T h e F e r mi G e V e x c e s s S i g n a l o r b a c k g r o u n d f o r D M s e a r c h e s ? → F . C a l o r e , I . C h o l i s & C W [ 1 4 0 9 . 0 0 4 2 ] , a c c e p t e d b y J C A P F . C a l o r e , I . C h o l i s , C . Mc C a b e & C W [ 1 4 1 1 . 4 6 4 7 ] , a c c e p t e d b y P R D S . C a r o n , A . A c h t e r b e r g , L . H e n d r i k s , R . R u i z d e A u s t r i & C W [ 1 5 0 2 . 0 5 7 0 3 ] C h r i s t o p h We n i g e r MIAPP Workshop, Munich, 24/02/2015 1
O v e r v i e w Introduction History of the “Fermi GeV Excess” Robust identification at higher latitudes Implications & ways forward Conclusions 2
I n t r o d u c t i o n 3
I n d i r e c t S e a r c h e s f o r D a r k Ma t t e r Gamma rays ● Very simple propagation (geodesics) ● Absorption negligible on Galactic scales ● Point towards their sources ? B-field Today's dark matter annihilation cross-section is roughly given by Charged cosmic rays ● Electrons/positrons, nuclei ● Propagation distorted by Conditions during freeze-out are galactic magnetic fields very different from today: The ● Sizable energy losses & velocity averaged annihilation interactions cross-sections can differ by orders of magnitude. Neutrinos ● Simple propagation ● But: hard to measure 4
D a r k Ma t t e r S i g n a l F l u x Velocity averaged annihilation cross-section Photon energy spectrum per annihilation Signal intensity: [photon flux per steradian per energy] Dark matter mass density Dark matter mass Line-of-sight integral Particle Physics Astrophysics Characteristic Morphology Characteristic Energy Spectrum (point-like, extended or diffuse) [review DM searches with gamma It is convenient to define a “J-value”: rays: Bringmann & Weniger (2012)] 5
D M a n n i h i l a t i o n p r o c e s s e s Gamma-ray lines: Bremsstrahlung: Two-body annihilation into Photon production in “hard photons process” DM DM DM DM Box-shaped spectra: Continuum emission: Photons from cascade decay Photons from neutral pion decay DM DM DM DM 6
E n e r g y s p e c t r u m o f p h o t o n s f r o m D M a n n i h i l a t i o n Continuum emission Gamma-ray lines Box-shaped spectra Internal Bremsstrahlung (IB) ● from two-body ● Cascade-decay into ● radiative correction to annihilation into photons monochromatic photons processes with charged final ● forbidden at tree-leve, ● already at tree level states ● Generically suppressed by generically suppressed by O( α² ) O( α ) 7
A n a l y t i c a l D a r k ma t t e r d e n s i t y p r o f i l e s The DM distribution very close (<1kpc) to the Galactic center is observationally only poorly constrained. Viable DM density profiles: Signal morphology (including substructure enhancement): [Pieri+ 0908.0195] [Cirelli et al. (2010)] 8
P o t e n t i a l t a r g e t s f o r s e a r c h e s wi t h p h o t o n s Signal is approx. proportional to column square density of DM: Point-like: Extended or diffuse: (for observations with (for observations with gamma rays) gamma rays) Galactic center (~8.5 kpc) - brightest DM source in sky Galactic DM halo - but: bright backgrounds - good S/N - difficult backgrounds - angular information DM clumps Extragalactic - w/o baryons - nearly isotropic - bright enough? - only visible close to - boost overall signal Galactic poles - angular information - Galaxy clusters? Dwarf Spheroidal Galaxies [review on N-body simulations: Kuhlen, - harbour small number of stars Vogelsberger & Angulo (2012)] - otherwise dark (no gamma-ray emission) 9
R e g i o n s o f i n t e r e s t ( ma x i mu m S / N ) Galactic Signal center Background DM profile Region with optimal signal/noise: 10
G a l a c t i c c e n t e r a n a l y s i s Constraints on WIMP Annihilation for Contracted Dark Matter in the Inner Galaxy with the Fermi-LAT [Gomez-Vargas+ 1308.3515] Constraints on the Galactic Halo Dark Matter from Fermi-LAT diffuse measurements [Ackermann+ 1205.6474] Tavakoli+ 2014; Gomez-Vargas+ 2014; Ackermann+ 2011; Hooper & Linden 2011 11
S t a c k i n g d wa r f s 12
100 GeV!!! [Slide from Matthew Wood, 14.10.14 SLAC, presented on 5 th Fermi Symposium] 13
