# X-ray Polarization of the Eastern Lobe of SS 433

Philip Kaaret NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
Riccardo Ferrazzoli INAF Istituto di Astrofisica e Planetologia Spaziali, Via
del Fosso del Cavaliere 100, 00133 Roma, Italy  Stefano Silvestri Istituto
Nazionale di Fisica Nucleare, Sezione di Pisa, Largo B. Pontecorvo 3, 56127
Pisa, Italy  Michela Negro Department of Physics and Astronomy, Louisiana
State University, Baton Rouge, LA 70803, USA  Alberto Manfreda Istituto
Nazionale di Fisica Nucleare, Sezione di Napoli, Strada Comunale Cinthia,
80126 Napoli, Italy  Kinwah Wu Mullard Space Science Laboratory, University
College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK  Enrico Costa
INAF Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del
Cavaliere 100, 00133 Roma, Italy  Paolo Soffitta INAF Istituto di Astrofisica
e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
Samar Safi-Harb University of Manitoba, Department of Physics & Astronomy,
Winnipeg, MB R3T 2N2, Canada  Juri Poutanen Department of Physics and
Astronomy, 20014 University of Turku, Finland  Alexandra Veledina Department
of Physics and Astronomy, 20014 University of Turku, Finland  Nordita, KTH
Royal Institute of Technology and Stockholm University, Hannes Alfvéns väg 12,
SE-10691 Stockholm, Sweden  Alessandro Di Marco INAF Istituto di Astrofisica e
Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
Patrick Slane Center for Astrophysics, Harvard & Smithsonian, 60 Garden St,
Cambridge, MA 02138, USA  Stefano Bianchi Dipartimento di Matematica e Fisica,
Università degli Studi Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy
Adam Ingram School of Mathematics, Statistics, and Physics, Newcastle
University, Newcastle upon Tyne NE1 7RU, UK  Roger W. Romani Department of
Physics and Kavli Institute for Particle Astrophysics and Cosmology, Stanford
University, Stanford, California 94305, USA  Nicolò Cibrario Istituto
Nazionale di Fisica Nucleare, Sezione di Torino, Via Pietro Giuria 1, 10125
Torino, Italy  Dipartimento di Fisica, Università degli Studi di Torino, Via
Pietro Giuria 1, 10125 Torino, Italy  Brydyn Mac Intyre  University of
Manitoba, Department of Physics & Astronomy, Winnipeg, MB R3T 2N2, Canada
Romana Mikus̆incová Dipartimento di Matematica e Fisica, Università degli
Studi Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy  Ajay Ratheesh
INAF Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del
Cavaliere 100, 00133 Roma, Italy  James F. Steiner Center for Astrophysics,
Harvard & Smithsonian, 60 Garden St, Cambridge, MA 02138, USA  Jiri Svoboda
Astronomical Institute of the Czech Academy of Sciences, Boční II 1401/1,
14100 Praha 4, Czech Republic  Stefano Tugliani Istituto Nazionale di Fisica
Nucleare, Sezione di Torino, Via Pietro Giuria 1, 10125 Torino, Italy
Dipartimento di Fisica, Università degli Studi di Torino, Via Pietro Giuria 1,
10125 Torino, Italy  Iván Agudo Instituto de Astrofísicade Andalucía – CSIC,
Glorieta de la Astronomía s/n, 18008 Granada, Spain  Lucio A. Antonelli INAF
Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone
(RM), Italy  Space Science Data Center, Agenzia Spaziale Italiana, Via del
Politecnico snc, 00133 Roma, Italy  Matteo Bachetti INAF Osservatorio
Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy
Luca Baldini Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, Largo B.
Pontecorvo 3, 56127 Pisa, Italy  Dipartimento di Fisica, Università di Pisa,
Largo B. Pontecorvo 3, 56127 Pisa, Italy  Wayne H. Baumgartner NASA Marshall
Space Flight Center, Huntsville, AL 35812, USA  Ronaldo Bellazzini Istituto
Nazionale di Fisica Nucleare, Sezione di Pisa, Largo B. Pontecorvo 3, 56127
Pisa, Italy  Stephen D. Bongiorno NASA Marshall Space Flight Center,
Huntsville, AL 35812, USA  Raffaella Bonino Istituto Nazionale di Fisica
Nucleare, Sezione di Torino, Via Pietro Giuria 1, 10125 Torino, Italy
Dipartimento di Fisica, Università degli Studi di Torino, Via Pietro Giuria 1,
10125 Torino, Italy  Alessandro Brez Istituto Nazionale di Fisica Nucleare,
Sezione di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy  Niccolò Bucciantini
INAF Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze,
Italy  Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze,
Via Sansone 1, 50019 Sesto Fiorentino (FI), Italy  Istituto Nazionale di
Fisica Nucleare, Sezione di Firenze, Via Sansone 1, 50019 Sesto Fiorentino
