Searching for TeV Gamma-ray Emission from Binary Systems with HAWC - PowerPoint PPT Presentation

Searching for TeV Gamma-ray Emission from Binary Systems with HAWC Chang Dong Rho University of Rochester ICRC 2017 BEXCO, Busan

2 Overview • γ -ray physics & γ -ray binary systems • The HAWC Observatory • Searches for emission from compact TeV binary systems (with highlighted results) • Closer look at HESS J0632+057

3 Why High-Energy γ rays? • γ rays are the most energetic form of EM radiation and have no electric charge. Greg Vance • There are multiple ways to generate them: 1. π 0 è γ + γ (hadronic) 2. e - + γ è e - + γ * (leptonic) • The hadronic process is seen in very high energy emissions and can tell us about CR. • Study new physics under extreme conditions (e.g. GRBs, pulsars, … ). CHANDRA

4 Observing γ rays with Air Showers • To observe γ rays at Earth, we can use air showers since they are absorbed in atmosphere: 1. Record Cherenkov light produced by charged particles in air showers (IACTs). 2. Sample charged particles at ground level (HAWC).

4 Observing γ rays with Air Showers • CR and γ rays are absorbed in the atmosphere, EM interactions! so we use air showers: 1. Record Cherenkov light produced by charged particles in air showers (IACTs). nuclear interactions! 2. Sample charged particles at ground level (HAWC).

High Altitude Water Cherenkov 5 (HAWC) Observatory • Latitude of 19°N, altitude of 4,100m • Sierra Negra near Puebla, Mexico • 300 WCDs – effective area of 22,000m 2 • 2 sr F.O.V. and >95% duty cycle • 300 GeV – 100 TeV

High Altitude Water Cherenkov 5 (HAWC) Observatory • Latitude of 19°N, altitude of 4,100m • Sierra Negra near Puebla, Mexico • 300 WCDs – effective area of 22,000m 2 • 2 sr F.O.V. and >95% duty cycle • 300 GeV – 100 TeV

TeV γ -ray Binary Sources 6 • Binaries are unusual since it is rare to have a natural mechanism that repeatedly accelerates particles. • There are many confirmed radio and X-ray binaries but only 5 γ -ray binaries (PSR B1259-63, LS 5039, LS I +61 303, HESS J0632+057, 1FGL J1018.6-5856, HESS J1832-093(?)) have been observed. • All 5 γ -ray binaries have been observed in TeV as point-like sources. • γ -ray binaries consist of compact Galactic objects in orbit with massive companion stars. • Do not fully understand the mechanism of γ -ray production and have unexplained mismatches in observations at different energy bands.

TeV γ -ray Binary Sources 6 • Binaries are unusual since it is rare to have a natural mechanism that repeatedly accelerates particles. • There are many confirmed radio and X-ray binaries but only 5 γ -ray binaries (PSR B1259-63, LS 5039, LS I +61 303, HESS J0632+057, 1FGL J1018.6-5856, HESS J1832-093(?)) have been observed. • All 5 γ -ray binaries have been observed in TeV as point-like sources. • γ -ray binaries consist of compact Galactic objects in orbit with massive companion stars. I. F. Mirabel • Do not fully understand the mechanism of γ -ray production and have unexplained mismatches in observations at different energy bands.

Table of Binary Candidates 7 • We looked at the following γ -ray binary candidates: – 3 known γ -ray binaries in HAWC FOV (red) – 28 XRBs with short orbital periods • I calculated TS after fitting a power law with a fixed idx of -2.7 and E piv at 7 TeV. • Then, post-trial significances are calculated for each of the sources (< 2 σ UL; > 2 σ LC).

95% UL Fluxes for 25 8 Candidates PRELIMINARY

95% UL vs. Dec with 9 Sensitivit y PRELIMINARY

10 HESS J0632+057

11 HESS J0632+057 • First discovered as a TeV source by H.E.S.S. in 2007. • Variability later found in X-rays (P orb = 321 ± 5 days) then also observed in TeV (P orb = 315 ± 5 days). • Only γ -ray binary observed by all three major IACTs (H.E.S.S., VERITAS & MAGIC). • No HE observation by Fermi/LAT.

