The gamma-ray flux from millisecond pulsars in dwarf spheroidal galaxies Justin Vandenbroucke, Keith Bechtol, Miles Winter (UW Madison) Gabrijela Zaharijas (University of Nova Gorica) TeVPA, CERN, September 12, 2016 arXiv:1607.06390 Outline • Motivation: dark matter search (inc. testing the Galactic Center excess) with dwarf spheroidal galaxies • If a signal is detected from dwarf galaxies, is there an astrophysical background? • Two mechanisms of millisecond pulsar (MSP) formation • MSP luminosity function • Predicted number of MSPs and gamma-ray flux from individual dwarf galaxies • Conclusion TeVPA 2016 Justin Vandenbroucke: millisecond pulsars in dwarf galaxies 2 The GeV excess from the Galactic center A. Albert for LAT Collaboration Daylan et al. 2016 IAU GC Symposium 2016 Dark matter? Millisecond pulsars? Something else? See Thursday afternoon session TeVPA 2016 Justin Vandenbroucke: millisecond pulsars in dwarf galaxies 3 Latest dark matter upper limits from dwarf galaxies 23 10− Pass 8 Combined dSphs Fermi-LAT MW Halo H.E.S.S. GC Halo 24 MAGIC Segue 1 10− Abazajian et al. 2014 (1σ) ) 1 Gordon & Macias 2013 (2σ) − s 3 Daylan et al. 2014 (2σ) 25 10− Calore et al. 2015 (2σ) (cm i v σ h Thermal Relic Cross Section 26 (Steigman et al. 2012) 10− 25 dwarf galaxies LAT collaboration, PRL 115 231301 (2015) b¯b 27 10− 101 102 103 104 DM Mass (GeV/c2) • Tension between dwarf limits and Galactic Center excess • Will a signal soon emerge from dwarf galaxies? • Would it be a smoking gun? TeVPA 2016 Justin Vandenbroucke: millisecond pulsars in dwarf galaxies 4 Additional dwarf galaxies continue to be discovered A. Drlica-Wagner et al. ApJ 813:109 (2015) 27 previously known Discovered in past 2 years: 17 by DES and 5 by others • DES much deeper than previous searches • DES (and SkyMapper, Pan-STARRS, LSST) will continue to provide new dwarfs, potentially with excellent J factors TeVPA 2016 Justin Vandenbroucke: millisecond pulsars in dwarf galaxies 5 Astrophysical backgrounds must exist from dwarf galaxies: are they negligible? • Stacked dwarf galaxy dark matter sensitivity is comparable to the Galactic Center, and growing as more galaxies are discovered • If a signal is seen from dwarf galaxies, is it confirmation of Galactic Center excess? • Or will we repeat the same discussions of dark matter vs. pulsars vs. other? • Be ready for this possibility by quantifying astrophysical backgrounds now TeVPA 2016 Justin Vandenbroucke: millisecond pulsars in dwarf galaxies 6 How do millisecond pulsars form? • Primordial channel – Binary is formed during stellar birth, before one becomes neutron star – Rate proportional to stellar density – Dominant in low stellar densities: Milky Way disk, dwarf galaxies • Dynamical channel – Existing neutron star is captured into orbit of companion – Rate proportional to stellar density squared – Dominant in high stellar densities: globular clusters, perhaps Milky Way bulge TeVPA 2016 Justin Vandenbroucke: millisecond pulsars in dwarf galaxies 7 Luminosity distribution of LAT millisecond pulsars 16 2PC Distances 14 ATNF Distances 12 10 8 Counts 6 4 2 0.1-100 GeV 0 1031 1032 1033 1034 1035 1036 Lγ (erg/s) 66 MSPs with distance estimates and not in globular clusters TeVPA 2016 Justin Vandenbroucke: millisecond pulsars in dwarf galaxies 8 