SPATIAL UNIFORMITY OF THE GALACTIC GAMMA-RAY EXCESS. Manoj Kaplinghat, UC Irvine

Transcription

1 SPATIAL UNIFORMITY OF THE GALACTIC GAMMA-RAY EXCESS Manoj Kaplinghat, UC Irvine

2 Bas Observed Extended Counts Source Model THE GALACTIC CENTER EXCESS GeV GeV GeV GeV Baseline Extended Model Residuals Source Counts * GC excess: Goodenough and Hooper (2009), Hooper and Goodenough (2010), Boyarsky, Malyshev, Ruchayskiy (2010), Hooper and Linden (2011), Abazajian and Kaplinghat (2012), Yusef-Zadeh et al (2012), Gordon and Macias (2013). * Hooper and Slatyer (2013), Daylan et al (2014) and Calore et al (2014) argued that Extended Full Source Model Model Residuals this signal is extended, at least to 10 degrees. FIG. 1. Shown in the top row are photon counts in four energy bins that have significant evidence for an extended source with a spectrum, morphology, and rate consistent with a 30 GeV mass WIMP annihilating to b b-quarks in the 7 7 region

3 Characterizing the Fermi excess in the inner galaxy Galactic Center: Innermost 7x7 deg Inner galaxy: innermost ~20x20 deg, without the Galactic Center

4 ASTROPHYSICAL GAMMA-RAY BACKGROUND MODEL Point sources Stellar remnants Blazars... Supermassive black hole Sag A* Extended sources: Cosmic rays impinging on gas in disk Upscattered starlight Extragalactic background Fermi bubbles

5 EXTRACTING THE GALACTIC CENTER EXCESS + Use Fermi tools for analyzing data. + Point sources refit simultaneously within the ROI GLAT GALACTIC LAT Galactic diffuse +1.4 GHz +Excess model + Free isotropic component GALACTIC LONG. Yusef-Zadeh et al GLON Method based on Abazajian and Kaplinghat 2012 and Abazajian, Canac, Horiuchi and Kaplinghat

6 EXTRACTING THE INNER GALAXY EXCESS + Use Fermi tool Complike2. + Bright point sources refit simultaneously. GLAT Galactic diffuse -50 +Bubble Free isotropic component. + Each energy bin analyzed separately, i.e., only use spatial information Excess model 2 0 GLON Horiuchi, Kaplinghat, Kwa, in prep

7 20 deg 26 deg Use Fermi tool Complike2 to tie astrophysical background model templates across inner galaxy subregions, separately for IC and π 0 +Bremsstrahlung. NFW-like template normalization allowed to vary between regions.

8 The di erential flux for a dark matter cand cross-section h A vi toward Galactic coordinat GAMMA RAYS FROM DARK MATTER ANNIHILATION h d (b, `) de d (b, `) de flux i = h Avi J(b, `) `) 11 dn dn 2 J 0 4 m 2 2 de de mass where dn /de is the gamma-ray spectrum p lation and m is the dark matter particle m quantity J is the cross integrated section mass density squ line-of-sight, x, spectrum Z J(b, `) =J 0 dx 2 (r gal (b, `,x)), where distance from the GC is given by qdark matter density profile r gal (b, `,x)= R 2 2xR cos(`) cos(b)+ Here, J 0 1/[8.5 kpc(0.3 GeV cm 3 ) 2 ] is a

9 COMPARING THE CENTRAL AND MORE EXTENDED REGIONS GC N S 10 6 (JT ot/jroi ) E 2 dn/de [GeV cm 2 s 1 ] NE SW 10 6 SE N2 NW S E [GeV] Good agreement between different regions at the factor of 2 level. Horiuchi, Kaplinghat, Kwa, in prep

10 COMPARING THE CENTRAL AND MORE EXTENDED REGIONS Model A Model E Model F =0.9 Background models (JT ot/jroi ) E 2 dn/de [GeV cm 2 s 1 ] =1.0 =1.1 =1.2 High energy tail of GC vs Inner galaxy spectra! GC prefers γ=1.1 but inner galaxy shows no preference GC Inner galaxy E [GeV] Horiuchi, Kaplinghat, Kwa, in prep Templates ~ r 2γ in the inner part.

11 FERMI-LAT COLLABORATION ANALYSIS Fermi-LAT analysis in 15x15 degree region around GC led by Troy Porter and Simona Murgia (arxiv: ): preference for an extended centrally concentrated component

12 UNRESOLVED POINT SOURCE EXPLANATION OF THE EXCESS? The excess spectrum is similar to the milli-second pulsar (MSP) spectrum [Abazajian 2010]! Both the spectrum and the spatial distribution could be consistent with a population of unresolved MSPs [Abazajian and Kaplinghat (2012)] * For plausibility arguments for and against all of the excess being due to pulsars see Hooper, Cholis, et al (2013), Petrovic et al (2014), Calore et al (2015) and others. They boil down to issues of the luminosity function of MSPs.

