles).

Calorimeter

I 1536 CsI(Tl) crystal; 8.6 radiationlengths on-axis.

I Hodoscopic, 3D shower profilereconstruction for leakage correction.


Looking at the future: the foggy part

sr)2Acceptance (m

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AGILE

AMS01

AMS02 AMS02 (CAL only)

ATIC

BESS

BESS-Polar

BETS

Buffington 1972-73

CALET (planned 2013)

CAPRICE94

COS-B

DAMPE (planned 2015)

Daugherty 1972

EGRET

Nishimura 1968-79

Fanselow 1965-66

Fermi LAT

Gamma 400 (planned 2018)

Golden 1976 Hartman 1977

HEAT

HERD (planned 2020)

H.E.S.S.

MASS

Meegan 1969-73

Muller 1984

PAMELA

PPB-BETS

Silverberg 1969-72

Tang 1980TS93

CalorimetricSpace-based

Balloon

SpectrometersSpace-based

Balloon

I Most of the forthcoming instruments focus on energy resolution.I And do it for good reasons. . .I . . . but keep in mind: there’s a trade-off with the acceptance.


How many high-energy gamma rays out there?

Rigidity (GV) or Energy (GeV)-110 1 10 210 310

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410Primary protonsAMS01 (Aguilar et al. 2002)Pamela (Adriani et al. 2011)ATIC-2 (Panov et al. 2006)CREAM (Yoon et al. 2011)

Primary electronsAMS01 (Aguilar et al. 2002)Pamela (Adriani et al. 2011)Fermi LAT (Abdo et al. 2010)H.E.S.S. (Aharonian et al. 2008)

Primary positronsPamela (Adriani et al. 2008)

Gamma-ray all-sky intensityFermi LAT (unpublished)

Gamma-ray extragalactic diffuseFermi LAT (Abdo et al. 2010)

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1 particle/hour

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I ∼ 1 γ ray/week (from anywhere in the sky) traversing Fermi.I You really need as much acceptance as possible to study high-energy

gamma rays in space!


il cielo dei raggi gamma - dipartimento di fisica ... · luca baldini (unipi and infn) pisa, april...

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