HAWC Search for High-Mass Microquasars
HAWC Collaboration, A. Albert, R. Alfaro, C. Alvarez, J. R. Angeles Camacho, J. C. Arteaga-Velazquez, K. P. Arunbabu, D. Avila Rojas, H. A. Ayala Solares, V. Baghmanyan, E. Belmont-Moreno, S. Y. BenZvi, C. Brisbois, K. S. Caballero-Mora, T. Capistran, A. Carraminana, S. Casanova, U. Cotti, J. Cotzomi, E. De la Fuente, C. de Leon, R. Diaz Hernandez, J. C. Diaz-Velez, B. L. Dingus, M. Durocher, M. A. DuVernois, R. W. Ellsworth, C. Espinoza, K. L. Fan, K. Fang, N. Fraija, A. Galvan-Gamez, J. A. Garcia-Gonzalez, F. Garfias, M. M. Gonzalez, J. A. Goodman, J. P. Harding, S. Hernandez, B. Hona, D. Huang, F. Hueyotl-Zahuantitla, P. Huntemeyer, A. Iriarte, A. Jardin-Blicq, V. Joshi, D. Kieda, A. Lara, J. Lee, W. H. Lee, H. Leon Vargas, J. T. Linnemann, A. L. Longinotti, G. Luis-Raya, J. Lundeen, K. Malone, O. Martinez, J. Martinez-Castro, J. A. Matthews, P. Miranda-Romagnoli, J. A. Morales-Soto, E. Moreno, M. Mostafa, A. Nayerhoda, L. Nellen, M. Newbold, M. U. Nisa, R. Noriega-Papaqui, L. Olivera-Nieto, N. Omodei, A. Peisker, Y. Perez Araujo, C. D. Rho, Y. J. Roh, D. Rosa-Gonzalez, F. Salesa Greus, A. Sandoval, M. Schneider, J. Serna-Franco, A. J. Smith, R. W. Springer, K. Tollefson, I. Torres, R. Torres-Escobedo, R. Turner, F. Urena-Mena, L. Villasenor, I. J. Watson, T. Weisgarber, E. Willox, H. Zhou
DDraft version January 25, 2021
Typeset using L A TEX twocolumn style in AASTeX63
HAWC Search for High-Mass Microquasars
A. Albert, R. Alfaro, C. Alvarez, J.R. Angeles Camacho, J.C. Arteaga-Vel´azquez, K.P. Arunbabu, D. Avila Rojas, H.A. Ayala Solares, V. Baghmanyan, E. Belmont-Moreno, S.Y. BenZvi, C. Brisbois, K.S. Caballero-Mora, T. Capistr´an, A. Carrami˜nana, S. Casanova, U. Cotti, J. Cotzomi, E. De la Fuente, C. de Le´on, R. Diaz Hernandez, J.C. D´ıaz-V´elez, B.L. Dingus, M. Durocher, M.A. DuVernois, R.W. Ellsworth, C. Espinoza, K.L. Fan, K. Fang,
15, 16, 14
N. Fraija, A. Galv´an-G´amez, J.A. Garc´ıa-Gonz´alez, F. Garfias, M.M. Gonz´alez, J.A. Goodman, J.P. Harding, S. Hernandez, B. Hona, D. Huang, F. Hueyotl-Zahuantitla, P. H¨untemeyer, A. Iriarte, A. Jardin-Blicq,
20, 21, 22
V. Joshi, D. Kieda, A. Lara, J. Lee, W.H. Lee, H. Le´on Vargas, J.T. Linnemann, A.L. Longinotti, G. Luis-Raya, J. Lundeen, K. Malone, O. Martinez, J. Mart´ınez-Castro, J.A. Matthews, P. Miranda-Romagnoli, J.A. Morales-Soto, E. Moreno, M. Mostaf´a, A. Nayerhoda, L. Nellen, M. Newbold, M.U. Nisa, R. Noriega-Papaqui, L. Olivera-Nieto, N. Omodei, A. Peisker, Y. P´erez Araujo, C.D. Rho, Y.J. Roh, D. Rosa-Gonz´alez, F. Salesa Greus,
7, 32
A. Sandoval, M. Schneider, J. Serna-Franco, A.J. Smith, R.W. Springer, K. Tollefson, I. Torres, R. Torres-Escobedo, R. Turner, F. Ure˜na-Mena, L. Villase˜nor, I.J. Watson, T. Weisgarber, E. Willox, and H. Zhou Physics Division, Los Alamos National Laboratory, Los Alamos, NM, USA Instituto de F´ısica, Universidad Nacional Aut´onoma de M´exico, Ciudad de Mexico, Mexico Universidad Aut´onoma de Chiapas, Tuxtla Guti´errez, Chiapas, Mexico Universidad Michoacana de San Nicol´as de Hidalgo, Morelia, Mexico Instituto de Geof´ısica, Universidad Nacional Aut´onoma de M´exico, Ciudad de Mexico, Mexico Department of Physics, Pennsylvania State University, University Park, PA, USA Institute of Nuclear Physics Polish Academy of Sciences, PL-31342 IFJ-PAN, Krakow, Poland Department of Physics & Astronomy, University of Rochester, Rochester, NY , USA Department of Physics, University of Maryland, College Park, MD, USA Instituto de Astronom´ıa, Universidad Nacional Aut´onoma de M´exico, Ciudad de