Cygnus X-3, among the X-ray binaries, is one of the brightest in the radio
band, repeatedly exhibiting huge radio flares. The X-ray spectra shows two
definite states, low (correspondingly hard) and high (correspondingly soft).
During the hard state the X-ray spectra shows a pivoting behaviour correlated
to the radio emission. In the high state the X-ray spectra shows a gamut of
behaviour which controls the radio flaring activity of the source. The complete
evolution of the X-ray spectra along with the radio flaring activity is
reported here, for the first time for this source.Comment: Bibliography has been correctly adde

Choudhury, M.

Rao, A. R.

English

arXiv

Manojendu Choudhury

Crossref

Evolution of X-ray spectra of Cygnus X-3 with radio flares

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Evolution of X-ray spectra of Cygnus X-3 with radio flares
Manojendu Choudhury∗ and A. R. Rao∗
∗Tata Institute of Fundamental Research, Mumbai – 400005. INDIA
Abstract. Cygnus X-3, among the X-ray binaries, is one of the brightestin the radio band, repeatedly exhibiting huge radio
flares. The X-ray spectra shows two definite states, low (correspondingly hard) and high (correspondingly soft). Duringthe
hard state the X-ray spectra shows a pivoting behaviour correlated to the radio emission. In the high state the X-ray spectra
shows a gamut of behaviour which controls the radio flaring activity of the source. The complete evolution of the X-ray spectra
along with the radio flaring activity is reported here, for the first time for this source.
INTRODUCTION
Cygnus X-3 is one of the most studied yet least under-
stood source among the X-ray binaries. The nature of
the compact object is still unresolved, although the X-
ray spectral energy distribution (SED) suggests it to be
a black hole candidate. Also, similar to the established
black hole candidates, it shows two main states of X-ray
emission, high (correspondingly ’soft’) and low (corre-
spondingly ’hard’), although these states can not be clas-
sified as the canonical black hole states as the individual
spectral components during these states are not those per-
taining to the classical black hole candidates, viz. Cygnus
X-1 [1]. Cygnus X-3 is one of the brightest and persistant
radio source among the X-ray binaries, exhibiting huge
radio flares quite frequently. These flares occur during
the X-ray high (and correspondingly soft) state.
General X-ray spectra
The X-ray SED of this source typically shows two dif-
ferent states, low (correspondingly hard) and high (cor-
respondingly soft), see Figure 1. There is very high in-
trinsic absorption of the X-ray emission in this system,
probably due to the dust originating from the winds of
the Wolf-Rayet type companion star. As a result the disk
black body component is not observable using theRXTE-
PCA during the low state, and the spectra is best de-
scribed by a comptonising component, CompST, and a
power law. The high state is generally characterised by
a thermal component, multi- coloured disk black body,
plus a comptonising component (CompST) [2], except
on a few cases when the disk black body component is
replaced by a power law, anologous to the low state (see
the following sections). Spectral analysis of the data ob-
FIGURE 1. Typical X-ray spectral energy distribution of
Cygnus X-3 during its high (correspondingly soft) and low
(correspondingly hard) states, along with their continuumco -
ponents.The high state is characterised by disk black body cm-
ponent and a comptonising component, (CompST). The low
state is charecterised by CompST and a power law.
tained from ASCA observatory also reveal three Fe emis-
sion lines at 6.36, 6.67 & 6.96 keV. These line features
are more prominent during the low state (Figure 1).
LONG TERM MONITORING IN X-RAY
AND RADIO BANDS
TheAll Sky Monitor (ASM)aboard theRXTEsatellite ob-
servatory, theBurst And Transient Sources Experiment
(BATSE)aboard theCGROsatellite observatory, and the
