SS433 is a binary system showing relativistic Doppler shifts in its two sets of emission lines. The origin of its UV continuum is not well established. We observed SS433 to determine the emission mechanism responsible for its far UV spectrum. The source was observed at several different phases of both its 13 d orbital period and 162.5 d precession period using the UVOT and XRT detector systems on Swift. The far UV spectrum down to 1880 Angstrom lies significantly above the spectral flux distribution predicted by extrapolating the reddened blackbody continuum that fits the spectrum above 3500 Angstroms. The intensity of the far UV flux varies over a period of days and the variability is correlated with the variability of the soft X-ray flux from the source. An emission mechanism in addition to those previously detected in the optical and X-ray regions must exist in the far UV spectrum of SS433

Boyd, P. T.

Cannizzo, J. K.

Dolan, J. F.

NASA Technical Reports Server

DRAFT VERSIONFEBRUARY 7,2007 
Preprint typeset using LAT@ style emulateapj v. 1 1/26/03 
Source of Acquisition 
NASA Goddard Space Flight Center 
SWIFTOBSERVATIONS OF A FAR UV LUMINOSITY COMPONENT IN SS433 
J. F. DOLAN"~,  P. T. B O Y D ~ ,  J. K. C A N N I Z Z O ~ ' ~  
Draft version February 7, 2007 
ABSTRACT 
SS433 is a binary system showing relativistic Doppler shifts in its two sets of emission lines. The origin of its 
UV continuum is not well established. We observed SS433 to determine the emission mechanism responsible 
for its far UV spectrum. The source was observed at several different phases of both its 13 d orbital period and 
162.5 d precession period using the UVOT and XRT detector systems on Swift. The far UV spectrum down to 
1880 A lies significantly above the spectral flux distribution predicted by extrapolating the reddened blackbody 
continuum that fits the spectrum above 3500 A. The intensity of the far UV flux varies over a period of days and 
the variability is correlated with the variability of the soft X-ray flux from the source. An emission mechanism 
in addition to those previously detected in the optical and X-ray regions must exist in the far UV spectrum of 
SS433. 
Subject headings: stars: individual: SS433 ultraviolet: stars X-rays: binaries 
1. INTRODUCTION 
SS433 is a binary system showing two sets of emission lines 
(Stephenson & Sanduleak 1977). The two emission line sys- 
tems exhibit relativistic Doppler shifts in opposite directions 
corresponding to a mass motion velocity v l c  = 0.26 (Margon 
1984). The two sets of lines vary regularly in redshift with 
a 162.5 d period ascribed to the precession of two counter- 
directed beams. We adopt here the phase of Margon & An- 
derson (1989): E($) =JD(2443562.37 f 0.30) + (162.50 f 
0.03)$, where E($) is the epoch at which the two sets of lines 
have the same redshift at integer values of $. The lines have a 
maximum redshift separation at phase $ = 0.631. A smaller 
maximal separation occurs at $ = 0.131, and a second epoch 
of redshift equality at $ = 0.262. The system also exhibits 
a 13 d period in photometric and spectroscopic observations. 
The 13 d period is attributed to orbital motion and eclipse 
phenomena in the binary (Cherepashchuk 1981; Stewart et a1 
1987). We adopt here the 13 d phase y of Goranskii et al. 
(1 998) relative to photometric minimum at integer values of 
9 :  E ( 9 )  =JD(2450023.62& 0.26) + (13.08211 f 0.00008) y. 
Observations of SS433 in the radio show jet-like linear 
structure on both sides of the central radio source, which is co- 
incident with the optical image (Hjellming & Johnston 1981; 
Neil1 et al. 1981). The position angle of the jets varies approx- 
imately sinusoidally with the 162.5 d precession period about 
a mean value of 100" f 2" with amplitude 19" f 3". The 
radio emitting material appears to propagate away from the 
central source in discrete packets. VLBI observations have es- 
tablished that these "blobs" possess the same velocity as those 
producing the redshifted emission lines and give a distance to 
the source of 4.85 i 0.2 kpc (Vermeulen et al. 1993). 
