Observing TeV Gamma Rays from the Jet Interaction Regions of SS 433 with HAWC

Observing TeV Gamma Rays from the Jet Interaction Regions of SS 433 with HAWC Chang Dong Rho University of Rochester TeVPA 2018 Berlin, Germany 08/28/2018

Overview 2 Microquasars as sources of TeV gamma rays HAWC detector Observation of TeV gamma rays from SS 433 interaction regions with HAWC Theoretical interpretation u Electron population with energies of 1 PeV. u How these particles are being produced? ² Note: results are under embargo so please refrain from posting about them on social media.

Microquasars 3 Compact Binaries are important astrophysical laboratories. They can be predictable, or flare unpredictably. Some binaries are microquasars, with accretion disks that can emit X-rays and γ rays. Microquasars also exhibit relativistic jets where particle acceleration can occur. Microquasars have been one of the candidates as possible Galactic cosmic-ray accelerators. I. F. Mirabel, 2006, Science They are also interesting scale models of AGN which we can observe at close range.

High Altitude Water Cherenkov (HAWC) Observatory Latitude of 19 N, altitude of 4,100m Sierra Negra near Puebla, Mexico 300 WCDs effective area of 22,000m2 2 sr F.O.V. and >95% duty cycle 300 GeV 100 TeV SS 433 transits 15 from zenith of HAWC every day 4

High Altitude Water Cherenkov (HAWC) Observatory Latitude of 19 N, altitude of 4,100m Sierra Negra near Puebla, Mexico 300 WCDs effective area of 22,000m2 2 sr F.O.V. and >95% duty cycle 300 GeV 100 TeV SS 433 transits 15 from zenith of HAWC every day 5

6 SS 433 IACTs SS 433: Known Galactic microquasar observed in radio / X-rays; jets terminate in nebula W50, producing Xray lobes (east and west). Strong TeV candidate, no discovery claimed before now. VERITAS: ~4σ pre-trial excess in termination regions (X-ray lobes). MAGIC + H.E.S.S.: no evidence of VHE γ-ray emission found. Upper limits computed for combined dataset. HAWC: extended elliptical hotspot around the location of SS 433 in 17 mo. data. Sig. increase in newer data. HAWC observation is interesting because previous searches for γ-ray emission from hotspots between 100 GeV and 10 TeV gave no detection.

Systematics: effect of GDE vs. semi-circular RoI; modeling of J1908 (diffusion vs. Gaussian vs. power law) HAWC SS 433 in 1017d of Data 7 Raw Map MGRO J1908+06 PRELIMINARY SS 433 Huge, extended blob is MGRO J1908+06 (J1908) in the center. Grey contours: X-ray emission from SS 433 east lobe, west lobe and the central binary. HAWC sees two hot spots to the either side of the known location of SS 433, spatially in coincidence with the X-ray contours. Simultaneously fit free parameters: 3 flux norms (J1908, east lobe, west lobe); 1 spectral index (J1908); 1 extent (J1908). Semi-circular RoI to reduce contamination from Galactic Diffuse Emission (GDE). Note: the source subtraction shown in steps but the fits are done simultaneously.

HAWC SS 433 in 1017d of Data 8 Res Map PRELIMINARY Simple power law spectrum with fixed index of -2.0 has been assumed for both lobes. Used nested models: TS(J1908 + lobes) TS(J1908) to separate the lobes from J1908. Figure on the left is the residual map after fitting and subtracting J1908 with X-ray termination regions marked e1, e2, e3 for east lobe and w1, w2 for west lobe. Hotspots outside the RoI can be ignored (Galactic Plane). The pre-trial significance distribution shows improvement by removing J1908 but highsignificance tail still exists.

HAWC SS 433 in 1017d of Data 9 Res Map PRELIMINARY PRELIMINARY

HAWC SS 433 in 1017d of Data 10 Res Map PRELIMINARY Figure on the left now shows residual map after subtracting the lobes as well as J1908. The residual significance distribution is now a zero-mean Gaussian, consistent with our background-only distribution. The nested fit of east and west lobes gives 5.4σ post-trial with HAWC s 1017 days of dataset at e1 and w1.

