On the Contribution of the 6.7 keV Line Emission of Algol Binary System to the 1 [619012]

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On the Contribution of the 6.7 keV Line Emission of Algol Binary System to the 1
6.7 keV Line Emission from the Galactic Ridge 2
*Ambrose C hukwudi EZE1, 2, Romanu s Nwachukwu Chijioke EZE2, 3& Sudum ESAENWI4 3
Jerry410001 @gmail.com , [anonimizat] , [anonimizat] 4
1Department of Physical and Geosciences, Faculty of Natural and Applied Sciences, Godfrey Okoye 5
University, Ugwuomu -Nike, P.M.B. 01014, Enugu State. Nigeria. 6
2Department of Physics and Astronomy, Faculty of Physical Sciences, University of Nigeria, 7
Nsukka, 410001, Nigeria. 8
3Institute of Space and Astronomical Science, Japan Aerospace Exploration Agency, 3 -1-1 9
Yoshinodai , Chou -ku, Sagamihara, Kanagawa 252 – 0222, Japan. 10
4NASRDA -Centre for Basic Space Science, Nsukka. 11
*Correspondence: [anonimizat] 12
Abstract: We carried out spectroscopic analysis of the extracted stellar f lare of the Algol 13
binary system and resolved a strong 6.7 keV line emission. The 6.7 keV line emission of the Algol 14
binary system is similar to the 6.7 keV line of the Galactic Ridge X -ray Emission (GRXE). The 15
equivalent width (EW) compared favorably with the EW of the 6.7 keV emission line obtained from 16
different Galactic ridge regions . In the Galaxy, we have a reasonable number of Algol binary 17
systems and many other stars as strong coronal X -ray emitters characterized by frequent quiescent 18
and super flari ng phases as observed by Suzaku, and these systems, could contribute to the 6.7 keV 19
emission line from the Galactic ridge. 20
Keywords : Binaries, Algol -stellar flares, X -ray, Galactic Ridge X -ray Emission. 21
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1. INTRODUCTION 1
Flare stars are variable stars that exhibit violent and sporadic flare activity. Satellite 2
observations across the Radio, Optical, and X-ray band regions have revealed the presence of dense 3
chromospheres , and coronae in the flare stars. The dynamo motion during the rapid rotation of flare 4
stars generate a magnetic field that is dissipated in the coronae [ 1]. The eruption in the magnetic 5
field generates stellar flares. The stellar flares last for a few minutes and this occur as a result of the 6
intense dramatic increase in the brightness during the rapid rotation of these stars. The magnitude of 7
coronal activities in the flare stars is about 3 times more energetic than in the Sun [2]. Spots are also 8
known to exis t on the surface of flare stars; therefore, the physical processes involved in the 9
atmosphere of flare stars are probably not distinct from those occurring in the Sun. There are 10
numerous X-ray flare stars including T. Tau stars, RS CVn ( RS Canum Venaticorum variables) 11
systems, Algols, W UMa and NU UMa systems [1, 2, 3, 4] that have been o bserved in the Milky 12
Way Galaxy. 13
Algol (Beta Persei) is a n X-ray binary system in the constellation Perseus . It is located about 14
92.8 lig ht years from the Sun with short orb ital period of ~ 2.9 days . Algol was first detected in X – 15
ray energy region by Small Astronomy Satellite (SAS) 3 in October 1975 [5]. The Sounding rocket 16
flight confirmed that Algol’s stellar flares are strong X -ray emitters. The Mass -transfer model ; 17
Roche Lobe overflow or stellar wind mechanisms , explain s the X -ray emission from the Algo l 18
binary system [6]. The presence of elemental abundance ( S, Si, Al, Mg, Ne, Fe, C, N, O) in the 19
convection zone (chromospheres/coronal) during the quiescent and peak flared phases of the Algol 20
binary system have been observed and confirmed by GIGA (Japane se for ‘galaxy’ X -ray), ASCA 21
(Advanced Satellite for Cosmology and Astrophysics) , and X MM (X -ray Multi -Mirror Mission) 22
Newton satellites [7, 8, 9] . ROSAT (Röntgensatellit) observation revealed that the X -ray luminosity 23
of the Algol binary system during the flared and quiescent phases are 32102 ergs-1 and 24