Mexico, Mexico Instituto Nacional de Astrof´ısica, ´Optica y Electr´onica, Puebla, Mexico Facultad de Ciencias F´ısico Matem´aticas, Benem´erita Universidad Aut´onoma de Puebla, Puebla, Mexico Departamento de F´ısica, Centro Universitario de Ciencias Exactase Ingenierias, Universidad de Guadalajara, Guadalajara, Mexico Department of Physics, University of Wisconsin-Madison, Madison, WI, USA Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), Stanford University, Stanford, CA 94305, USA NHFP Einstein Fellow Tecnologico de Monterrey, Escuela de Ingenier´ıa y Ciencias, Ave. Eugenio Garza Sada 2501, Monterrey, N.L., Mexico, 64849 Department of Physics and Astronomy, University of Utah, Salt Lake City, UT, USA Department of Physics, Michigan Technological University, Houghton, MI, USA Max-Planck Institute for Nuclear Physics, 69117 Heidelberg, Germany Department of Physics, Faculty of Science, Chulalongkorn University, 254 Phayathai Road, Pathumwan, Bangkok 10330, Thailand National Astronomical Research Institute of Thailand (Public Organization), Don Kaeo, MaeRim, Chiang Mai 50180, Thailand Erlangen Centre for Astroparticle Physics, Friedrich-Alexander-Universit¨at Erlangen-N¨urnberg, Erlangen, Germany Natural Science Research Institute, University of Seoul, Seoul, Republic of Korea Department of Physics and Astronomy, Michigan State University, East Lansing, MI, USA Universidad Politecnica de Pachuca, Pachuca, Hgo, Mexico Centro de Investigaci´on en Computaci´on, Instituto Polit´ecnico Nacional, Ciudad de Mexico, Mexico Dept of Physics and Astronomy, University of New Mexico, Albuquerque, NM, USA Universidad Aut´onoma del Estado de Hidalgo, Pachuca, Mexico Instituto de Ciencias Nucleares, Universidad Nacional Aut´onoma de Mexico, Mexico City, Mexico Department of Physics, Stanford University: Stanford, CA 94305–4060, USA
Corresponding author: Chang Dong [email protected] author: Ke [email protected] a r X i v : . [ a s t r o - ph . H E ] J a n HAWC Collaboration Instituto de F´ısica Corpuscular, CSIC, Universitat de Val`encia, E-46980, Paterna, Valencia, Spain Facultad de Ciencias F´ısico Matem´aticas, Benem´erita Universidad Aut´onoma de Puebla, Puebla, Mexico Tsung-Dao Lee Institute & School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai, China
Submitted to The Astrophysical Journal LettersABSTRACTMicroquasars with high-mass companion stars are promising very-high-energy (VHE; 0.1-100 TeV)gamma-ray emitters, but their behaviors above 10 TeV are poorly known. Using the High AltitudeWater Cherenkov (HAWC) observatory, we search for excess gamma-ray emission coincident withthe positions of known high-mass microquasars (HMMQs). No significant emission is observed forLS 5039, Cygnus X-1, Cygnus X-3, and SS 433 with 1,523 days of HAWC data. We set the moststringent limit above 10 TeV obtained to date on each individual source. We have also performedsource-stacking searches, considering two different scenarios: I) gamma-ray luminosity is a fraction (cid:15) γ of the microquasar jet luminosity, and II) very-high-energy gamma rays are produced by relativisticelectrons up-scattering the radiation field of the companion star in a magnetic field B . We obtain (cid:15) γ < . × − for scenario I, which tightly constrains models that suggest observable high-energyneutrino emission by HMMQs. In the case of scenario II, the non-detection of VHE gamma raysyields a significant magnetic field, B (cid:38)
22 G, which excludes synchrotron radiation as the dominantmechanism of the microquasar emission between 10 keV and 10 MeV.