Green Bank Interferometer (GBI)serendipitously moni-
tored the source (quasi-)simultaneously in the soft X-ray
(2-12 keV), hard X-ray (20-100 keV) and the radio (2.2
FIGURE 2. Soft X-ray (RXTE-ASM, 2-12 keV), hard X-
ray (CGRO-BATSE, 20-100 keV) & radio (GBI, 2.2 GHz)
monitoring of the source. Region 1, 3 & 4 correspond to the
low state whereas region 2 correspond to the high state of the
X-ray emission.
GHz) band, respectively, during the period MJD 50400
– 51500. ThereafterCGRO-BATSEstopped its operation
while GBI andRXTE-ASMcontinued the simultaneous
monitoring untilGBI stopped its operation around MJD
51700.
Figure 2 shows the light curves of the source as ob-
served by the three observatories. The regions 1, 3 & 4
correspond to the hard (as well as ‘low’) state of the X-
ray emission, with correlated radio emission [3, 1]. The
region 2 correspond to the high (correspondingly ‘soft’)
state of the X-ray emission. Within the precincts of the
hard (low) state of the X-ray emission, the soft X-ray
(RXTE-ASM) and the radio (GBI) are highly correlated,
while the hard X-ray (CGRO-BATSE) is anti-correlated
to both the soft X-ray and the radio. This is due to the piv-
oting in the X-ray spectra, correlated to the radio emis-
sion. In the high (soft) state, we report the evidence of
ejection of the central non-thermal Compton cloud, giv-
ing rise to very bright radio flares.
FIGURE 3. Pivoting of the X-ray spectra, correlated to the
radio emission, in the low (correspondingly hard) state. The
spectra becomes softer with increased radio emission.
LOW (HARD) STATE: PIVOTING OF
NON-THERMAL SPECTRA
The low (as well as hard) state spectral energy distribu-
tion of the X-ray emission is generally non-thermal in
nature (Figure 1). Figure 3 shows the pivoting of the X-
ray spectra, which is correlated to the radio emission, in
this state. The soft X-ray flux increases with the radio
emission, inversely the hard X-ray flux spectra hardens
with decrease in radio flux, with the pivot point lying in
the region 10−20 keV [3, 1].
Assuming that the region of the Comptonization is
confined to a small volume near the compact obejct, the
pivoting of the spectra can be qualitatively explained
by the Two Component Accretion Flow model of [4].
The Comptonisation component of the spectra originates
from a region close to the compact object, confined
within the Centrifugal Boundary Layer (CENBOL). At
low accretion rate, the CENBOL is far away from the
compact object and the spctrum is harder with lower out-
flow [5]. On increasing the accretion rate the CENBOL
comes closer to the compact object with greater outflow,
giving rise to increased radio emission and decreased
non-thermal hard X-ray emission. The radio emission,
from a core jet, is inversely proportional to the compres-
sion ratio which decreases as the spectra softens, in the
low (hard) state.
HIGH (SOFT) STATE: X-RAY
SPECTRAL EVOLUTION DRIVES THE
RADIO EMISSION
The soft X-ray (RXTE-ASM, 2 – 12 keV) and the radio
(GBI, 2.2 GHz) monitoring shows a more complicated
evolution of the high energy emission with respect to
the emission in the radio band (Figure 4), in the high
(correspondingly soft) state of X-ray emission. The X-
ray spectra in this state is generally dominated by the
thermal multicoloured disk black body component, along
with a hard component best described by a CompST
model [6], except for the post flare phase, when the
spectral shape hardens in the soft X-ray region. As shown
in Figure 5, the salient features of the spectra are:-
The radio quiescent phase.The X-ray spectra has
a strong disk black body and an equally strong
Comptonising component.
Pre-radio flare. The Comptonising component van-
ishes, resulting in a flare. The flare may result in
a time scale of a day or less.
Post-radio flare. The succession of radio flares, both
minor as well as major, is stopped by the change in
the X-ray spectrum, with the spectral shape harden-