(FI), Italy  Fiamma Capitanio INAF Istituto di Astrofisica e Planetologia
Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy  Simone
Castellano Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, Largo B.
Pontecorvo 3, 56127 Pisa, Italy  Elisabetta Cavazzuti Agenzia Spaziale
Italiana, Via del Politecnico snc, 00133 Roma, Italy  Chien-Ting Chen Science
and Technology Institute, Universities Space Research Association, Huntsville,
AL 35805, USA  Stefano Ciprini Istituto Nazionale di Fisica Nucleare, Sezione
di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
Space Science Data Center, Agenzia Spaziale Italiana, Via del Politecnico snc,
00133 Roma, Italy  Alessandra De Rosa INAF Istituto di Astrofisica e
Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
Ettore Del Monte INAF Istituto di Astrofisica e Planetologia Spaziali, Via del
Fosso del Cavaliere 100, 00133 Roma, Italy  Laura Di Gesu Agenzia Spaziale
Italiana, Via del Politecnico snc, 00133 Roma, Italy  Niccolò Di Lalla
Department of Physics and Kavli Institute for Particle Astrophysics and
Cosmology, Stanford University, Stanford, California 94305, USA  Immacolata
Donnarumma Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma,
Italy  Victor Doroshenko Institut für Astronomie und Astrophysik, Universität
Tübingen, Sand 1, 72076 Tübingen, Germany  Michal Dovčiak Astronomical
Institute of the Czech Academy of Sciences, Boční II 1401/1, 14100 Praha 4,
Czech Republic  Steven R. Ehlert NASA Marshall Space Flight Center,
Huntsville, AL 35812, USA  Teruaki Enoto RIKEN Cluster for Pioneering
Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan  Yuri Evangelista INAF
Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere
100, 00133 Roma, Italy  Sergio Fabiani INAF Istituto di Astrofisica e
Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
Javier A. García California Institute of Technology, Pasadena, CA 91125, USA
Shuichi Gunji Yamagata University,1-4-12 Kojirakawa-machi, Yamagata-shi
990-8560, Japan  Kiyoshi Hayashida  Deceased  Osaka University, 1-1 Yamadaoka,
Suita, Osaka 565-0871, Japan  Jeremy Heyl University of British Columbia,
Vancouver, BC V6T 1Z4, Canada  Wataru Iwakiri International Center for Hadron
Astrophysics, Chiba University, Chiba 263-8522, Japan  Svetlana G. Jorstad
Institute for Astrophysical Research, Boston University, 725 Commonwealth
Avenue, Boston, MA 02215, USA  Department of Astrophysics, St. Petersburg
State University, Universitetsky pr. 28, Petrodvoretz, 198504 St. Petersburg,
Russia  Vladimir Karas Astronomical Institute of the Czech Academy of
Sciences, Boční II 1401/1, 14100 Praha 4, Czech Republic  Fabian Kislat
Department of Physics and Astronomy and Space Science Center, University of
New Hampshire, Durham, NH 03824, USA  Takao Kitaguchi  RIKEN Cluster for
Pioneering Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan  Jeffery J.
Kolodziejczak NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
Henric Krawczynski Physics Department and McDonnell Center for the Space
Sciences, Washington University in St. Louis, St. Louis, MO 63130, USA  Fabio
La Monaca INAF Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso
del Cavaliere 100, 00133 Roma, Italy  Luca Latronico Istituto Nazionale di
Fisica Nucleare, Sezione di Torino, Via Pietro Giuria 1, 10125 Torino, Italy
Ioannis Liodakis NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
Simone Maldera Istituto Nazionale di Fisica Nucleare, Sezione di Torino, Via
Pietro Giuria 1, 10125 Torino, Italy  Frédéric Marin Université de Strasbourg,
CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, 67000 Strasbourg,
France  Andrea Marinucci Agenzia Spaziale Italiana, Via del Politecnico snc,
00133 Roma, Italy  Alan P. Marscher Institute for Astrophysical Research,
Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA  Herman L.
Marshall MIT Kavli Institute for Astrophysics and Space Research,
Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA
02139, USA  Francesco Massaro Istituto Nazionale di Fisica Nucleare, Sezione
di Torino, Via Pietro Giuria 1, 10125 Torino, Italy  Dipartimento di Fisica,
Università degli Studi di Torino, Via Pietro Giuria 1, 10125 Torino, Italy
Giorgio Matt Dipartimento di Matematica e Fisica, Università degli Studi Roma
Tre, Via della Vasca Navale 84, 00146 Roma, Italy  Ikuyuki Mitsuishi  Graduate
School of Science, Division of Particle and Astrophysical Science, Nagoya