Keywords:
Gamma-ray sources(633), High mass x-ray binary stars (733) INTRODUCTIONMicroquasars are radio emitting X-ray binaries(XRBs) with relativistic outflows or jets (Mirabel &Rodr´ıguez 1999). Powered by stellar-mass compactobjects, they mimic extragalactic quasars on smallerscales and present accretion and formation of jets. Mi-croquasars with high-mass companion stars (or, high-mass microquasars, HMMQs) share many similaritiesin geometry and observational behaviors (Paredes et al.2002). A typical HMMQ has a young O or B type starwith mass greater than 10 M (cid:12) , and experiences masstransfer between the companion and the compact ob-ject via stellar winds. In addition, they usually showpersistent radio emission (Romero et al. 2017).HMMQs are suggested to be promising TeV γ -rayemitters (Marcote et al. 2015, also see the review byDubus 2013 and the references therein). Indeed, afew of them have been observed in high-energy (HE;0.1-100 GeV) and/or very-high-energy (0.1-100 TeV)gamma rays, including LS I +61 ◦
303 (Albert et al.2006), LS 5039 (Mariaud et al. 2016), Cygnus X-3 (onlyin HE; Fermi-LAT Collaboration 2009), and Cygnus X-1(possibly only in HE; Zdziarski et al. 2017). Althoughnot all HMMQs are detected in gamma rays, the HMMQbranch of the gamma-ray binaries raises an interestingquestion as to whether γ -ray emission is a common fea-ture in the HMMQ population. The γ -ray production mechanism of the known binarysystems is largely unknown. The emission has been sug-gested to be produced by either the accretion-poweredmicroquasar jets and outflows or the rotation-poweredpulsar winds (Dubus 2013). The origin of the gammarays is also debated to be either from the decay of neu-tral pions via hadronic interactions or from the inverseCompton scattering of optical-to-UV photons from thedonor star by relativistic electrons.Motivated by these questions, we search for VHE γ -ray emission from HMMQs using the High AltitudeWater Cherenkov (HAWC) observatory. HAWC ob-serves VHE gamma rays via the induced extensive airshowers produced from a series of pair production andBremsstrahlung. It provides an unprecedented sensi-tivity for the observation of VHE gamma rays above ∼
10 TeV. For each source in our target list, we de-rive upper limits on the VHE emission and comparethese to existing multi-wavelength observational dataof the source. By stacking the likelihoods of the fit-ted γ -ray emission from all known HMMQs accessible toHAWC, assuming that they produce gamma rays via acommon mechanism, the absence of detection stronglyconstrains the VHE emission efficiency and the mag-netic field strength in the relativistic outflows of micro-quasars.This work is different from Abeysekara et al. (2018)where VHE γ -ray emission from the extended jets ofSS 433 is studied. Here, we focus on the gamma-ray AWC Search for High-Mass Microquasars ∼ . § § .
1, the analysis of HAWC data § .
2, and the stacking of likelihoods § .
3. The results arepresented in § § METHODS2.1.
Source Selection
We select target sources based on two criteria: i) itis a confirmed X-ray binary system with steady radioemission, i.e. a microquasar, within the sky coverageof HAWC, and ii) it does not present transient X-rayoutbursts like XTE J0421+560 (Frontera et al. 1998).Applying these conditions to the high-mass X-ray binarycatalog (Liu et al. 2006), we are left with four HMMQsas target sources: LS 5039, Cygnux X-1, Cygnus X-3,and SS 433. Although LS I +61 ◦
303 may also seem tosatisfy our criteria, it is at the edge of the HAWC field-of-view (FOV). Due to the poor detector sensitivity inthat region, we do not include LS I +61 ◦
303 in thetarget list.Table 1 lists the relevant properties of the four HM-MQs studied in this paper.2.2.
HAWC Analysis
HAWC is a high duty cycle, wide field-of-view par-ticle sampling array consisting of 300 water Cherenkovdetectors (WCDs) covering a combined geometrical areaof ∼ ,
000 m (Abeysekara et al. 2017b). It is locatedat a latitude of ∼ ◦ N and at an altitude of ∼ , γ -ray events is estimated from the numberof hit PMTs per gamma-ray event. The expected en-ergy and angular resolutions are ≥
20% and ≥ . ◦ ,respectively, based on the Crab Nebula analysis (Abey-sekara et al. 2017b). More details about the HAWCsetup, data, and general source analysis procedures canbe found in Abeysekara et al. (2017b).Likelihood fitting with given spatial and spectral mod-els is used to compute the γ -ray energy spectrum. Ineach energy bin i , a simple power-law spectral model isused to describe the γ -ray spectrum,Φ i = A i (cid:18) EE i, piv (cid:19) − α i , (1) where Φ i is the differential flux at the pivot energy E i, piv , A i is the flux normalization, E is the photonenergy, and α i is the spectral index. For this analy-sis, we use four quasi-differential energy bins as listedin Table 2. Within each bin, we adopt a spectral index α i = 2 .