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31107.0 ergs-1 [10, 11] . Algo l consists of a B -star and a sub -giant K -star. The B-star has a mass of 1
3.7 M ʘ, whereas the mass of the K -star is 0.81 M ʘ. The binary separation between these stars is 2
14.14 R ʘ, and t heir individual radius is about 2.9 R ʘ, and 3.5 R ʘ respective ly (Mʘ =1.988 × 1030 kg, 3
Rʘ = 6.957×105 m; [12, 9]). The B-star is in the main sequence p hase, wh ereas K-star is on the 4
evolutionary stage and this can give rise to an Algol Paradox. In the stellar evolution model, when 5
the K -star fills its Roche Lobe, most of its mass is transferred onto the B -star during accretion. The 6
K-star have active cor ona, and most of the X -ray emissions from the Algol binary system are 7
contributed by the K -star [13, 14] . The stellar flaring activ ities occur as a result of chromospheric 8
and coronal activities; chromospheric evaporation. This generates magnetic activitie s that transport 9
magnetic energy into the corona [10]. The stellar flares from Algol manifests with luminosities that 10
last for long duration , and this suggests that stellar flares from Algol belong to a class of “2 – 11
ribbons”or arcade stellar flares [15]. 12
GRXE is an X-ray emission along the galactic plane with hard energy spectra in the range; 2 13
-10 keV, as basic properties , and t he emission lines from elements such as C, N, O, Ne, Mg, Si, Sr, 14
Ar, Ca, and Fe have been observed in these energy spectra [16, 17 , 18, 19] . GRXE traces the 15
distribution of stellar mass in the Milky Way Galaxy. The integrated emission fr om faint galactic 16
point sources explains the origin of GRXE , and each population source contributes different X -ray 17
energy to the total luminosity of the G RXE [20, 21, 22]. Energy injection and confinement in the 18
Galactic plane is difficult to achieve [23, 24, 25] . Therefore, the origin of the GRXE cannot be 19
explained by the diffuse source scenario. In 6 – 7 keV energy range, thermal iron emission line (Fe 20
Kα-line) is the prominent emission line among other line emissions observed in the Galactic ridge 21
[26]. This Fe Kα -line was resolved into; 6.4 keV, 6.7 keV and 7.0 keV line emission s [19, 22, 27] . 22
The 6.7 keV and 7.0 keV emission lines are broadening spectra while 6.4 keV line emission has a 23
narrow spectrum. The 6.4 keV line emission originates from the reflection of incident X -rays by cold 24
gas [28], and it is consistent with the fluorescence of cool matter . The 6.7 keV and 7.0 keV GRXE 25

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are due to photo ionization/collisional excitation in the vicinities of the white dwarfs [e.g. 29, 30, 31] 1
associated with the numerous discrete/faint coronally X -ray sources (discrete X -ray stars ). Some of 2
the detected point sources (Magnetic CVs; intermediate polars (IPs) and coronally active stars 3
(ABs) ) exhibit spectral morphology (shape) that is similar to the GRXE and Fe Kα -line emission 4
[26, 22, 28, 30, 32] . Recent researches have sho wn that these stellar point sources contribute most of 5
the Fe Kα -line emission in the Galactic ridge [28, 30, 33, 34 ], but the total luminosity of the Fe Kα – 6
line of the GRXE has not been accounted for. Is there an additional galactic source whose spectral 7
component could contribut e to the total luminosity of the Fe Kα -line of the GRXE? High mass X -ray 8
binaries rather than CVs [26], and other yet to be identified galactic binary systems reside very close 9
to the galactic plane and these Galactic point sources can contribute to the total luminosity of Fe Kα- 10
line of the GRXE. 11
In this work, we resolved a strong 6.7 keV line emission from the extracted stellar flare of 12
Algol and found that the EW compares favorably with the EW of the 6.7 keV line emission from the 13
Galactic ridge. We are, therefore, suggesting that a collection of stellar flare s in our galaxy, which 14
emit such 6.7 keV line could account for the total luminosity of the 6.7 keV line from the GRXE. 15
2. DATA ACQUISITION 16
We retrieved the observed Algol stellar flare data used in this research work from Suzaku 17
data Public Archive. The o bservation of Algol’s stellar flares was performed with X -ray Imaging 18
Spectrometer (XIS) at the focal planes of the X -ray Ray Telescope (XRT) on board the Suzaku 19
Satellite on March 8th – 10th, 2007 with a net exposure time of about 512 kilo seconds and 20
observation identification (Obs Id; 401093010). The XIS contains three functional sets of X -ray 21
Charge Coupled Device (CCD) camera systems (XIS 0, 1, and 3). XIS 0 and 3 have front – 22
illuminated (FI) CCDs; while XIS 1 has a back -illuminated (BI) CCD. Details of Suzaku Satellite, 23
the XRT and XIS of Suzaku satellite are found in [35, 36, 37] respectively. 24