H . E . S . S . o b s e r v a t i o n s o f G a l a c t i c c e n t e r arXiv:1103.3266 For Atmospheric Cherenkov Telescopes , the backgrounds are completely dominated by Signal region unrejected electron and proton Crs → Isotropic backgrounds! Limits on a WIMP annihilation signal Flux from search region (green) compared to flux from background region (red). → the fluxes are consistent → upper limits on DM signal. Background Abramowski et al. 2011 Limits hold only for cuspy profiles! Fermi dwarfs Thermal cross-section ` 14
T h e F e r mi G e V E x c e s s 15
F i r s t a p p e a r a n c e i n 2 0 0 9 First clear statements about properties of excess emission (morphology, spectrum etc, subject to some changes in later analyses): First very cautious comments by the LAT team, without any detailed characterization of the residual : 16
F o l l o w u p s t u d i e s At the Galactic center (roughly 7deg x 7deg) Goodenough & Hooper 2009 Hooper & Goodenough 2011 Hooper & Linden 2011 Boyarsky+ 2011 Abazajian & Kaplinghat 2012 Gordon & Macias 2013 Macias & Gordon 2014 Abazajian+ 2014 Daylan+2014 [Daylan+ 2014] In the inner Galaxy (roughly |b|>1 deg to tens of deg) Hooper & Slatyer 2013 Huang+ 2013 Zhou+ 2014 Daylan+ 2014 [Hooper & Slatyer 2013] 17
N o n - e x p l a n a t i o n s GeV emission from HESS GC HESS Hooper & Goodenough 1010.2752; Boyarsky+ 1012.5839 ● Emission is not a point source 21cm radio continuum, GBT Bremsstrahlung of CR electrons from GC on molecular gas Yusuf-Zadeh+ 1206.6882; Gordon & Macias 1306.5725; Macias & Gordon 1312.6671; Abazajian+ 1402.4090 ● Emission extends outside of the inner 250 pc (central molecular zone), and is not correlated with molecular gas maps Neutral pion decay from proton interaction with molecular gas Linden+ 1203.3539; Abazajian+ 1207.6047; Abazajian+ 1402.4090 ● Requires proton spectrum with break to produce spectrum Yusef-Zadeh+ 2012 ● Gives rise to wrong morphology Point source subtraction, general background systematics, etc Boyarsky+ 1012.5939 ● Proven not relevant in subsequent studies 18
P o s s i b l e A s t r o p h y s i c a l i n t e r p r e t a t i o n s Milli-second pulsars: ● Spectrum of known MSPs agrees reasonably well with claimed GCE spectrum (except at sub-GeV energies) ● Observed luminosity function is claimed to be incompatible with GCE (we don't see resolved MSPs at GC) Hooper+, Calore+, Cholis+ 2013 ● Compatible with distribution of low-mass X-ray binaries (possible MSP progenitors) Cholis+ 2014 Contribution from MSPs to GC flux, Wang, Jiang, Cheng astro-ph/0501245 Consistency of inner Galaxy observations with MSPs, Abazajian, 1011.4275 Abazajian, Kaplinghat, 1207.6047 “Pulsars cannot account for the Inner Galaxy's GeV Excess”, Hooper, Cholis, Linden, Siegal-Gaskins, Slatyer, 1305.0830 Could be new population of MSPs, Mirabal, 1309.3428 Abazajian, Canac, Horiuchi, Kaplinghat 1402.4090 Very broad population study, Yuan and Zhang, 1404.2318 Disk population cannot account for GC MSPs, Calore, Di Mauro, Donato, 1406.2706 Challenges for MSPs, Cholis, Hooper, Linden, 1407.5625 Population study and importance of ICS, Petrovic, Serpico, Zaharijas, 1411.2980 VHE gamma rays, Yuan and Ioka, 1411.4363 Statistical analysis, Mirabal, 1411.7410 LMXB population in M31, Voss, Gilfanov, astro-ph/0610649 Gordon, Macias, 1306.5725 19
P o s s i b l e A s t r o p h y s i c a l i n t e r p r e t a t i o n s Hadronic activity: Carlson+ 2014 ● Recent injection of protons at Galactic center, 100 Kyr – 2 Myr ago ● Diffusion causes approx. spherical CR distribution, but target material is not spherical Leptonic activity: Petrovic+ 2014 ● Recent injection of hard electrons at Galactic center, ~1 Myr ago ● Diffusion → approx. spherical profile & emission ● Can “naturally” explain peaked spectrum ● The morphology, especially emission above 10 deg (1.5 kpc) is hard to reproduce, since the energy loss time of electrons is < 1 Myr. 20
D a r k Ma t t e r A n n i h i l a t i o n Previous results were inconsistent References : Goodenough & Hooper 0910.2998; Hooper & Goodenough 1010.2725; Hooper & Linden 1110.0006; Abazajian & Kaplinghat 1207.6047; Hooper & Slatyer 1302.6589; Gordon & Macias 1306.5725; Huang+ 1307.6862; Abazajian+ 1402.4090; Daylan+ 1402.6703; Abazajian+ 1410.6168; Calore+ 1409.0042; Calore+ 1411.4647; S. Murgia Fermi Symposium 2014 + >O(100) models that explain the excess. 21
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