7, which is a good approximation for point-likeHAWC sources (Abeysekara et al. 2017a). The system-atic uncertainties due to the unknown spectral indexand detector response functions will be discussed be-low. Since the binary systems have a typical size of0 . B , Figure 3. The residual maps, as shown inFigure 1, are obtained with the following steps. Wefirst fit background sources using their known locationsand spectral indices from the 3HWC Catalog (Albertet al. 2020). In particular, we fit point-like backgroundsources such as 3HWC J1819-150 and 3HWC J1913+048with a point source model. The regions of interestalso contain four extended sources. We use a sim-ple Gaussian morphology for 3HWC J2006+340 and3HWC J1908+063. The 3HWC J1825-134 area con-sists of two pulsar wind nebulae, HESS J1825-137 andHESS J1826-130 (Abdalla et al. 2019), positioned aboveand below the location of 3HWC J1825-134. Hence, weapply an asymmetrical Gaussian morphological modelto 3HWC J1825-134 with its semi-major axis positionedalong the line joining the three VHE source locations.Finally, the Cygnus cocoon’s gamma-ray profile is “flat”(Hona et al. 2020). Hence we adopt a disk-like morpho-logical model for 3HWC J2031+415.The obtained best-fit models for the nearby sourcesare then subtracted from the original HAWC 1,523 tran-sit maps to produce the residual maps as shown in Fig-ure 1. Then, we fit for the flux normalization of eachHMMQ to find their flux upper limits.We calculate a test statistic (TS) for γ -ray detectionbased on the logarithm of the likelihood ratio when fit-ting with the residual maps with and without the targetsource in all energy bins,TS ≡ (cid:104) ln L ( ˆ A ) − ln L ( A = 0) (cid:105) , (2)where L is the Poisson likelihood function and ˆ A isthe best-fit normalization found from the maximum- HAWC Collaboration
Table 1.
List of high-mass microquasars in the HAWC FOV and their properties, includingthe location (RA, DEC) and distance D of the binary system, the companion star’s temper-ature T ∗ , radius R ∗ , separation from the compact object d ∗ , and the compact object’s jetpower L jet . Name RA DEC T ∗ R ∗ d ∗ Jet kinetic power L jet Distance D [10 K] [ R (cid:12) ] [AU] [erg s − ] [kpc]LS 5039 a ◦ e b ◦ − × c ◦ < d ◦ f a Perucho & Bosch-Ramon 2008 b Gallo et al. 2005; Heinz 2006; Russell et al. 2007; Ziolkowski 2014 c Zdziarski et al. 2012; Koljonen et al. 2018 d Wagner 1986; Begelman et al. 2006 e It has also been suggested that γ -ray emission in this source is powered by pulsar winds (Dubus 2006). f Based on the mass of the donor star 12 . M (cid:12) (Kubota et al. 2010) and the stellar mass-radius relation(Eker et al. 2018). Table 2.
Quasi-differential energy bins.
Energy Bin Energy Range Pivot Energy[TeV] [TeV]1 1.0-3.2 1.82 3.2-10.0 5.63 10.0-31.6 17.84 > likelihood estimators. We obtain a priori statistical sig-nificance for a given location in the sky via, σ ≈ ±√ TS . (3)The best-fit normalization, ˆ A , is used as an input to aMarkov-Chain Monte Carlo (MCMC). MCMC then esti-mates the distribution of the posterior likelihood aroundthe maximum value of the likelihood with a positive uni-form prior assumed. From the obtained MCMC distri-bution, we can finally compute the 95% credible upperlimit on the flux normalization.The HAWC data analysis involves forward-folding ofthe assumed morphological and spectral models throughthe detector response to obtain the expected gamma-raycounts. Following Abeysekara et al. (2019), we evaluatethe detector systematic uncertainties by applying var-ious versions of the detector response. Also, differentspectral indices between 2.0 and 3.0 with an interval of0.1 are applied to study the source spectrum. The sys- tematic errors on the flux normalizations due to differ-ent detector responses and astrophysical spectral indicesare computed for each source at each quasi-differentialenergy bin and for one full energy bin containing datafrom all four bins. The errors are shown in Table 4 inAppendix D. 2.3. Stacking of Likelihoods
Due to their similarity in the source structure, suchas the accretion disk–jet configuration, and in the radi-ation background, such as thermal photons from donorstars of similar star type, temperature, and size, theHMMQ population could, in principle, produce gammarays with one same mechanism (Dubus 2013). By com-bining the observations of all HMMQs in the HAWCFOV, we can constrain the common factors that impactthe γ -ray production in these microquasars.Below we consider two generic models, referred to asscenarios I and II, for VHE γ -ray emission in micro-quasar jets. In the first scenario, we assume that γ -rayluminosity is proportional to the kinetic power of thejets, L γ = (cid:15) γ L jet . (4)This is a general assumption which may be satisfied bydifferent γ -ray production models such as neutral piondecay from hadronic interactions. The γ -ray flux in sce-nario I can be written asΦ γ = (cid:15) γ L jet π D K p (cid:18) EE piv (cid:19) − p , (5) AWC Search for High-Mass Microquasars D is the distance to the source, K p =(2 − p ) E − p piv / ( E − p max − E − p min ) is a normalization factor forspectral index p , and K p = E − / log( E max /E min ) for p = 2. Also, E min = 1 TeV, and E max = 100 TeV arethe boundaries of the energy bin used for the stackinganalysis (instead of the quasi-differential bins used inSec. 