11 HESS J0632+057 • First discovered as a TeV source by H.E.S.S. in 2007. PRELIMINARY • Variability later found in X-rays (P orb = 321 ± 5 days) then also observed in TeV (P orb = 315 ± 5 days). A. R. • Only γ -ray binary observed by all three major IACTs (H.E.S.S., VERITAS & MAGIC). • No HE observation by Fermi/LAT.

11 HESS J0632+057 • First discovered as a TeV source by H.E.S.S. in 2007. PRELIMINARY • Variability later found in X-rays (P orb = 321 ± 5 days) then also observed in TeV (P orb = 315 ± 5 days). A. R. • Only γ -ray binary observed by all three major IACTs (H.E.S.S., VERITAS & MAGIC). • No HE observation by Fermi/LAT.

12 17Months – Light Curve of HESS J0632+057 (P orb = 135 days) PRELIMINARY

13 Observation of HESS J0632+057 PRELIMINARY

13 Observation of HESS J0632+057 Power law: ~ 3 years of data PRELIMINARY

13 Observation of HESS J0632+057 Power law: ~ 3 years of data Cutoff 5TeV : ~ 8 years of data PRELIMINARY

14 Summary • Upper limits for 25 TeV binary candidates < 2 sigma. • Light curve analysis on 6 candidates > 2 sigma (no results shown). • Upper limits for HESS J0632+057 computed and presented alongside VERITAS results. We expect to see it with ~3 years of data (power law), ~8 years of data (cutoff @ 5 TeV). • For phase stacking analysis, check C. Brisbois poster (GA231, board 136) .

15 Reference 1. “H.E.S.S. observations of LS 5039”, de Naurois, M. et al., 2007, ApSS, 309, 277-284 2. “Long-term TeV Observations of the Gamma- ray Binary HESS J0632+057 with VERITAS”, Maier, G. et al., 2015, arXiv 1508.05489 3. “IRAS observations of SS 433 and W 50.”, Band, D. L., 1987, PASP, 99, 622, 1269

Back up

24 Source Fitting • To search for γ -ray sources we do spatial + spectral fits to the map: 1. We assume a morphology (shape) for a source (e.g. Point, Disk). 2. We assume a spectrum for a source (e.g. power law, cutoff power law). α ⎛ ⎞ dN E = A ⎜ ⎟ ⎜ ⎟ dE E piv ⎝ ⎠ 3. “Forward fold” a model through detector response. 4. Find the free model parameters that maximize maximum likelihood and calculate the likelihood ratio (TS) and statistical significance. Sig ~ TS

25 Maps • To search for γ -ray sources, a data map is generated and compared with a background map to remove CR that have survived the γ – hadron separation. • (raw) Data map contains photon counts after the γ – hadron separation. • Background map contains CR that passed γ – hadron cuts. • One parameter fit for each pixel. The sqrt of the calculated maximized TS gives significance. • 25 month HAWC data used for analysis. Only 17 months of data available for daily maps (light curve).

26 Likelihood Calculation • Events are binned according to the fHits. • Logarithm of likelihood is computed with the binned data: AllBins ( ) ( ) ∑ ( ) = ( ) i θ ln L θ ; N obs ln f N obs i = 0 • θ that maximizes ln L estimates the optimum set of free parameters. TS is used to compare two hypotheses: ( ) L H 1 ; N obs TS = 2ln ( ) L H 0 ; N obs

27 Forward Folding • Take a theoretical spectrum, smear it, and then compare the result to the data. The best fit gives you the true spectrum. • Compare observed count to expected count in a bin. • But, we use fhit bins (energy variable). – Fundamental problem: data in reconstructed energy space vs. expectations in true energy space • Hence, fold (convolve) the expected counts distributed in true energy space using a model spectrum. • Det res used to compute the expectations in the reconstructed bins given hypothesis (model spectrum, e.g. power law) about the true energy distribution.

28 LS 5039

29 LS 5039 • Identified in 1997 as a massive X- ray binary system with OB star. PRELIMINARY • Detected in 2005 by H.E.S.S. as a TeV binary (P orb = 3.90678 ± 0.0015 days). • Max flux near inferior conjunction. A. R. • GeV observation by Fermi/LAT (P orb = 3.90532 ± 0.0008 days).

30 17Months – Light Curve of LS 5039 PRELIMINARY

31 17Months – Periodogram of LS 5039 Similar calculations were done for the other 5 candidates but no orbital PRELIMINARY modulations were observed. Check C. Brisbois poster (GA231, board 136) for phase stacking analysis.