University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan  Tsunefumi
Mizuno Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1
Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan  Fabio Muleri INAF
Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere
100, 00133 Roma, Italy  Chi-Yung Ng Department of Physics, University of Hong
Kong, Pokfulam, Hong Kong  Stephen L. O’Dell NASA Marshall Space Flight
Center, Huntsville, AL 35812, USA  Nicola Omodei Department of Physics and
Kavli Institute for Particle Astrophysics and Cosmology, Stanford University,
Stanford, California 94305, USA  Chiara Oppedisano Istituto Nazionale di
Fisica Nucleare, Sezione di Torino, Via Pietro Giuria 1, 10125 Torino, Italy
Alessandro Papitto INAF Osservatorio Astronomico di Roma, Via Frascati 33,
00040 Monte Porzio Catone (RM), Italy  George G. Pavlov Department of
Astronomy and Astrophysics, Pennsylvania State University, University Park, PA
16801, USA  Abel L. Peirson Department of Physics and Kavli Institute for
Particle Astrophysics and Cosmology, Stanford University, Stanford, California
94305, USA  Matteo Perri Space Science Data Center, Agenzia Spaziale Italiana,
Via del Politecnico snc, 00133 Roma, Italy  INAF Osservatorio Astronomico di
Roma, Via Frascati 33, 00040 Monte Porzio Catone (RM), Italy  Melissa Pesce-
Rollins Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, Largo B.
Pontecorvo 3, 56127 Pisa, Italy  Pierre-Olivier Petrucci Université Grenoble
Alpes, CNRS, IPAG, 38000 Grenoble, France  Maura Pilia INAF Osservatorio
Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy
Andrea Possenti INAF Osservatorio Astronomico di Cagliari, Via della Scienza
5, 09047 Selargius (CA), Italy  Simonetta Puccetti Space Science Data Center,
Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy  Brian
D. Ramsey NASA Marshall Space Flight Center, Huntsville, AL 35812, USA  John
Rankin INAF Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del
Cavaliere 100, 00133 Roma, Italy  Oliver J. Roberts Science and Technology
Institute, Universities Space Research Association, Huntsville, AL 35805, USA
Carmelo Sgrò Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, Largo B.
Pontecorvo 3, 56127 Pisa, Italy  Gloria Spandre Istituto Nazionale di Fisica
Nucleare, Sezione di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy  Douglas
A. Swartz Science and Technology Institute, Universities Space Research
Association, Huntsville, AL 35805, USA  Toru Tamagawa RIKEN Cluster for
Pioneering Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan  Fabrizio
Tavecchio INAF Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807
Merate (LC), Italy  Roberto Taverna Dipartimento di Fisica e Astronomia,
Università degli Studi di Padova, Via Marzolo 8, 35131 Padova, Italy  Yuzuru
Tawara  Graduate School of Science, Division of Particle and Astrophysical
Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602,
Japan  Allyn F. Tennant NASA Marshall Space Flight Center, Huntsville, AL
35812, USA  Nicholas E. Thomas NASA Marshall Space Flight Center, Huntsville,
AL 35812, USA  Francesco Tombesi Dipartimento di Fisica, Università degli
Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma,
Italy  Istituto Nazionale di Fisica Nucleare, Sezione di Roma “Tor Vergata”,
Via della Ricerca Scientifica 1, 00133 Roma, Italy  Department of Astronomy,
University of Maryland, College Park, Maryland 20742, USA  Alessio Trois INAF
Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius
(CA), Italy  Sergey S. Tsygankov Department of Physics and Astronomy, 20014
University of Turku, Finland  Roberto Turolla Dipartimento di Fisica e
Astronomia, Università degli Studi di Padova, Via Marzolo 8, 35131 Padova,
Italy  Mullard Space Science Laboratory, University College London, Holmbury
St Mary, Dorking, Surrey RH5 6NT, UK  Jacco Vink Anton Pannekoek Institute for
Astronomy & GRAPPA, University of Amsterdam, Science Park 904, 1098 XH
Amsterdam, The Netherlands  Martin C. Weisskopf NASA Marshall Space Flight
Center, Huntsville, AL 35812, USA  Fei Xie Guangxi Key Laboratory for
Relativistic Astrophysics, School of Physical Science and Technology, Guangxi
University, Nanning 530004, China  INAF Istituto di Astrofisica e Planetologia
Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy  Silvia Zane
Mullard Space Science Laboratory, University College London, Holmbury St Mary,
Dorking, Surrey RH5 6NT, UK

