Observations of theMicroquasar Cygnus X-3
by Fermi LAT
Stéphane Corbel(University Paris Diderot / CEA Saclay /Institut Universitaire de France )R. Corbet, G. Dubus, M. Kerr
E. Koerding

on behalf of the Fermi LAT
collaboration
Paper submitted (embargo on all results)

Cygnus X-3
High Mass X-ray Binary with a Wolf Rayet companion star,short orbital period (4.8 hours), distance ~ 7 kpc
Compact object is probably a black hole but may be aneutron star
Frequent radio outbursts associated with relativistic jets
resolved in radio: microquasar
Long history of “detections” from MeV to PeV (EeV?) in the70s and early 80s, not confirmed later. Motivated thedevelopment of high energy astronomy
AGILE report detection of a transient source in Cygnus
regionWhat can Fermi-LAT say about this ?

The Cygnus region
PSR J2032+4127
664

Abdo et al. 2009

2200-2-2

A crowded and complex fieldwith strong diffuse emission
and 3 bright pulsars, incl.
PSR J2032+4127 (cross) only
30’ from Cyg X-3 (circle)

Cyg X-3-4-4-68684

82
8078
Galactic longitude (deg)76counts/pixel

100.2

1.0

2.2

4.074

76

78

80

82

84

86-6

Galactic latitude (deg)474

Turning off the pulsar byselecting the LAT photons byphase: keep 80% of LATexposure

The Cygnus region

20

A bright source at the location ofCyg X-3: ~ 29 -4-4-679

No off-pulse detection ofPSRJ2032+4127

-2-2

80

79

81

81

80

Cyg X-30002286

84

8280
78
Galactic longitude (deg)76counts/pixel

100.2

1.0

2.2

4.074

76

78

80

82

84

86-6

Galactic latitude (deg)114426674

Average flux (>100 MeV): 1.19 +/0.06 (sta) +/- 0.37 (sys) 10-6 ph s-1cm-2Soft spectrum: PL index: 2.70 +/0.05 (stat) + 0.20 (syst)

Fermi light-curve of the LAT source
2008 Aug2009 Feb

2009 Sep

The LAT source is variable with two main active periods : Oct toDec 2008 and June to Aug 2009
Active phases seem consistent with one or several flares
Peak flux can be as high as 2 x10-6 ph s-1 cm-2, corresponds toGamma-ray luminosity L ~ 5 x 1036 (d / 7 kpc)2 erg s-1

2

Orbital period search

2Power in 10-13

Orb 2 Orb(ph cm-2 s-1)2Cyg X-3 orbital period = 4.8hours (red arrows)

Active periods!#$$"!!#! 3

Data points weighted by their
exposure%"!!#!!#$2#"!!#!!#$1

No significant orbital
modulation in the entire dataset!"!
$"!!#!!#$3

Significant detection of Cyg X-3
orbital period in the two activeperiods (probability of a false
detection ~2 10-9)

All data%"!!#!!#$2

Identification of the LAT sourcewith Cyg X-3 !!

#"!!#!!#$1!"! !3#!
1#! !$#! !%#! !##! !
!#-1)4*)56)7/8+,.'6*1
2
Frequency (hours()*+,-./+012+34ï5+6ï$7

ï%'"#!$#!%'"#!$#&"#!$#LAT

RXTE/ASM and LAT: same
asymmetric shape: slow rise
followed by a faster decayï%&"#!$#!%!"#!$#

ï%LAT data: consistent with a 100 %modulationï$)@A+BC.D<+E=<?+03<6+6 7!"#!$#!%$'!$)@A+BC.D<+E=<?+03<6+6 7()*+,-./+012+34!5+6!$7

Folded light-curves of Cyg X-3

Important: the LAT maximum isshifted by 0.3 to 0.4 in phase from
X-ray maximum

ASM

$'

$#

$#'

#"##"'

$"#

$"'

5"#

89:;<=-+>2=6?

'#"#

#"'

$"#
89:;<=-+>2=6?

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5"#

LAT

LAT

Swift/BAT count rate (in 10-2 c/s/cm2)

LAT detection of Cyg X-3 in the soft state

The two active periods of Cyg X-3 as seen by LAT correspond to thesoft X-ray state of Cyg X-3
Connection to the ultra-soft state associated with relativistic ejections

Relativistic jets
2008 Aug2009 Feb2009 Sep

The LAT active periods of Cyg X-3 occur close to radio (AMI +OVRO) flares
Positive correlation as indicated by the discrete crosscorrelation function
Lag of the radio light-curve not well constrained: 5 +/- 7 days

Origin of the modulationline of sight (observer)
gamma

e-Superior conjunctionuv

e-lessscattering

Inferior conjunction

-rays can not originate too close to accretion disk (pair production)
but within system: Modulation due to inverse Compton (IC) scatteringon UV photons. More IC at superior conjunction (head-on collisions)
Consistent with X-ray minima and phasing of orbit (Hanson et al. 00)
e- in corona (= base of jets?). Extension of hard X-ray power-law to 100
MeV consistent with Fermi (but steepening).

Conclusions

Detection of a LAT source positionally consistent with Cyg X-3
Identification with Cyg X-3 secured by the detection of its
orbital period. First detection of a microquasar at high energy
Variability in gamma-rays : associated with the soft X-ray state
and also with periods of relativistic ejection events. Clear linkwith relativistic jets ejection events.
Gamma-ray emission = inverse Compton of stellar photons onelectrons in corona (=base of jets ?)Maximum gamma-ray modulation when X-ray ~ minimum :consistent with IC emission at superior conjunction.
WR wind + compact binary -> huge density -> hadronicinteractions ? Cosmic rays protons in microquasars ?