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2.1 DATA ANALYSIS 1
The spectroscopic data analysis of the extracted Algol stellar flares’ was done using version 2
2.0 of the off -line standard Suzaku pipeline pr oducts and the tools provided in HEASoft version 6.10 3
packages. The Algol flux was extracted from a circular region using 180 arc -seconds radius, and the 4
extracted flux was saved. During the extraction, we ensured that this arc -second radius covered 5
about 90% of the Algol’s flux. The background spectra (flux) were extracted from a circular region 6
center using 100 arc -seconds radius with no apparent sources, and we save the extracted background 7
flux. We subtracted the background spectra from the source spect ra, and generated the light -curve. 8
We then extracted only the portion (between 40 to 90 kilo seconds of the light -curve, see the purple 9
broken lines in Fig. 1) where the Algol binary system showed stellar flares. The Redist ribution 10
Matrix File (RMF), and Ancillary Response File (ARF), was created for the XIS sensors (XIS 0, XIS 11
1, XIS 3) using the Flexible Image Transport System Tools (FTOOLS), X -ray imaging spectrometer 12
response matrix file generator ( xisrmf -gene) and X -ray imaging spectrometer Ancillary ma trix 13
response file generator ( xissamrf -gene) respectively. We merged the spectral data of XIS 0, and 3, 14
and referred it as XIS front illuminated (FI) spectrum. We referred XIS1 spectrum as XIS back – 15
illuminated (BI) spectrum. 16
The spectral analysis was perf ormed using XSPEC version 12.8. We modeled the spectrum using 17
thermal bremsstrahlung model with a Gaussian line. Our spectral fitting covers 4.5 – 7.5 keV for 18
both the XIS FI and XIS BI. We were not able to measure the absorption (hydrogen column density, 19
NH) in both full and partial covering matters due to low photon counts in the Algol’s extracted stella r 20
flares spectra and primarily to the 4.5 keV lower limits to our fits. 21
3. RESULTS 22
The Table below shows the best -fit spectral parameters and the error i n each parameter are 23
estimated at the 90% confidence ranges. Fig. 1 shows the background subtracted light -curve of the 24

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Algol binary system. Fig. 2 shows the resolved 6.7 keV emission line of stellar flares of Algol binary 1
system. The 6.7 keV emission line corresponds to the peak of the spectrum curve of both XIS FI and 2
XIS BI. 3
4. DISCUSSION OF RESULTS 4
The light -curve of the Algol binary system shows a typical signature of a stellar flare. An 5
inspectio n of the light curve shows that between 0 – 34 kilo secon ds there is no significant change in 6
the observed stellar flares. This is the quiescent phase of the observed stellar flares . The stellar flares 7
rise gradually from 35 kilo seconds with dramatic increase in brightness and reach its peak at 60 kilo 8
seconds, and start decaying with a decrease in brightness. The stellar flares start to rise again at 15 0 9
kilo seconds for another cycle as K-star rotate rapidly around the B – star. The 6.7 keV emission line 10
is produced as a result of photo ionization/collisional excitation in the hot plasma . The 11
Bremstrahlung model with a Gaussian line for the 6.7 keV emission line gave a statistically 12
acceptable fit (see Table 1). We resolved 6.7 keV line emission from stellar flares of Algol binary 13
system that is similar to the 6.7 keV line emission obtained from the Galactic ridge . 14
We c ompared the equivalent width (EW= 510.18 eV) of the 6.7 keV line emission of Algol with that 15
of the equivalent width of the 6.7 keV line emitted in the Galactic ridge. We found that the 16
equivalen t width of Algol compares favorably with the equivalent widths (300 eV – 980 eV) of the 17
6.7 keV emission line obtained from a different Galactic plane/ ridge regions [19, 25, 27, 32] . 18
Therefore, the Algol binary system might be among the probable galactic s ources whose stellar 19
flares contribute to the 6.7 keV GRXE. 20
4.1 CONTRIBUTION OF STELLAR FLARES FROM ALGOL BINARY SYSTEM TO 21
THE 6.7 keV EMISSION LINE FROM THE GALACTIC RIDGE 22