###### Abstract

How astrophysical systems translate the kinetic energy of bulk motion into the
acceleration of particles to very high energies is a pressing question. SS 433
is a microquasar that emits TeV $\gamma$-rays indicating the presence of high-
energy particles. A region of hard X-ray emission in the eastern lobe of SS
433 was recently identified as an acceleration site. We observed this region
with the Imaging X-ray Polarimetry Explorer and measured a polarization degree
in the range 38% to 77%. The high polarization degree indicates the magnetic
field has a well ordered component if the X-rays are due to synchrotron
emission. The polarization angle is in the range $-12\arcdeg$ to $+10\arcdeg$
(east of north) which indicates that the magnetic field is parallel to the
jet. Magnetic fields parallel to the bulk flow have also been found in
supernova remnants and the jets of powerful radio galaxies. This may be caused
by interaction of the flow with the ambient medium.

††facilities: IXPE, XMM-Newton, NuSTAR††software:  ixpeobssim (Baldini et al.,
2022), xspec (Arnaud, 1996), ds9 (Smithsonian Astrophysical Observatory,
2000), astropy (Astropy Collaboration et al., 2013)

##  1 Introduction

A variety of astrophysical systems are capable of translating the kinetic
energy of bulk motion into the acceleration of particles to very high
energies. Some examples include supernova remnants (Vink, 2012), the jets of
supermassive black holes and stellar-mass compact objects (Blandford et al.,
2019; Mirabel & Rodríguez, 1999), and explosive events such as $\gamma$-ray
bursts (Kumar & Zhang, 2015). Understanding the physical conditions within the
acceleration regions is key to deciphering the acceleration process.

SS 433 is a stellar binary system exhibiting jets that generate TeV
$\gamma$-rays revealing the presence of energetic particles (Margon, 1984;
Abeysekara et al., 2018). SS 433 contains a stellar-mass compact object
(likely a black hole) accreting from a supergiant companion (Fabrika, 2004)
and lies within the Milky Way at a distance of 5.5 kpc (Hjellming & Johnston,
1981; Blundell & Bowler, 2004). The compact object launches bipolar jets (with
a position angle of $100\arcdeg$ and inclined $80\arcdeg$ to our line of
sight) containing ionized matter moving at a bulk speed of $0.26c$ nearly
perpendicular to our line of sight (Margon, 1984). The jets precess with a
period of 162.5 days and an opening angle of $20\arcdeg$ and interact with the
surrounding supernova remnant, W50, creating two lobes east and west from SS
433 and a unique morphology denoted as the
‘Manatee’.111https://www.nrao.edu/pr/2013/w50/

The jet X-ray emission has a thermal spectrum with prominent line emission
within 0.16 pc of SS 433 (Marshall et al., 2002; Migliari et al., 2002). The
spectrum is non-thermal in the east (“e1”) and west (“w1”) lobes starting
about 30 pc from SS 433 (Safi-Harb & Ögelman, 1997; Yamauchi et al., 1994;
Safi-Harb & Petre, 1999). Soft thermal emission is seen from the termination
shock at the jet end (Safi-Harb & Ögelman, 1997; Brinkmann et al., 2007). The
X-ray spectra of the e1 and w1 lobes are consistent with synchrotron radiation
from energetic electrons (Watson et al., 1983; Safi-Harb & Petre, 1999). The
High Altitude Water Cherenkov (HAWC) observatory, in Mexico, detected very
high-energy (VHE) $\gamma$-ray emission from the e1 and w1 lobes extending up
to at least 25 TeV indicating the presence of particles with energies of 100’s
of TeV (Abeysekara et al., 2018). The VHE gamma-ray emission has been
confirmed and more accurately localized with the Large High Altitude Air
Shower Observatory (LHAASO) (Cao et al., 2023). Detection of gamma-ray
emission from the lobes with the High Energy Stereoscopic System (H.E.S.S.)
has also been reported (Olivera-Nieto, 2022). The TeV emission and the non-
thermal X-ray emission is localized to the lobes which suggests that particle
acceleration occurs in the jet at significant distances away from SS 433.

Using XMM-Newton and NuSTAR, Safi-Harb et al. (2022) identified a region (the
‘Head’, see the lower panel of Fig. 1) of nonthermal hard X-ray emission (3–30
keV) at $\sim 18\arcmin$ (29 pc) east of SS 433 coincident with the onset of
X-ray emission in the e1 lobe closest to SS 433. Assuming the hard X-rays are
synchrotron radiation, the hard spectrum (photon index $\Gamma\approx 1.6$)
suggests that this is the site of the acceleration of the high energy
particles. This is supported by the morphology of the X-ray emission which
shows the hardest X-ray emission closest to SS 433, with lower energy X-rays
forming a cometary tail pointed away from SS 433. The ’Head’ does not appear
to have counterparts at longer wavelengths, thus no polarization information
is available.

We observed the Head region using the Imaging X-ray Polarimetry Explorer
(IXPE) in order to measure the polarization of the X-ray emission and
constrain the geometry of the magnetic field. We describe the observations and
our analysis in Section 2 and present the X-ray polarization results in
Section 3. We discuss the results and the implications for the acceleration
mechanism in Section 4.