2.2) with E piv = 7 TeV as the pivot energy. L jet and D of the target sources are listed in Table 1.In the second scenario, we consider the model sum-marized in Dubus (2013), where gamma rays are pro-duced when relativistic electrons accelerated by the jetsupscatter optical photons from the donor star. See Ap-pendix A for more details regarding the modeling of γ -ray production. In this model, the inverse Comptonemission of a HMMQ is expected to peak at TeV ener-gies and the corresponding synchrotron emission is typ-ically at 10 keV–10 MeV. The energy flux of the twocomponents, F syn and F IC , are connected by F syn F IC ≈ u B u f KN , (6)where u is the energy density of the radiation field ofthe star (equation A1) and u B = B / π is the magneticenergy density. Since the stellar radiation field is inthe optical band, the inverse Compton emission of VHEelectrons are in the Klein-Nishina regime. The unitless f KN factor, evaluated at the inverse Compton break en-ergy, E IC , bk , (equation A3) accounts for the suppressionof the inverse Compton cross section.The γ -ray flux in scenario II can be expressed asΦ γ = F syn u f KN u B K p (cid:18) EE piv (cid:19) − p . (7)We estimate the synchrotron flux F syn using the mea-sured X-ray (or MeV γ -ray) energy flux, F obs , bk , be-tween 0 . E syn , bk and 10 E syn , bk , where E syn , bk (equa-tion A5) is the peak energy of the synchrotron emissionsuggested by models fitted to the multiwavelength data.The energy density of the radiation field u is derivedfrom observed properties, including stellar temperature,radius, and separation from the compact object as listedin Table 1. The u and f KN used in the analysis arelisted in Table 3 in Section A of the Appendix.In both scenarios, the γ -ray flux of the i th source canbe written as Φ i = K C i (cid:18) EE piv (cid:19) − p , (8) where C i is the source-dependent contribution fac-tor , C i = K p L jet , i / π D i in scenario I and C i = K p u ,i f KN , i F syn , i in scenario II. K is the “weightingfactor” shared between the sources, specifically, K = (cid:15) γ in scenario I and K = 1 /u B in scenario II.We perform a likelihood fit for each target HMMQ toobtain their best-fit flux normalization. The sources arethen stacked in the likelihood space weighted by theirrelevant contribution factors C i ,ln L ( p, K ) = (cid:88) i ln L i ( p, K, C i ) . (9)The credible interval of the “linked” flux normaliza-tion for a given p is obtained using MCMC following thesteps described in § .
2. By scanning the index p between2 . . . K and the best-fit ˆ p that corresponds to the peak ofthe maximum log-likelihood ln L ( p, ˆ K ( p )). The largestdifference in K obtained when varying p is used as anestimation of the statistical error due to the scanningof the index. The detector systematic uncertainties areevaluated by applying various versions of the detectorresponse for the cases with the best-fit spectral index.Finally, the stacked flux is used to derive the limitson the weighting factor K . Note that the stacked fluxdepends on the definition of the contribution factor in aphysical model. The fluxes in different scenarios are notdirectly comparable. RESULTS3.1.
Upper Limits on Individual Sources
Figure 2 shows the spectral energy distribution ofour target sources ranging from X-rays to multi-TeVgamma-rays. LS 5039 is currently the only source inour list that has been detected at TeV energies (Mari-aud et al. 2016). Our limits below 10 TeV are consis-tent with the observation of this source by the IACTs.For CYG X-1 and CYG X-3, the upper limits fromMAGIC (Zdziarski et al. 2017) and VERITAS (Archam-bault et al. 2013) are more constraining at 1 TeV butapproach the HAWC upper limits as the energy goes up.Finally for SS 433, our limits are slightly less constrain-ing in the first quasi-differential bin but become com-parable to the combined MAGIC-H.E.S.S. data (Ahnenet al. 2018) at higher energies. This could be due to apotential contribution from the SS 433 west lobe (Abey-sekara et al. 2018), which is not included in the 3HWCCatalog (Albert et al. 2020). The contribution factor is sometimes referred to as the “J-factor”in the literature.
HAWC Collaboration
Figure 1.
Residual significance maps of the regions centered around LS 5039 (top left), CYG X-1 (top right), CYG X-3(bottom left), and SS 433 (bottom right) produced using 1,523 days of HAWC data. We also show in these maps the labelled3HWC sources fitted and subtracted. These significance maps have been made by fitting, per pixel, an E − . spectrum and apoint-like source morphology. AWC Search for High-Mass Microquasars Energy [eV]10 − − − − − − − − E d N / d E [ e r g c m − s − ] LS 5039
Other ExperimentsHAWC 95% CIHAWC H MedianHAWC H
95% ContainmentHAWC H
68% Containment Energy [eV]10 − − − − − − − − E d N / d E [ e r g c m − s − ] Cygnus X-1
Other ExperimentsHAWC 95% CIHAWC H MedianHAWC H
95% ContainmentHAWC H
68% Containment Energy [eV]10 − − − − − − − − E d N / d E [ e r g c m − s − ] Cygnus X-3
Other ExperimentsHAWC 95% CIHAWC H MedianHAWC H
95% ContainmentHAWC H
68% Containment Energy [eV]10 − − − − − − − − E d N / d E [ e r g c m − s − ] SS 433
Other ExperimentsHAWC 95% CIHAWC H MedianHAWC H
95% ContainmentHAWC H
68% Containment
Figure 2.