FIGURE 4. The soft X-ray (RXTE-ASM, 2-12 keV) and radio (GBI, 2.2GHz) monitoring of the source during the X-ray high
state. The days for which the X-ray spectra obtained from thepointed observations usingRXTE-PCAare reported here are indicated
by arrows. The insets in the two right hand panels highlight the minor flares.
FIGURE 5. The X-ray spectral energy distribution (SED) and the individual continuum components, during the radio quiescent,
pre-radio flare & post-radio flare phases. The quiescent phase disk black body and Comptonising component at near equal ratio,
the pre-radio flare has vanishingly small Comptonising compnent, and the post-radio flare has the disk black body component
replaced by a simple power law.
TABLE 1. Model parameters of the continuum components and their flux contributions of the X-ray SED
Quiescent Radio Emission
Disk Black Body CompST
MJD Total Flux kTin (keV) Flux % of Total Flux kTE (keV) τ Flux % of Total Flux
50500 7.9 1.8 1.9 24.05 23.93 2.22 6.0 75.95
51587 6.6 1.44 3.4 51.51 13.39 1.59 3.2 48.49
51588 5.1 1.42 3.7 72.55 21.69 1.39 1.4 27.45
Pre-Radio Flare
Disk Black Body CompST
MJD Total Flux kTin (keV) Flux % of Total Flux kTE (keV) τ Flux % of Total Flux
50604 6.6 1.53 6.0 90.91 18.27 4.07 0.6 9.09
50624 6.9 1.55 5.9 85.51 18.34 3.31 1.0 14.49
51586 5.3 1.56 4.4 83.02 42.31 3.41 0.9 16.98
51589 4.5 1.53 3.8 84.44 53.53 2.40 0.7 15.56
51646 3.6 1.63 3.3 91.67 54.33 2.53 0.3 8.33
51650 5.8 1.70 5.0 86.21 80.79 9.91 0.8 13.79
Post-Radio Flare
Power law CompST
MJD Total Flux Γ Flux % of Total Flux kTE (keV) τ Flux % of Total Flux
50495 8.5 2.43 4.2 49.41 4.03 8.12 4.3 50.59
50632 9.9 2.62 4.0 40.40 5.12 7.05 5.9 59.60
51676 7.9 2.62 2.7 34.18 6.18 6.36 5.2 65.82
ing in the soft X-ray region. During this phase the
disk black body component becomes insignificant,
the spectral shape is explained by the similar com-
bination of non-thermal components, viz. CompST
and power law.
The evolution of the X-ray spectral changes with re-
spect to the radio emission can be explained as follows:-
1. The radio quiescent emission is marked by the radio
emission (2.2 GHz) bordering around 110 mJy and
below. The X-ray spectra has strong CompST com-
ponent with the amounting 75%−25% of the total
flux.
2. The vanishing of the CompST component (flux go-
ing below 15%) always precedes a minor flare, with
the radio flux around≈ 150−800 mJy, suggesting
the ejection of the central Compton cloud resulting
in the flare. The stronger the ejection, the louder the
flare.
3. The minor flare may be followed by the filling of
the central Compton cloud, i.e. increase in CompST
flux, causing the radio emission to become quies-
cent. Otherwise, if the continuous accretion persists
with the central cloud unfilled, i.e. the CompST flux
remains low, a major radio flare (2.2 GHz, flux> 1
Jy) follows.
4. The continuing series of minor and major flares
come to an end only with the change in the X-ray
spectra, i.e. hardening of the soft X-ray band, with
the flux level remaining high. This is the most inter-
esting state of the X-ray spectra with the shape be-
ing best fit by the model of the low (correspondingly
hard) state, i.e. power law and CompST, although
the soft X-ray flus remains high. This change in the
X-ray spectra puts a brake in the episodes of radio
flaring.
REFERENCES
1. Choudhury, M., Rao, A. R., Vadawale, S. V., and Jain,
A. K., Astrophysical Journal, 593, 452 (2003).
2. Choudhury, M., and Rao, A. R.,Journal of Astrophysics
and Astronomy, 23, 39 (2002).
3. Choudhury, M., Rao, A. R., Vadawale, S. V., Jain, A. K.,
and Ishwara-Chandra, C. H.,Astronomy and Astrophysics,
383, L35 (2002).
4. Chakrabarti, S. K.,Physics Reports, 266, 229 (1996).
5. Das, T., and Chakrabarti, S. K.,Classical and Quantum
Gravity, 16, 3879 (1999).
6. Sunyaev, R. A., and Titarchuk, L.,Astronomy and
Astrophysics, 86, 121 (1980).


Evolution of X-ray spectra of Cygnus X-3 with radio flares

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