HAWC SS 433 in 1017d of Data 11 Res Map PRELIMINARY PRELIMINARY

Key Issues 12 This is the first time astrophysical jets have been spatially resolved at such high energies. But there still are many questions: Ø Composition and spectrum of the particles generating the gamma rays: hadronic (π 0 decay) or leptonic (IC) origin? Ø Acceleration in magnetic fields or by standing shocks? Ø Is there enough energy to accelerate high-energy particles?

Broadband Spectral Energy Distrib. of e1 13 Leptonic: radio + X-ray photons are produced via synchrotron emission in a magnetic field and TeV γ rays observed by HAWC are produced via IC scattering of the same e -. Multiwavelength spectral fit (solid + dashed line) of leptonic scenario assumes dn de E α exp( E ), E max in a magnetic field of strength B. We inferred E max > 1 PeV. K. Fang

Key Points 14 Leptonic model does a good job of explaining the gamma ray emission, requires ~0.5% of jet power going into electron acceleration. Acceleration is occurring in the jets, not in the central binary: 1. Emission region is ~40 pc from central binary. 2. Diffusion length scale is ~35 pc at these energies, assuming ISM diffusion coefficient (which may be much too large in this region). 3. Advection length scale is ~4 pc. HAWC observation disfavors hadronic-only scenario because: 1. With ISM diffusion coefficient, protons would have spread to a few degrees. 2. Hadronic-only scenario can hardly meet the energy budget; ~100% of jet energy must go into accelerating protons to explain the observed gamma-ray emission. Note: We are not ruling out cosmic-ray acceleration entirely, but data do not support a purely hadronic origin for the gamma rays.

v But, even with an extremely small diffusion coefficient and a very hard spectral index, the hadronic-only scenario requires > 30% of jet power going into protons. Fraction of Jet Power Hadronic Scenario 15 The blue region shows the energy injection rate of protons in order to produce the observed HAWC results. Anything above the red line is not allowed since that would require jet power >100%. H. Zhang The dashed grey lines are for source lifetime and confinement time of 200TeV protons in a 30pc region in the ISM.

Acceleration Power? 16 SS 433 is an object which we expect particle acceleration: presence of jets and interaction regions make it a good candidate accelerator. But does this system have the power to produce ~1 PeV electrons? Acceleration in magnetic fields: possible up to a few hundred TeV. Above that, acceleration time exceeds cooling time for 16 µg fields. Acceleration in standing shocks (Fermi acceleration): can reach PeV energies, but at present no multiwavelength evidence for large shocks in the interaction regions. Explaining the emission from SS 433 is a challenge for current acceleration models!

Summary 17 HAWC has observed SS 433 jet lobes at its known X-ray interaction regions, e1 and w1, with > 5σ post-trial significance (combined). There still are properties that we do not fully understand (e.g. acceleration mechanism) Spectra of SS 433 jet lobes with future measurements from HAWC. Paper Status (Nature, publication date: early October) Please do not share these results yet! (Nature embargo)

Reference & Acknowledgements 18 1. Mirabel, I. F., 2006, Science, 312, 1759 2. VERITAS Observations of High-Mass X-Ray Binary SS 433, Kar, P. et al., 2017, Proceeding for ICRC 2017 3. Constraints on Particle Acceleration in SS433/W50 from MAGIC and H.E.S.S. Observations, Ahnen, M. L. et al., 2018, Astron. Astrophys. 612, A14 4. ROSAT observations of the W 50/SS 433 system, Brinkmann, W., Aschenbach, B. & Kawai, N., 1996, Astron. Astrophys. 312, 306 316 Thanks to HAWC members. Also, Ke Fang (UMD), Haocheng Zhang (LANL) and Hui Li (LANL) for the modeling of leptonic and hadronic emission.

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Abstract

Source Fitting 21 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). dn de = A E α E piv 3. Forward fold a model through detector response function. 4. Find the free model parameters that maximize maximum likelihood and calculate the likelihood ratio (TS) and statistical significance. Sig ~ TS = 2 ln L N obs ln L N obs! θalt TS! ( ( ) ln L( N obs θ0 ))! θ 9 m j,b ( ) = ln P N obs B=1 j=1! θ ( )