The X-ray emission in SS433 originates near the base of 
the jets, which adiabatically expand and cool (Marshall et al. 
1979; Brinkman et al. 1991; Kotani et al. 1996). The X-ray 
luminosity is N erg s-l. The spectrum between 1 and 
10 keV shows multiple emission lines from several different 
species of highly ionized elements superposed on a continuum 
(Marshall et al. 2002). Marshall et al. modeled the continuum 
between 8 and 0.8 keV as a single power law with absorption, 
dn 
-= 0.015~-1.35~-"(E)N~ 
dE photons ~ r n - ~  s-I keTJ-', 
/ I  \ 
\ I /  
where NB = 9.5 x lo2' cm-2 is the hydrogen column density 
in the direction of SS433 and o(E) is the opacity of interstel- 
lar matter with normal cosmic abundances. 
Spectra of SS433 from the infrared to the U band are ac- 
ceptably represented by reddened blackbody emisslon (Cies 
et al. 2002) ascribed to the super-Eddington accretion dask 
around the compact object in the system and the dense part 
of an outflowing wind associated with the disk. UV observa- 
tions with the High Speed Photometer (HSP) on the Hubble 
Space Telescope (HST) found excess emission above that pre- 
dicted by extrapolating the reddened blackbody spectrum to 
2770 A (Dolan et al. 1997). Far UV observations of SS433 
provide only upper limits on the flux between 1 170 and 1575 
14 (Gies et al. 2002) because of the large interstellar absorp- 
tion along the line of sight. (SS433, at 1 = 3g0, b = -ZO, 
is highly reddened: Av = 7.8 f 0.5 [Wagner 1986 and ref- 
erences cited therein]). This non-detection provides few sig- 
nificant constraints on the nature of any additional far-UV lu- 
minosity component. We therefore observed SS433 with the 
UV/Optical Telescope (UVOT) and X-Ray Telescope (XRT) 
on board the Swift satellite to confirm the existence of an addi- 
tional luminosity component in the far UV. The observations 
might also locate the origin of this component in the binary 
system and constrain the physics of its emission mechanism. 
2. OBSERVATIONS 
The Swift mission, described by Gehrels et al. (2004), is 
devoted primarily to the detection of gamma-ray bursts and 
includes a UV/Optical telescope (UVOT) and an X-ray tele- 
scope (XRT) in its complement of instruments. The UVOT 
can observe photometrically in six different bandpasses cen- 
tered between 1880 A and the V band (cf. Table I ) :  its ou- 
I Department of Astronomy, San Diego State University San Diego, CA eration is described by Roming et al. (2005). The XRT i s  
921 82-1221 sensitive between 0.2 and 10 keV. Its operation is described 
NASAlGoddard Space Flight Center, Exploration of the Universe Divi- by B~~~~~~ et al. (2005). sion, Greenbelt, MD 20771 
Joint Center for Astrophysics, University of Maryland, Baltimore We observed SS433 with the UVOT and the XRT at the 
County, Baltimore, ME) 21250 six epochs given in Table 2. The 13 d (orbital) phase and 
https://ntrs.nasa.gov/search.jsp?R=20070034818 2019-08-30T01:50:26+00:00Z
Dolan, Boyd, & Cannizzo 
TABLE I .  UVOT BANDPASSES 
Filter Sensitivity A, (A) FWHM(A) " 
V 5440 750 2.24 220 
B 4390 980 1.31 85 
U 3450 875 1.49 60 
UVW 1 2510 700 3.52 75 
UVM2 2170 510 6.89 110 
W W 2  1880 760 6.04 70 
"Inverse Sensitivity(1): 10-l6 erg cm-2 s-I A-l/(ct S-l) 
bii.iverse Sensitivity(2): pJyl(ct s-') 
(JD 2 453 000+)" 
162.5 d (precession) phase at the start of each observation is 
also given there. Observations 014 and 015 included multiple 
exposures in each filter and extended over 35 ks and 115 ks, 
respectively. The data were reduced using the standard Swift 
analysis programs (HEASoft 6.0). 