13 0.5 MSP ENERGY SPECTRUM 47 Tuc M62 N6440 ωcen Μ28 N6388 N6652 Terzan5 2 dnêde Harbitrary unitsld Globular cluster spectra compared to excess in the inner galaxy (out to 10 degrees) obtained by Daylan et al GeV 2 GeV 8 GeV

14 HYPOTHESIS FOR THE SPATIAL PROFILE OF UNRESOLVED MILLISECOND PULSARS Assume MSPs trace Low Mass X-ray Binaries steepening with respect to Bulge stellar distribution R towards M31 center =1.5 kpc distance from M31 center =10 deg towards MW center Voss+Gilfanov 2007 Orange line is same as best-fit excess template (R 1.2 in projection implies r 2.2 de-projected)

15 HYPOTHESIS FOR THE SPATIAL PROFILE OF UNRESOLVED MILLISECOND PULSARS Assume MSPs trace LMXBs GC signal requires ~ 4000 MSPs if each has the same E>2 GeV luminosity as (1/30) of 47 Tucanae globular cluster. ~30 MSPs in 47 Tuc Created from Voss+Gilfanov 2007 data

16 HAVE THE UNRESOLVED POINT SOURCES BEEN DISCOVERED? b, Gal. latitude [deg] l, Gal. longitude [deg] Bartels et al Lee et al 2015 FIG. 2: (Left) Best-fit source-count functions within 10 of the median and 68% confidence intervals are shown for each of the fo (solid, blue), and isotropic (dotted, green). The number of observed for the di use emission in the fit is consistent with that at high la within 10 of the GC with b 2 arising from the separate PS co the result of removing the NFW PS template from the fit. Dashed Issue: are all of the detected point sources real or are they due to small-scale structure in the diffuse background? See Fermi-LAT paper for a discussion on this issue.

17 FIG. 2. Spectral energy distribution for the GCE modelled with 10 GeV WIMPs annihilating into Model I, democratic leptons ( 1 3 e+ e µ+ µ ). IC and Bremss stand for inverse Compton and bremsstrahlung emission respectively. Black and 3 red error bars refer to the LAT (1 ) statistical and systematic errors, respectively. The fit and plot only consider energy bins with TS 1. Left Panel: shows the results of a bin-by-bin analysis when secondaries di erent morphologies were not POSSIBLE accounted for in determining SECONDARY the bins. Right Panel: EMISSION Displays the results of thefrom bin-by-bin analysis ELECTRONS when the full spectral and spatial information from secondaries was considered. AND POSITRONS New excess E 2 dn/de [GeV cm 2 s 1 ] Millisecond Pulsars Spectra + Spatial Total emission Prompt IC Bremss New excess Abazajian et al 2015 Lacroix et al 2015 Figure 3. Shown here is an example 8 GeV dark matter annihilation model with equal branching to all charged leptons, e ±,µ ±, ±, with the residual spectra of the prompt GCE (blue square), IC (golden triangle), and bremsstrahlung (pink circle) sources. The blue (dashed) GCE spectrum is is determined by the particle mass and annihilation rate fit to the observations. The solid predicted resultant spectra for this annihilation channel s IC (golden) and bremsstrahlung (pink) cases are in solid lines. ULTRACLEAN class photons are used for this analysis. The source was spatial used. profile and spectrum consistent with IC off of starlight due to e + e with E < GeV. The sharp cut-off in energy required distinguishes this from the other cosmic ray components. that absorb the spectral dependence of the excess. The e ± in the products created by dark matter annihilation lose energy through three distinct process [28]: (1) IC, which leads to upscattering of the interstellar radiation field (ISRF) photons, ondary (2) bremsstrahlung emission. (Br) radiation o the gas, and (3) synchrotron radiation in the Galactic magnetic field. We focus on the first two components in this letter. The di erential flux of photons for these two components may be written as, E dn Z Z IC,Br de = C. d Model Z m d` III, dn e de e MSPs dp IC,Br (3.1) FOV 4 LOS E min de e de where FOV and LOS indicate integration over the field-of-view and line-of-sight respectively, dp IC /de and dp Br /de are the di erential power emitted per electron due to IC and bremsstrahlung processes. For bremsstrahlung, we include energy losses from atomic H and He. To get the source energy distribution of electrons, positrons and gamma rays, we use the software PPPC4DMID [29]. The number density of electrons and positrons per unit E [GeV] FIG. 3. Spectral energy distribution for the GCE modelled with Model III (MSPs). This scenario assumes monochromatic injection of e ± at 20 GeV [48]. Line and color conventions used in the panels are the same as in Fig. 2. Left Panel: Results of a bin-by-bin analysis when the di erent spatial morphologies of the three-component source spectra were not considered [47]. Right Panel: Shows the results of a bin-by-bin analysis when the full spectral and spatial information of the three-component As seen from Table I, the spectral-only analysis would not reveal the need for secondary component for the MSP case (Model III ) as the p-value is well above the 10 3 threshold before or after adding the secondaries. This is due to the three parameters of the prompt component However, the broadband analysis results in a high TS for both the total spectrum and the secondaries as shown in Table II. Therefore, this case is distinct from Model I

18 SUMMARY The status of a centrally concentrated extended source that is bright in GeV photons seems secure. Many questions remain: Is it all unresolved point sources? If yes, are they dominantly MSPs? What is the spatial profile of the source in the inner galaxy? Is there a change in the spectrum as one moves away from the GC? Does the spectrum in the inner galaxy extend beyond 10 GeV? How does the new IC-like excess fit in the big picture?