Figure 1: X-ray images of the eastern lobe of SS 433. (Top) Mosaic of XMM-
Newton data in the 0.3–10 keV band. The $\times$ marks the location of the
binary. The dashed line indicates the jet axis (Hjellming & Johnston, 1981)
and the dashed rectangle marks the jet termination shock. The blue contour is
25 counts pixel-1 in the XMM-Newton image. The black contour is the IXPE
source region. The black circle is the HAWC error circle and has a radius of
$15\arcmin$. (Bottom) X-ray image from IXPE in the 2–5 keV band. C11 marks the
black contour used as the IXPE source region. The partially-overlapping
magneta contours indicate the forward region (overlapping the Head) and
trailing region (overlapping the Cone) used for spatially-resolved
polarization analysis. The blue, dashed contour marks the background region.
The blue circle is the Head region and the blue quadrilateral is the Cone
region from Safi-Harb et al. (2022). The black arrows have a length of 2′(3.2
pc).

##  2 Observations and Data Reduction

IXPE is the first satellite mission dedicated to X-ray polarimetry in the 2–8
keV band (Weisskopf et al., 2022). It contains three X-ray telescopes, each
consisting of a Mirror Module Assembly (Ramsey et al., 2022) and a
polarization-sensitive gas-pixel detector unit (DU) (Costa et al., 2001;
Soffitta et al., 2021; Baldini et al., 2021), that enable imaging X-ray
polarimetry of extended sources. IXPE provides an angular resolution of $\sim
30\arcsec$ (half-power diameter). The overlap of the fields of view of the
three DUs is circular with a diameter of $12\farcm{9}$. The DUs record the
tracks of photoelectrons produced by X-rays to enable measurement of linear
X-ray polarization.

IXPE observed the eastern lobe of SS 433 from 2023 April 19 to 2023 May 9 with
a gap on 2023 May 2 due to observations of another target. The target
coordinates (RA = $288\fdg 27799$, DEC = $4\fdg 93573$) were selected to cover
the region of hard X-ray emission identified by Safi-Harb et al. (2022). The
target is $5\farcm 7$ from the LHASSO position and within the error circle.

Because our target is extended and relatively faint, the photon event list was
processed to reduce the instrumental background that is induced by Earth
albedo and primary energetic particles and secondaries produced in their
interactions in the detectors or elsewhere on the spacecraft. The rejection
algorithm is described in Di Marco et al. (2023) and removes $\sim$40% of the
background events and a negligible fraction ($\sim$1%) of good events. The
main components of the residual background are secondary positrons and a few
primary protons that cannot be morphologically distinguished from X-ray
initiated photoelectron tracks (Xie et al., 2021). We also removed times of
high background found following bright solar flares (likely associated with
coronal mass ejections) and surrounding intervals when IXPE crosses the South
Atlantic Anomaly. These backgrounds appear as short term increases in counting
rate and we removed times when the count rate was higher than the mean rate
plus three times the rms of the rate (Forsblom et al., 2023). The technique
has been important for the study of faint sources such as Galactic Center
clouds and supernova remnants with IXPE (Marin et al., 2023; Zhou et al.,
2023). After filtering, the total livetime was 800.7, 791.8, and 789.2 ks for
DU1, DU2, and DU3, respectively. The background is higher at the field of view
edges of each DU, but relatively uniform across the central $10\arcmin$
diameter (Di Marco et al., 2023). We limit our analysis to this central
region.

We selected an energy band to optimize the signal from the source versus the
background, both instrumental and from the cosmic X-ray background. We
extracted a source spectrum using the ‘Head’ region defined by Safi-Harb et
al. (2022) and a background spectrum from a nearby region without strong
diffuse emission. Scaling for the region sizes, the source spectrum is above
the background spectrum in the energy band of 2 to 5 keV, which we use for all
subsequent analysis.

The IXPE image in this band is shown in Fig. 1 along with an XMM-Newton mosaic
in the 0.3–10 keV band (Safi-Harb et al., 2022). We define an IXPE source
region, that we refer to as ‘C11’, by drawing a contour corresponding to 11
counts per pixel in the IXPE map using the SAOImage DS9 image display tool
with the smoothing parameter set to 16 (Smithsonian Astrophysical Observatory,
2000). The region covers most of the ‘Head’ and ‘Cone’ regions. We define a
background region (shown as a dashed line in Fig 1) that avoids areas of
strong diffuse emission and is within the central $10\arcmin$ diameter of the
image.

We extracted events within the source and background regions using ixpeobssim
version 30.5.0 (Baldini et al., 2022). We calculated polarization data cubes
in the 2–5 keV energy band using the pcube algorithm in the xpbin tool and
spectra of $I$, $Q$, and $U$ Stokes parameters with PHA1, PHAQ1, and PHAU1
algorithms in xpbin. We performed spectropolarimetric analysis using xspec
version 12.13.1. We used version 12 of IXPE instrument response files and
performed an unweighted analysis for both pcube and xspec.