3. RESULTS 
3.1. Photometry 
Figure 1, based on Fig. 11 of Gies et al. (2002), shows 
previously observed fluxes from SS433 in the optical and ul- 
traviolet. The diamonds and squares are taken from Wagner 
(1986) for the bright and faint states of SS433 in the visible. 
The 20 upper limits below 1700 A are from Gies et al. (2002). 
The error bars at 2770 A give the maximum and minimum 
fluxes reported by Dolan et al. (1997) using HSTJHSP. The 
spectral flux distribution of a reddened blackbody acceptably 
represents the observations in the visible. 
The mean flux at each epoch of observation (cf. Table 2) 
is shown in Fig. 1 in each UVOT bandpass. These fluxes 
agree with previous observations in the visible, and confirm 
the excess flux detected in the UV by Dolan et al. (1997). 
The mean flux at 1880 A is detected at the 120 level of sig- 
nificance in observation 015. It lies more than 20 times above 
the reddened blackbody spectral flux distribution extrapolated 
frcrn the optical. No flux is detected in the UVM2 bandpass at 
2370 A, the wavelength of maximum interstellar absorption; 
the error bars shown are 20  upper limits. 
3.2. Variability 
The mean flux in the five UV and optical bandpasses in 
which we detected the source varied from observation to ob- 
servation, as expected for SS433. Historically, a bright state 
FIG. 1 .- The spectral flux distribution observed from SS433. The data 
were taken at different epochs and different precessional and eclipse phases. 
Our individual W O T  observations for all visits in the six bandpasses given 
in Table 1 are shown as open triangles, except for the UVM2 data for which 
the three short horizontal line segments indicate upper limits. All other data 
shown is taken directly from Fig. 11 of Gies et al. (2002). The meaning 
of the other symbols is given in Sec. 3.1 in the text. The spectral flux dis- 
tribution of a reddened blackbody acceptably represents the observations in 
the visible. Fits with T = 72,000, 49,000, 45,000, and 21,000 K and 
Av = 8.4, 8.2, 7.8, and 7.8 are shown as dotted, dashed, dot-dash, and 
solid lines, respectively. 
of SS433 occurs at precession phase 0.8 f 0.05 (near max- 
imum redshift separation of the two sets of emission lines) 
at the binary phase of greatest elongation (9 = 0.3 i 0.15) 
and a faint state occurs when photometric eclipse occurs, es- 
pecially when the accretion disk around the X-ray source is 
nearly edge-on (y5 = 0.27 f 0.1) (Panferov et al. 1997). The 
lowest flux in the V bandpass was detected in observation 002, 
during photometric eclipse. Only an upper limit on the flux in 
the UVW2 bandpass was obtained at this epoch. (Observa- 
tions in the other bandpasses were not made at this epoch.) 
Variability of SS433 over a timescale of minutes has been 
reported in the optical (Zwitter et al. 1991; Revnivtsev et al. 
2004; Cherepashchuk et al. 2005). Observations 013A and 
013B, taken N 1.5 h apart, suggest that the far UV flux varies 
on a time scale of hours. We therefore monitored the source at 
N 90 min intervals over 0.4 d in observation 014 and over 1.3 
d in observation 015 to investigate UV variability on this time 
scale. The fluxes from SS433 during these two observations in 
the UV and optical bandpasses and in the 0.2 - 10 keV band- 
pass are shown as a function of time in Fig. 2. The flux den- 
sity is variable on a time scale of hours in all six bandpasses. 
The increase in flux density occurring at JD 2453917.6 is cor- 
related in the optical, UV, and X-ray bandpasses, rising to a 
FIG. 2.- The flux density in pJy of SS433 during observations 014 and 
01 5. The five UVOT bandpasses in which the source was detected are shown, 
along with the simultaneous 0.2-10 keV flux from the XRT. 
maximum over a timescale of - 10 ks. The flux density then 
decreases to its starting value over the next 40 ks in the optical 
and UV bandpasses. The X-ray flux density remains elevated 
for N 70 ks after the X-ray maximum before decreasing to its 
pre-maximum value. 