Accounting for completeness fraction in LAT detection of Galactic millisecond pulsars Detected MSPs are nearby: Monte Carlo of true distribution: LAT 2PC • Model density of Milky Way MSPs as exponential in R and z • Scale height z0 = 0.6 (+0.6, -0.3) kpc • Scale radius R0 = 3 (+3, -1) kpc • Generate MSPs according to spatial model and determine which fraction would be detected by LAT • Repeat for many spatial models according to uncertainty in (z0, R0) TeVPA 2016 Justin Vandenbroucke: millisecond pulsars in dwarf galaxies 9 Calculated completeness fraction for LAT millisecond pulsar detection 100 1 10− 2 10− 3 10− 4 10− Completeness Fraction 5 10− median central 68% 10 6 − 1032 1033 1034 1035 Lγ (erg/s) TeVPA 2016 Justin Vandenbroucke: millisecond pulsars in dwarf galaxies 10 Accounting for completeness fraction to determine luminosity function for Milky Way stellar mass 1039 1038 1037 (erg/s) γ 1036 dN/dL 2 γ L 1035 best-fit LF (this study) statistical uncertainty systematic uncertainty 34 10 Hooper & Mohlabeng (2016) LF: fixed lum. Hooper & Mohlabeng (2016) LF: fixed MSP LAT MSPs: 2PC dist. w/ mean completeness 1033 1031 1032 1033 1034 1035 1036 Lγ (erg/s) TeVPA 2016 Justin Vandenbroucke: millisecond pulsars in dwarf galaxies 11 What about the chance occurrence of a foreground MSP along the line of sight to a dwarf galaxy? • Estimated using the same spatial distribution and Milky Way luminosity function • Used Monte Carlo to determine probability of one MSP lying along line of sight, and if it occurs what is the PDf for its flux • Most often there is no MSP along line of sight • When there is one, it is typically brighter than MSP emission from dwarf TeVPA 2016 Justin Vandenbroucke: millisecond pulsars in dwarf galaxies 12 100 100 Segue 1 Draco Foreground Foreground 1 dSph 1 dSph 10− 10− (l,b)=(220.5,50.4) (l,b)=(86.4,34.7) <NMSP >=0.004 <NMSP >=6.1 2 2 10− 10− 3 Underflow Bin 3 Underflow Bin 10− 10− Fraction Per Bin 4 Fraction Per Bin 4 10− 10− 5 5 10− 16 14 12 10 8 10− 16 14 12 10 8 10− 10− 10− 10− 10− 10− 10− 10− 10− 10− 2 1 2 1 Flux > 500 MeV (ph cm− s− ) Flux > 500 MeV (ph cm− s− ) Results: fornax 100 100 Sculptor LAT 6 year Fornax Foreground Foreground 1 dSph 1 threshold dSph 10− 10− (l,b)=(287.5,-83.2) (l,b)=(237.1,-65.7) <NMSP >= 44.2 <NMSP >= 278.7 2 2 10− 10− 3 Underflow Bin 3 Underflow Bin 10− 10− Fraction Per Bin 4 Fraction Per Bin 4 10− 10− 5 5 10− 16 14 12 10 8 10− 16 14 12 10 8 10− 10− 10− 10− 10− 10− 10− 10− 10− 10− 2 1 2 1 Flux > 500 MeV (ph cm− s− ) Flux > 500 MeV (ph cm− s− ) • Underflow bin: no foreground MSP • Expect on average ~300 MSPs with luminosity above 1031.5 erg/s • Brightest expected MSP flux among all dwarfs investigated • Still below current LAT sensitivity and Galactic center DM model prediction • Very small probability of a foreground source, but if there is one it is likely to be brighter than MSP signal from Fornax TeVPA 2016 Justin Vandenbroucke: millisecond pulsars in dwarf galaxies 13 100 100 Segue 1 Draco Foreground Foreground 1 dSph 1 dSph 10− 10− (l,b)=(220.5,50.4) (l,b)=(86.4,34.7) <NMSP >=0.004 <NMSP >=6.1 2 2 10− 10− 3 Underflow Bin 3 Underflow Bin 10− 10− Fraction Per Bin 4 Fraction Per Bin 4 10− 10− 5 5 10− 16 14 12 10 8 10− 16 14 12 10 8 10− 10− 10− 10− 10− 10− 10− 10− 10− 10− 2 1 2 1 Flux > 500 MeV (ph