Figure 2: Contour plots of the polarization degree and angle measured with
pcube. Position angles are measured positive east of north. The $\times$ marks
the measured values. The contours are at the 68.27%, 95.45%, and 99.73%
confidence intervals. The blue arrow indicates the direction from the lobe to
the compact object while the orange arrow is perpendicular.

##  3 Polarization Results

Figure 2 shows the polarization of the events from the C11 region as measured
using the model-independent pcube algorithm. The observed polarization degree
(PD) is $17.3\%\pm 4.3\%$ and the electric vector position angle (EVPA) is
$-0\fdg 2\pm 7\fdg 0$ (all uncertainties are 68% confidence). Position angles
are measured anticlockwise from north in the equatorial coordinate system as
per the convention of the International Astronomical Union. The probability
that the signal is a random fluctuation from an unpolarized source is
$2.4\times 10^{-4}$. This is equivalent to a $3.7\sigma$ confidence level. We
note that the energy band and source region selection were based on the
intensity data alone, thus there is only one trial for calculation of the
significance of the polarization detection. The events include the X-ray
emission from the lobe of SS 433, the cosmic X-ray background, and the
residual instrumental background. All are expected to be unpolarized except
for the emission from the lobe. Thus, the result indicates a highly probable
detection of polarization in the X-ray emission of the lobe.

Scaling from the background region, we estimate that 30% of the counts are
from the lobe and 70% are from instrumental or astrophysical backgrounds.
Utilizing the additive nature of the Stokes parameters, we calculated the
Stokes $I$, $Q$, and $U$ for the source and background regions then subtracted
the background from the source after scaling for the different region areas.
After background subtraction, we estimate the PD of the lobe emission as
$62\%\pm 15\%$ and the EVPA as $2\fdg 8\pm 7\fdg 0$. As expected for
unpolarized backgrounds, the EVPA is consistent with the results before
background subtraction. The PD is consistent with that calculated using the
polarization dilution equation (eq. 2) in Di Marco et al. (2023).

We also divided the C11 region into two parts, see Fig. 1, and performed a
background-subtracted polarization analysis. The circular region covering the
forward region has $\rm PD=59\%\pm 17\%$, $\rm EVPA=6\arcdeg\pm 8\arcdeg$, and
a chance probability of 0.0044. While the polygon covering the trailing region
does not provide a significant detection, yielding a chance probability of
0.033, the PD ($48\%\pm 18\%)$ and EVPA ($4\arcdeg\pm 10\arcdeg$) are
consistent with those of C11. The lower detection significance of the forward
region relative to C11 suggests that the polarization signal is not
concentrated in the forward region.

We measured the polarization of the events from the background region and
found that it is consistent with zero. The same background region is used for
the spectropolarimetic analysis described below. We also investigated three
rectangular background regions placed to the North, West, and South of C11
with an area equal to that of C11. The inferred PD and EVPA for the C11
emission are consistent within the statistical uncertainties regardless of the
background region used. We also investigated other procedures for the removal
of intervals of high background. While the statistical significance of the
polarization signal varies, the PD and EVPA after background subtraction
remain consistent.

Figure 3: IXPE X-ray spectra of the C11 region. (Top) Intensity (Stokes $I$)
spectra. Data from the three IXPE DUs are shown as crosses: DU1 (black), DU2
(orange), and DU3 (blue). The fitted models, identical except for the
normalization, are in the corresponding color. The lower panel shows the fit
residuals. (Bottom) Stokes $Q$ (black) and $U$ (orange) spectra. Points are
shifted on the x-axis by $\pm 20$ eV for clarity. Data from the three DUs are
added. The fitted model is shown as a solid line in the same color.

Figure 3 shows the IXPE background-subtracted X-ray spectra for the C11
region. We performed spectropolarimetric analysis using Xspec. This differs
from the pcube analysis in that it fully takes into account the response of
the instrument including the non-diagonal portion of the energy response. The
Stokes $I$ spectra have 80 eV bins below 3 keV and 120 eV above. The Stokes
$Q$ and $U$ spectra have 400 eV bins. We modeled the spectrum using an
absorbed power law with constant polarization multiplied by a constant allowed
to vary between the three DUs. The column density for the tbabs absorption
model using the Wilms et al. (2000) abundances was fixed to $1.18\times
10^{22}\,\rm cm^{-2}$ as for the ‘Cone’ region in Safi-Harb et al. (2022). The
normalization for DU1 was fixed to unity and those for the other DUs were
allowed to vary. We find a good spectropolarimetric fit ($\chi^{2}/\rm
dof=137.0/123$) with a photon index of $\Gamma=1.57\pm 0.14$, consistent with
that found by Safi-Harb et al. (2022). The observed flux is $5.8\times
10^{-13}\,\rm\,erg\,cm^{-2}\,s^{-1}$ in the 2–5 keV band. This corresponds to
a luminosity of $1.3\times 10^{34}\,\rm\,erg\,s^{-1}$ in the 0.3–30 keV band.
The EVPA is $-3\fdg 7\pm 7\fdg 8$, which is consistent with the pcube results.
The PD is $52\%\pm 14\%$, which is consistent with the background-subtracted
pcube result. The allowed range for PD considering the two analyses is 38% to
77%.