The additional flux appearing in each bandpass at the time 
of maximum flux density increases monotonically with fre- 
quency from - 40% of the quiescent flux in the V bandpass 
to - 75%, - 180%, -- 250%, and N 300% of the quies- 
cent flux in the B, U, UVWI, and UVW2 bandpasses. The 
increase in the X-ray bandpass is - 200% of the flux before 
and after maximum. 
The X-ray spectrum we observed between 0.2 - 10 keV 
during observation 015 is acceptably represented by the ab- 
sorbed power law type of spectrum given in eqn. [I]. The 
minimum X2 spectral fit to our data has photon s ectral index 4' 
-1.52 k 0.04 and NH = 6.4 f 0.3 x lo2' cm- . 
4. DISCUSSION 
A luminosity component in addition to the reddened black- 
body component that dominates the emission in the visible is 
Brinkman, W., Kawai, N., Matsuoka, M., h Fink, H. H. 1991, ABA, 241, 
112 
Burrows, D. N., et al. 2005, Sp. Sci. Rev., 120, 165 
Charles, P. A,, Barnes, A. D., Casares, J., Clark, J. S., Clarkson, W. 
I., Harlaftis, E. T., Hynes, R. I., Marsh, T. R., B Steeghs, D. 2004, 
RevMexAA, 20,50 
Cherepashchuk, A. M. 1981, MNRAS, 194,761 
clearly present in the far UV spectrum of SS433. Thrs far UV 
component extends into the visible, as shown by the corre- 
lated variability we detected in the UV and optical bandpasses 
during observations 014 and 015. The reddened blackbody 
component dominates in the optical and near UV, I-iowever; 
the fractional contribution to the luminosity of the variability 
ascribed to the far UV component decreases monotonrca!ly 
with frequency into the optical. 
Based on their correlated variability, the far UV component 
also extends into the soft X-ray region, but the power law 
continuum component must dominate there. Marshall et al. 
(2002) note that the continuum model they fit to their data at 
higher energies falls below the observed spectrum at wave- 
lengths > 7 A (cf. their Fig. 7). We suspect that this ad- 
ditional luminosity may arise from the same component we 
detect in the far UV. The absorbed X-ray specerunl given by 
eqn. [I] extrapolated to 1880 A lies two orders of magnitude 
below the flux we detect in the UVW2 bandpass. 
The similarity between the jet plus accretion disk geometry 
of SS433 and that present in OVV quasars has led to SS433 
being referred to as a microquasar (e.g., Cherepashchuk et a1 
2003; Charles et al. 2004). We note that a lurninositj~ compo- 
nent in the far UV in addition to those present in the optlcal 
and UV has been detected in the OVV quasar 3C345 (Dolan 
et al. 1994). The far UV component in 3C345 is produced by 
optically thin synchrotron radiation. A similar origin of t'ne 
far W component in SS433 would require its emission to be 
significantly polarized. 
The location of the far UV component in the binary system 
can be determined by monitoring its flux as a functlon of 13 
d and 162.5 d periods. In particular, if the far UV component 
is eclipsed near superior conjunction of the X-ray source, it 
must originate near the compact object itself, or at the base 
of the jets. The only non-detection of SS433 at 1880 oc- 
curred during photometric eclipse (observat~on 002), but the 
upper limit on the flux density at that epoch can not ~ule  out a 
flux consistent with that observed at other times in the UVW2 
bandpass. 
5. CONCLUSIONS 
A luminosity component in addition to those previously de- 
tected in the optical and X-ray regions must exist in the far 
UV spectrum of SS433. 
We acknowledge assistance from the Swift team, espe- 
cially Neil Gehrels and Sally Hunsberger for approving and 
scheduling the observations, and we thank Doug Gies for pro- 
viding the data to reproduce Fig. 11 from Gies et al. (2002). 
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SWIFT Observations of a Far UV Luminosity Component in SS433

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