cm− s− ) Flux > 500 MeV (ph cm− s− ) Results: Sculptor 100 100 Sculptor Fornax LAT 6 year Foreground Foreground 1 threshold dSph 1 dSph 10− 10− (l,b)=(287.5,-83.2) (l,b)=(237.1,-65.7) <NMSP >= 44.2 <NMSP >= 278.7 2 2 10− 10− 3 Underflow Bin 3 Underflow Bin 10− 10− Fraction Per Bin 4 Fraction Per Bin 4 10− 10− 5 5 10− 16 14 12 10 8 10− 16 14 12 10 8 10− 10− 10− 10− 10− 10− 10− 10− 10− 10− 2 1 2 1 Flux > 500 MeV (ph cm− s− ) Flux > 500 MeV (ph cm− s− ) • Underflow bin: probability of there being no foreground MSP • Expect on average ~40 MSPs with luminosity above 1031.5 erg/s in Sculptor • Very small probability of a foreground source, but if there is one it is likely to be brighter than MSP signal from Sculptor TeVPA 2016 Justin Vandenbroucke: millisecond pulsars in dwarf galaxies 14 Results: Draco 100 100 Segue 1 LAT 6 year Draco Foreground threshold Foreground 1 dSph 1 dSph 10− 10− (l,b)=(220.5,50.4) (l,b)=(86.4,34.7) <NMSP >=0.004 <NMSP >=6.1 2 2 10− 10− 3 Underflow Bin 3 Underflow Bin 10− 10− Fraction Per Bin 4 Fraction Per Bin 4 10− 10− 5 5 10− 16 14 12 10 8 10− 16 14 12 10 8 10− 10− 10− 10− 10− 10− 10− 10− 10− 10− 2 1 2 1 Flux > 500 MeV (ph cm− s− ) Flux > 500 MeV (ph cm− s− ) • Underflow bin: probability of there being no foreground MSP (or MSP in dSph) 100 • Expect on average100 ~6 MSPs with luminosity above 1031.5 erg/s in Draco Sculptor Fornax Foreground• Moderate Poisson probability of zero MSPs in DracoForeground 1 dSph 1 dSph 10− • Very small probability10− of a foreground source, but if there is one it is likely to be (l,b)=(287.5,-83.2)brighter than MSP signal from Fornax (l,b)=(237.1,-65.7) <NMSP >= 44.2 <NMSP >= 278.7 2 2 10− 10− TeVPA 2016 Justin Vandenbroucke: millisecond pulsars in dwarf galaxies 15 3 Underflow Bin 3 Underflow Bin 10− 10− Fraction Per Bin 4 Fraction Per Bin 4 10− 10− 5 5 10− 16 14 12 10 8 10− 16 14 12 10 8 10− 10− 10− 10− 10− 10− 10− 10− 10− 10− 2 1 2 1 Flux > 500 MeV (ph cm− s− ) Flux > 500 MeV (ph cm− s− ) Results: Segue 1 100 100 LAT 6 year Segue 1 Draco threshold Foreground Foreground 1 dSph 1 dSph 10− 10− (l,b)=(220.5,50.4) (l,b)=(86.4,34.7) <NMSP >=0.004 <NMSP >=6.1 2 2 10− 10− 3 Underflow Bin 3 Underflow Bin 10− 10− Fraction Per Bin 4 Fraction Per Bin 4 10− 10− 5 5 10− 16 14 12 10 8 10− 16 14 12 10 8 10− 10− 10− 10− 10− 10− 10− 10− 10− 10− 2 1 2 1 Flux > 500 MeV (ph cm− s− ) Flux > 500 MeV (ph cm− s− ) • Underflow bin: probability100 of there being no foreground MSP (or MSP in dSph) 100 • Expect on average 0.004 MSPs with luminosity aboveSculptor 1031.5 erg/s in Segue 1 Fornax • Most likely to have zero MSPs above 1031 erg/s Foreground Foreground 1 dSph 1 dSph 10− 10− (l,b)=(287.5,-83.2) (l,b)=(237.1,-65.7) <NMSP >= 44.2 <NMSP >= 278.7 TeVPA 2016 Justin Vandenbroucke: millisecond pulsars in dwarf galaxies2 2 16 10− 10− 3 Underflow Bin 3 Underflow Bin 10− 10− Fraction Per Bin 4 Fraction Per Bin 4 10− 10− 5 5 10− 16 14 12 10 8 10− 16 14 12 10 8 10− 10− 10− 10− 10− 10− 10− 10− 10− 10− 2 1 2 1 Flux > 500 MeV (ph cm− s− ) Flux > 500 MeV (ph cm− s− ) Calculated MSP flux for 30 individual dwarf Millisecond Pulsars in Dwarf Spheroidal Galaxies 7 galaxies/candidates with good stellar mass estimatesTABLE 1 Estimated GeV Contribution from MSPs in MW dSphs 2 1 GeV2 a Galaxy D(kpc) log10(M /M )Flux> 500 MeV (ph cm− s− )log10 J 5 Ref.