Suzaku Observation of the Fermi Cygnus Cocoon

Tsunefumi Mizuno, Toshiaki Tanabe, Hiromitsu Takahashi (Hiroshima Univ.), Katsuhiro Hayashi (ISAS/JAXA), Ryo Yamazaki (Aoyama Gakuin Univ.), Isabelle

Grenier (CEA Saclay) and Luigi Tibaldo (SLAC National Laboratory)

1. Introduction: Fermi Cygnus Cocoon• Extended hard GeV g-ray excess [1] in Cygnus-X.

• Its spectrum and morphology imply an interstellar origin => CR production in star-forming region

• Fermi-LAT data alone cannot determine the particle type => X-ray observation by Suzaku [2]

2. Observation: Suzaku View of the Cocoon

3. Analysis: Spectra of Extended Emission

4. Discussion: Constraint on X-rays and the Cocoon MW Spectrum

5. Summary:

• Two source observations (positions with strong GeV g-ray excess)

• Two background observations to estimate the contribution from CXB (Cosmic X-ray Background) and GRXE (Galactic Ridge X-ray Emission; [3])

Employ a model to represent local emission+GRXE+CXB and search for possible non-thermal excess from the cocoon. (We used thin-thermal model vapec/apec [4] to represent local emission/GRXE)

kT=0.1 keV (Local Bubble or SWCX; [5])

kT<=1 keV (soft component of GRXE)

kT>=1 keV (hard componentof GRXE)

CXB (parameters taken from 　 [6])

• Low-energy lines at ~0.55 keV (forbidden OVII)=>significant contribution from SWCX(Solar Wind Charge eXchange)

• Spectra of all four regions are successfully reproduced by our model (no significant excess)

• High-temp plasma model might obscure the emission from the cocoon => examine latitude dependence

• Uncertainty of CXB and NXB taken into account

• Monotonic decrease with increasing latitude => most of the extended emission from GRXE

• Robust upper limit in source positions obtained by subtracting intensity of BG2

• MW spectrum from radio[7], X-ray(this study), GeV g-ray[1] and TeV g-ray[8]• Model curves of electron scenario with an amplification

factor of 120@50 GeV and cutoff at 1, 5, 10, 50 and 100 TeV. l=25 pc (1o at 1.4 kpc) and B=20 mG (pressure balance with gas) assumed.

• X-ray data requires a cutoff (in electron scenario) at <=30 TeV, independent confirmation of cutoff at <=5 TeV inferred by TeV data

Positions of obs. on a g-ray excess count map (E>=10 GeV)

S1

S2BG2

BG1

soft band (0.7-2 keV) hard band (2-10 keV)

Source1

Source2

Background1

Background2

• We identified several point sources and small-scale structures and removed them. Unusable area excluded & non-X-ray background subtracted. => Study extended emission (sec.3)

(~1/2 of point sources show absorbed hard spectrum and likely to be background AGNs)

vapec1 + wabs2*apec2 + wabs3*apec3 + wabs*Power-law

XIS1 of Source1

Source1(XIS0, XIS1, XIS3)

Source2

Background1 Background2

(radio) (X-ray) (GeV) (TeV)

• We conducted a series of deep X-ray observations of Fermi Cygnus Cocoon using Suzaku-XIS

• Most of extended X-ray emission above 2 keV comes from GRXE.• X-ray upper limit requires a cutoff (in electron scenario) at <=30 TeV, consistent with a cutoff inferred from TeV data.

• Paper in preparation.

Reference :[1] Ackermann+11, Science 334, 1103[2] Mitsuda+07, PASJ 59, S1[3] Worall+82, ApJ 255, 111; Warwick+85, Nature 317, 218; Koyama+86, PASJ 38, 121[4] Smith+01, ApJL 556, 91

[5] Fujimoto+07, PASJ 59, 133[6] Kushino+02, PASJ 54, 327[7] Taylor+03, ApJ 125, 3145[8] Abdo+12, ApJ 753, 159;

AbstractThe origin of Galactic cosmic-rays (GCRs) remains unconfirmed, but promising candidates for the source of GCRs are star-forming regions. We report a series of in-depth observations of the nearby star-forming region Cygnus-X in X-rays by Suzaku-XIS, focusing on the GeV g-ray excess revealed by Fermi-LAT. After excluding point sources and small-scale structures and subtracting the non-X-ray and cosmic X-ray background, the 2-10 keV X-ray intensity distribution was found to monotonically decrease with increasing Galactic latitude. This indicates that most of the remaining extended emission constitutes Galactic ridge X-ray emission. The upper limit of the possible diffuse X-rays from the g-ray excess in the 2-10 keV range, derived in two source observations were found to be incompatible with a pure electron scenario if the CR electron spectrum extends up to 100 TeV as a power-law, but are consistent with the spectral cutoff at 5 TeV inferred by the multiwavelength spectrum including TeV data.