Spectral energy distribution of LS 5039 (top left), CYG X-1 (top right), CYG X-3 (bottom left), and SS 433(bottom right), in comparison with the upper limits on VHE γ -rays derived in this work from the HAWC observation. The bluedata points below ∼ . E syn , bk and E IC , bk (see equations A4and A5). The shaded grey band, spanning from 0 . E syn , bk to 10 E syn , bk , is used to evaluate F syn in Section 2.3. The spectralenergy distribution plots zoomed in at energies between 10 GeV and 100 TeV are presented in Appendix C (Figure 4). HAWC Collaboration
In Figure 2, containment bands are displayed to in-dicate the HAWC sensitivity at each location. A pointsource model is fitted in the empty regions of the skyalong the same declination band as the target HMMQ tocalculate the expected upper limits containing 68% and95% in yellow and green, respectively. For the calcula-tion of the sensitivities, regions with VHE gamma-raysources such as the Galactic plane have been excluded.Indeed our upper credible intervals, in red, are at mostabout 2sigma above the expected HAWC limit if therewere no emission (dashed black line). Hence, we do nothave a clear detection of the HMMQs.For the four sources discussed in this work, HAWCprovides the most stringent upper limits above 10 TeV.3.2.
Stacking Analysis
In neither stacking analysis does the combination ofthe four sources result in a significant detection. How-ever, the stacked flux limits allow us to set limits onparameters of the scenarios.In scenario I, we find the best-fit fluxnorm K (cid:80) i C i = (2 . ± . stat ± . sys ) × − TeV − cm − s − with TS = 4 . p = 2 .
2. It corresponds to a 95%Confidence Interval (C.I.) limit on the stacked fluxΦ γ ( E piv ) = 4 . × − TeV − cm − s − . Through equa-tion 5, we obtain the limit on the jet emission efficiencyabove 1 TeV, (cid:15) UL γ = 5 . × − . (10)This TeV emission efficiency is 3–5 orders of magni-tude lower than the emission efficiency of HMMQs in0.5–10 keV X-rays, which typically reaches 10 − − − (Marti et al. 1998; Fabrika 2004; Cadolle Bel et al. 2006).Our TeV γ -ray emission efficiency constrains the high-energy neutrino emission efficiency (cid:15) ν of HMMQs. IfVHE gamma rays are produced by the decay of neu-tral pions, the same hadronic process should producecharged pions that decay into high-energy neutrinoswith an emission efficiency (cid:15) ν ≈ (cid:15) γ /
2. The (cid:15) γ derivedin equation 10 suggests that a mean-orbital (cid:15) ν ∼ . § . F obs , bk , the stacking analysis yields the best-fit flux norm, K (cid:80) i C i = (6 . ± . stat ± . sys ) × − TeV − cm − s − with TS < p = 2 .
1. The 95% C.I. upperlimit on the stacked γ -ray flux is Φ γ ( E piv ) = 2 . × − TeV − cm − s − , which corresponds to a lowerlimit on the magnetic field strength, B LL = 22 (cid:16) (cid:15) syn
10 % (cid:17) / G , (11)where (cid:15) syn is an unknown factor denoting the ratio ofthe actual synchrotron emission by the electron popula-tion that emits VHE gamma rays to the total observed10 keV–10 MeV flux, (cid:15) syn ≡ F syn /F obs , bk .The derived magnetic field strength agrees with thefinding of Dubus et al. (2015), where B ≈
20 G wasobtained by fitting a relativistic hydrodynamics modelto the multi-wavelength observation of LS 5039. Dubuset al. (2015) concludes that a high B is unavoidableto explain the COMPTEL flux level of LS 5039. Ourresult extends the conclusion to all HMMQs accessibleto HAWC, and suggests that the large gap between theenergy flux in 10 keV–10 MeV and in VHE gamma rayscould be a universal feature of HMMQs. Such a highmagnetic field challenges the existing models of γ -raybinaries (Bosch-Ramon et al. 2008), and suggests thatthe synchrotron component is a small fraction, (cid:15) syn (cid:46)
10% of the observed flux between 10 keV and 10 MeV. Afew caveats should however be noted when interpretingthis result as discussed in § CONCLUSIONS AND DISCUSSIONThe highest-energy behaviors of the “mini” quasarsin our Galaxy are poorly understood, despite the ob-servational and theoretical indications that they provideplausible particle acceleration sites (Marcote et al. 2015;Mariaud et al. 2016; Abeysekara et al. 2018). A lot ofmicroquasars are located close to bright and extendedTeV sources, making their observations challenging. Byfitting and removing background sources from the re-gions of interest observed by the HAWC observatory, weprovide the most stringent limits on the γ -ray emissionsfrom LS 5039, CYG X-1, CYG X-3 and SS 433 above10 TeV. By stacking the chance of excess emission fromall HMMQs accessible to HAWC, we derive an upperlimit of the γ -ray emission efficiency of HMMQs above1 TeV, which also constrains the high-energy neutrinoemission efficiency of these sources. A second stackingsearch, applying a