##  4 Discussion

The high polarization degree found from the C11 region suggests the magnetic
field has a well-ordered component, assuming that the X-rays are produced via
synchrotron radiation. The maximum polarization for synchrotron radiation is
$\Pi=(p+1)/(p+7/3)$ where the electron spectral index is $p=2\Gamma-1$. For
the C11 region, $\Gamma=1.57$ thus $p=2.14$ and $\Pi=0.70$. The ratio of the
PD after background correction to the maximum polarization is approximately
equal to the ratio of the energy in the magnetic field component with uniform
direction to that in the total field (Burn, 1966). For the C11 region, the
lower bound on the PD of 38% indicates at least $\approx 50$% of the field is
in the uniform component.

The energy of the electrons producing the emission can be calculated given the
magnetic field strength. Assuming equipartition between the energies of the
relativistic electrons and the magnetic field, the latter can be estimated
from the X-ray luminosity and volume as $B\approx 10\,\mu$G (Safi-Harb et al.,
2022). To produce X-rays of energy $E_{\gamma}$, the electron energy needed is
$E_{e}\approx({\rm 50\,TeV})(B/10\,\mu{\rm G})^{-1/2}(E_{\gamma}/{\rm
1\,keV})^{1/2}$. For $E_{\gamma}=4\,$keV and $B=10\,\mu$G, electrons must be
accelerated to 100 TeV. The same electrons also produce $\gamma$-rays near 25
TeV via inverse-Compton scattering on cosmic microwave background photons.
Thus, IXPE probes the magnetic field configuration in the same region
producing the VHE emission. The gyroradius for 100 TeV electrons in a
$10\,\mu$G magnetic field is 0.011 pc which is much smaller than the
transverse diameter of the C11 region of 3 pc. The synchrotron lifetime is
1200 yr (Reynolds, 1998). Thus, either the magnetic field is significantly
higher than the equipartition estimate or some other loss mechanism dominates
over the length of the region.

For synchrotron radiation, the EVPA is perpendicular to the magnetic field.
Thus, the magnetic field lies along the direction of the main bulk motion of
the jet. The key result of the IXPE observations is that the magnetic field
near the acceleration region contains a significant component that is well-
ordered and parallel to the flow direction. This result places constraints on
models of the acceleration process and raises questions on the origin of the
magnetic field.

The magnetic field could be intrinsic to the jet. However, while radio
polarization measurements show the magnetic field is aligned with the ridge
line of the precessing jet near SS 433 (Stirling et al., 2004), at larger
distances, the field is instead parallel to the motion of individual knots
(Miller-Jones et al., 2008). Thus, the field appears to be determined by the
interaction between the individual knots and the ambient medium (Blundell et
al., 2018).

The powerful jets in Fanaroff-Riley Class II (FR II) radio galaxies have
magnetic fields that are predominantly parallel to the jet axis all across the
jet (Bridle & Perley, 1984; Asseo & Sol, 1987). The radio PDs are high,
typically 20%–40% and reaching 50%, e.g., in a knot in the jet of 4C 32.69
that may be analogous to the knot considered here (Potash & Wardle, 1980).
Velocity shear due to the interaction of the jet with the ambient medium is a
natural means to produce a magnetic field aligned with the direction of bulk
motion (Blandford, 1983).

Young, shell-type supernova remnants show radio EVPAs perpendicular to the
flow near the shocks in the outer rims. Recent IXPE observations show similar
EVPAs, typically with higher PDs, in the X-ray band (Vink et al., 2022;
Ferrazzoli et al., 2023; Zhou et al., 2023). These results indicate magnetic
fields parallel to the flow, as seen here. Giacalone & Jokipii (2007) and
Inoue et al. (2013) suggest that the magnetic field may stretch as the shock
propagates around density fluctuations in the interstellar medium. This
stretching can both amplify and align the field with the flow. Again, velocity
shear is key to aligning the magnetic field with the flow. Alternatively, West
et al. (2017) suggest that the overall field is disordered and that the mean
EVPA represents the magnetic field only in localized regions where electrons
are most efficiently accelerated because the magnetic field is parallel to the
flow.