standard γ -ray binary model, fur-ther allows us to tightly constrain the contribution ofsynchrotron emission by relativistic electrons between10 keV and 10 MeV.The emission mechanism of hard X-rays / MeVgamma rays from HMMQs has been under debate sincethe detection of HMMQs by INTEGRAL and COMP-TEL (e.g., Cadolle Bel et al. 2006; Hoffmann et al. 2009).The data can be explained both by thermal Comptoniza-tion models where thermal electrons on the accretion AWC Search for High-Mass Microquasars γ -ray band wherethermal models become difficult.Our model does not account for the γγ absorption bythe stellar photon field, which could play an importantrole if the TeV emitter is deep inside the binary system(Bosch-Ramon et al. 2008). The effect of the γ -ray ab-sorption does not seem to be devastating in the case ofLS 5039, whose VHE γ -ray emission is still observableat the system’s superior conjunction when the absorp-tion is the strongest. Electromagnetic cascades initiatedin the pair production process could lead to secondaryelectrons that emit additional X-ray synchrotron emis-sion, which would further deepen the tension found byour analysis. Our model assumes non- or mildly rela-tivistic outflows like the jets of SS 433. If jets or pul-sar winds have a Lorentz factor of a few, Γ > Abdalla, H., et al. 2019, Astron. Astrophys., 621, A116Abeysekara, A., et al. 2017a, Astrophys. J., 843, 40—. 2019, Astrophys. J., 881, 134Abeysekara, A. U., et al. 2017b, ApJ, 843, 39Abeysekara, A. U., Albert, A., Alfaro, R., et al. 2018,Nature, 562, 82, [Erratum: Nature 564, E38 (2018)]Ahnen, M., et al. 2018, Astron. Astrophys., 612, A14Albert, A., Alfaro, R., Alvarez, C., et al. 2020, arXive-prints, arXiv:2007.08582Albert, J., Aliu, E., Anderhub, H., et al. 2006, Science, 312,1771Archambault, S., et al. 2013, Astrophys. J., 779, 150 Bai, X., Bi, B. Y., Bi, X. J., et al. 2019, arXiv e-prints,arXiv:1905.02773Begelman, M., King, A. R., & Pringle, J. 2006, Mon. Not.Roy. Astron. Soc., 370, 399Bosch-Ramon, V., Khangulyan, D., & Aharonian, F. A.2008, A&A, 489, L21Cadolle Bel, M., Sizun, P., Goldwurm, A., et al. 2006,A&A, 446, 591Cherenkov Telescope Array Consortium, Acharya, B. S.,Agudo, I., et al. 2019, Science with the CherenkovTelescope Array, doi:10.1142/10986 HAWC Collaboration
AWC Search for High-Mass Microquasars A. MODEL OF γ -RAY EMISSIONRelativistic electrons in the outflow of the compact object lose energy due to both synchrotron and inverse Comptonradiation. The target photon field for the inverse Compton process is dominated by the thermal radiation of thecompanion star. The energy density u of the photon field from a star with temperature T ∗ , radius R ∗ , and distance d ∗ from the compact object can be written as (Dubus 2013): u = σ SB T ∗ c R ∗ d ∗ = 260 (cid:18) T ∗ × K (cid:19) (cid:18) R ∗ R (cid:12) (cid:19) (cid:18) d ∗ . (cid:19) − erg cm − (A1)where σ SB is the Stefan-Boltzmann constant. As the thermal radiation peaks at (cid:15) = 3 k B T ∗ = 10 (cid:18) T ∗ × K (cid:19) eV , (A2)the inverse Compton process of electrons above E e, KN ≈ ( m e c ) / (4 (cid:15) ) = 6 . kT ∗ /
10 eV) − GeV is in the Klein-Nishina regime. The factor f KN ( γ e ) in equation 6 accounts for the Klein-Nishina suppression for electrons at energy γ e m e c that inverse Compton scatter a radiation field with differential energy density du/d(cid:15) (Moderski et al. 2005), f KN = 1 u (cid:90) d(cid:15) F KN ( b ) dud(cid:15) (A3)where b ≡ γ e (cid:15) / ( m e c ) and F KN ( b ) ≈ / (2 b ) (log b − / du/d(cid:15) ∝ (cid:15) / [exp( (cid:15) m e c /kT ∗ ) −
1] and u = (cid:82) d(cid:15) du/d(cid:15) .The dominant energy loss channel changes from the inverse Compton emission at low energy to the synchrotronemission at high energy, with ˙ γ syn = ˙ γ IC happening at E e, bk ≈ . (cid:18) B (cid:19) (cid:18) u
260 erg cm − (cid:19) − / TeV . (A4)This electron energy corresponds to a break in the Synchrotron spectrum at E syn , bk = 0 . B/ (u /
260 erg cm − ) − / MeV (A5)and a break in the inverse Compton spectrum at E IC , bk ≈ E e, bk . (A6)The fluxes of the inverse Compton and Synchrotron radiation at the break energy roughly scale as F X F γ ≈ ˙ γ B ˙ γ IC ≈ u B u f KN . (A7)The derived source properties, including u , F KN , E syn , bk , and E e, bk are listed in Table 3. B. SIGNIFICANCE MAPSFigure 3 presents the significance maps of the regions of the four HMMQs in equatorial coordinates before removingphoton counts from nearby 3HWC sources. C. FLUX UPPER LIMIT WITH A SINGLE ENERGY BINFigure 4 displays the spectral energy distributions of the four HMMQs between 10 GeV and 200 TeV as a zoom inview of Figure 2.In Figure 5, we also show the flux upper limits and HAWC sensitivities when using one single energy bin with
E >
HAWC Collaboration
Table 3.