We consider several scenarios for acceleration and subsequent propagation and
radiation of the energetic particles in the eastern lobe of SS 433. We note
that the jet speed of 0.26$c$ implies a bulk Lorentz factor of 1.036. The
magnetic field of $10\,\mu$G estimated above and a jet density of
$0.05\rm\,cm^{-3}$ (Panferov, 2017) implies an Alfvén Mach number of $\sim
900$ and a magnetization of $\sigma\sim 2\times 10^{-4}$. Thus, the
interaction is non-relativistic with a very high Mach number and the jet is
hydrodynamical.

One possible scenario is stochastic acceleration in a shock (see, e.g.,
Caprioli & Spitkovsky, 2014; Park et al., 2015) at the leading edge of the e1
lobe followed by synchrotron emission from gradually cooling electrons flowing
eastward (Sudoh et al., 2020). In this scenario, acceleration would be
localized; Safi-Harb et al. (2022) show a compact region at the leading edge
that is bright in the 10-20 keV band and could be the acceleration region.
Ordering of the parallel magnetic field would occur downstream of the
turbulent shock region. The energetic electrons would suffer radiative loss
while propagating downstream from the shock. Thus, the photon index of the
nonthermal X-ray emission should gradually steepen eastward as is observed.

A second scenario is stochastic acceleration by turbulence (Petrosian, 2012)
in the shear interfaces between the jet and ambient medium. Such models have
been proposed for FR II radio galaxies and predict hard particle spectra
consistent with those observed (Stawarz & Ostrowski, 2002; Rieger & Duffy,
2004). The acceleration is stochastic, hence the electron energy spectrum
would be a power law with an index similar to that observed. Acceleration is
not localized and can occur along the extent of the lobe where there is strong
shear, thus steepening of the spectrum would not be expected over that region.
The polarized emission would be predominantly from particles diffused out from
the turbulent acceleration shear layer into the interior region with parallel
magnetic field (Tavecchio, 2021).

A third scenario is acceleration through plasma processes (see, e.g., Melrose,
1990) associated with magnetic field reconnection. This may occur at magnetic
loops produced by shear (Romanova & Lovelace, 1992; Sironi et al., 2021) or at
quasi-separatrix layers of adjacent magnetic flux ropes (see Aulanier et al.,
2006). The overall field reconfiguration process should be continual and
gradual to maintain a quasi-equilibrium between particle acceleration, escape
and radiative loss in order to produce a power-law spectrum with a relatively
stable spectral index. While acceleration can occur wherever there is strong
shear, the electron spectral index may vary due to local variations in the
intensity of reconnection.

We note that Thomson scattering of radiation from the binary system, as
alternative to synchrotron emission, would naturally produce highly polarized
X-rays with an EVPA perpendicular to jet axis. Such a scenario requires an
X-ray bright central source and SS 433 may be an ultraluminous X-ray source
with the X-ray beam along the jet axis. Waisberg et al. (2019) place an upper
bound on the X-ray luminosity of $\lesssim 5\times 10^{36}\rm\,erg\,s^{-1}$ in
a beam with a $1\arcdeg$ opening angle assuming the X-ray spectrum of a hard
ultraluminous source (Kaaret et al., 2017). C11 viewed from SS 433 has an
opening angle of $3\fdg 2$, so the illuminating beam would have a luminosity
of $\lesssim 5\times 10^{37}\rm\,erg\,s^{-1}$. An optical depth of $3\times
10^{-4}$ within C11 would be required to produce the observed luminosity,
which is much larger than the expected optical depth of $3\times 10^{-7}$
given the density estimate of Panferov (2017). We note that the differing
morphology in different energy bands seen with NuSTAR (Safi-Harb et al., 2022)
may also present issues with the scattering interpretation.

It may be possible to distinguish between these various models with future
IXPE and other X-ray and multiwavelength observations. Deeper observations of
the e1 lobe could enable spatially-resolved polarization measurements while
observations of the e2 lobe could provide information on downstream evolution
of the magnetic field. IXPE observation of the w1 lobe would be of great
interest to determine if it also shows an EVPA perpendicular to the jet.

The Imaging X-ray Polarimetry Explorer (IXPE) is a joint US and Italian
mission. The US contribution is supported by the National Aeronautics and
Space Administration (NASA) and led and managed by its Marshall Space Flight
Center (MSFC), with industry partner Ball Aerospace (contract NNM15AA18C). The
Italian contribution is supported by the Italian Space Agency (Agenzia
Spaziale Italiana, ASI) through contract ASI-OHBI-2017-12-I.0, agreements ASI-
INAF-2017-12-H0 and ASI-INFN-2017.13-H0, and its Space Science Data Center
(SSDC), and by the Istituto Nazionale di Astrofisica (INAF) and the Istituto
Nazionale di Fisica Nucleare (INFN) in Italy. A.V. acknowledges support from
the Academy of Finland grant 355672.

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