Derived Source Properties.
Name u f KN at E e, bk E syn , bk E e , bk [erg cm − ] [keV] [TeV]LS 5039 820 4 . × − . × − . × − . × − Table 4.
Systematic uncertainties due to detectorresponse and the choice of spectral index for quasi-differential bins and for a single full-energy bin, respec-tively.
Energy Bin Systematics (detector, index)LS 5039 CYG X-1 CYG X-3 SS 4331 6%, 7% 12%, 9% 12%, 18% 11%, 10%2 15%, 4% 19%, 7% 14%, 13% 13%, 3%3 27%, 11% 21%, 10% 12%, 4% 22%, 4%4 28%, 16% 86%, 51% 15%, 5% 22%, 4%Full 19%, 72% 13%, 53% 16%, 36% 10%, 34% D. SYSTEMATIC EFFECTSTable 4 shows the systematic effects on the flux normalizations for the HMMQs. We evaluate the impact of twosystematic errors. The first is the uncertainty due to the detector response, and the second is due to the choice of thespectral index in our power-law model. The uncertainty due to detector response is at the level of 10 −
20% for mostsources and energy bins except the fourth bin of the CYG X-1 analysis. The statistics for this source above 30 TeV isdeficient and the fits are not adequately converged. The uncertainty due to the choice of the spectral index is < AWC Search for High-Mass Microquasars Figure 3.
Significance maps of LS 5039 (top left), CYG X-1 (top right), CYG X-3 (bottom left), and SS 433 (bottom right)produced using 1,523 days of HAWC data. HAWC Collaboration Energy [eV]10 − − − − − − E d N / d E [ e r g c m − s − ] LS 5039
HAWC 95% CIHAWC H MedianHAWC H
95% ContainmentHAWC H
68% ContainmentH.E.S.S. Energy [eV]10 − − − − − − E d N / d E [ e r g c m − s − ] Cygnus X-1
MAGICHAWC 95% CIHAWC H MedianHAWC H
95% ContainmentHAWC H
68% Containment Energy [eV]10 − − − − − − E d N / d E [ e r g c m − s − ] Cygnus X-3
VERITASHAWC 95% CIHAWC H MedianHAWC H
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68% Containment Energy [eV]10 − − − − − − E d N / d E [ e r g c m − s − ] SS 433
MAGIC-H.E.S.S.HAWC 95% CIHAWC H MedianHAWC H
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Figure 4.
Spectral energy distribution of LS 5039 (top left), CYG X-1 (top right), CYG X-3 (bottom left), and SS 433(bottom right). Features Gamma-ray data from various IACTs in blue in comparison with the upper limits on VHE γ -raysderived from the HAWC observation. AWC Search for High-Mass Microquasars Energy [eV]10 − − − − − − E d N / d E [ e r g c m − s − ] LS 5039
HAWC 95% CIHAWC H MedianHAWC H
95% ContainmentHAWC H
68% ContainmentH.E.S.S. Energy [eV]10 − − − − − − E d N / d E [ e r g c m − s − ] Cygnus X-1
MAGICHAWC 95% CIHAWC H MedianHAWC H
95% ContainmentHAWC H
68% Containment Energy [eV]10 − − − − − − E d N / d E [ e r g c m − s − ] Cygnus X-3
VERITASHAWC 95% CIHAWC H MedianHAWC H
95% ContainmentHAWC H
68% Containment Energy [eV]10 − − − − − − E d N / d E [ e r g c m − s − ] SS 433
MAGIC-H.E.S.S.HAWC 95% CIHAWC H MedianHAWC H
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68% Containment