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Our Galaxy hosts luminous point sources ; cataclysmic variable and active bi naries (with an 1
intrinsic X -ray luminosity range; 1030-33 ergs-1) as the plausible candidates for the origin of the 6.7 2
keV emission line because their large EW spectra is similar to the 6.7 keV spectrum observed in the 3
Galactic ridge [32, 38], but these p oint sourc es have not completely explained the total luminosity of 4
6.7 keV emission line. X-ray flaring stars and X -ray binaries (e.g. Algol) whose luminosity range; 5
1030-33 ergs-1 could also be probable candidate sources that can contribute to the total l uminosity of 6
6.7 keV emission line. 7
In 2009, [22] resolved about 80% of the Galactic ridge sources (473) detected by Chandra 8
Observatory in energy range; 0.5 -7 keV into many point s/discrete sources (accreting White dwarfs 9
of luminosity; L 2-10 keV ~1031-32 ergs-1, and binary stars with strong coronal activity; coronally active 10
stars, of luminosity; L 2-10 keV < 1031 ergs-1) of strong 6.7 keV line emission , but the study did not 11
consider if stellar flares from other flaring stars and X -ray binaries (e.g. Alg ol) with luminosity 12
range; 1031-33 ergs-1 are capable of generating Fe Kα -line on the Galactic ridge. Stellar flares from RS 13
Canum Venaticorum variables (RS CVn’s), Algols, and other flare stars with luminosity range; 1031-14
33 ergs-1 generates Fe Kα -line si milar to th ose of GRXE [39]. The stellar flares f rom Algol during the 15
quiescent and the flaring phases have revealed high Fe abundant at the 6.7 keV line emission [9]. 16
The photometric and spectroscopic analysis of cataclysmic variables ( CVs), Symbiotic sta rs, and X- 17
ray active stars show s strong Fe Kα -line at 6.4 keV, 6.7 keV , and 7.0 keV [29, 30, 33, 40], and the 18
shape of the spectra of these sources resemble those of the GRXE . It is widely believed that these 19
population sources are the major contributo rs of Fe Kα -line. The contributions of some of these 20
point sources to the Fe Kα -line in the Galactic ridge have been estimated [33]. [34] reported that 21
about ~ 80% of the 6.7 keV – and 7.0 keV emission lines of the GRXE are contributed by coronally 22
active star and binaries (ASBs) and cataclysmic variables (CVs). Further argument by [34] suggested 23
that X -ray emission from Galactic X -ray binaries (XRBs) and young galactic stellar source s might 24
contribute ~20% of the GRXE, and in view of this, Algol is an X -ray bin ary system. ASBs and CVs 25

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are considered as the major contributors of the Fe Kα emission line of the GRXE because of their 1
higher population mass density, mass transfer rate, and hard spectra [e.g. 30] when compared to 2
other observed point sources. The contributions of hard X -ray emitting symbiotic stars [e.g. 33, 34, 3
40] and other yet to be identified Galactic point sources, to the Fe Kα emission line in the Ga lactic 4
ridge are not negligible. The challenges we have now are that a large number of these point sources 5
which could contribute to the uncounted Fe Kα emission line in the Galactic ridge have not been 6
observed by present X -ray telescopes. Moreover, [38] were of the view that the equivalent width of 7
the 6.7 keV line emission emitted by any point source is expected to be large. The equivalent width 8
of the 6.7 keV line emi ssion from point sources must be comparable to the equivalent width of 6.7 9
keV emission line observed in the Galactic plane /ridge [19, 25, 27, 32]. 10
In this present work, we discovered that t he equivalent width of the Algol (Beta Persei), 510.18 eV 11
compares favorably with the equivalent width of the 6.7 keV GRXE in the range; 300 eV – 980 eV 12
depending on the galactic po sitions, and this implies that a collection of sources like Algol binaries 13
are probable X -ray sources that may explain the total luminosity o f the 6.7 keV emission line from 14
the GRXE. 15
In order to determine the actual contribution of the 6.7 keV line emission of Algol and stars that 16
flares to that of the GRXE, the procedure by [21, 30, 34, 40, 41 ] should be fol lowed where the stellar 17
density in the galaxy is taken into consideration in determining the total luminosity of the 6.7 keV 18
line of the stars to be compared with that of the GRXE. However, this is beyond the scope of this 19
present work, but Eze et al., in preparation will adequately address the issue. 20
5. CONCLUSION 21
We analyzed Algol’s (Beta Persei) stellar flare data observed with Suzaku. The light -curve of 22
the Algol binary system shows a typical signature of a stellar flare. We resolved 6.7 keV emission 23

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line of stellar flares of Algol bina ry system. The 6.7 keV emission line of stellar flares of Algol is 1
similar to the 6.7 keV emission lines from the Galactic ridge in different regions. 2
We observed that the equivalent width (EW) of the 6.7 keV emission line of the Algol binary 3
system, 510 e V, compares favorably with the EW of the 6.7 keV emission lines from the Galactic 4
ridge. We are of the view that collection of similar systems (short period Algols; U Cep, TW Dra, 5
RZ Cas, δ Lib, RW Ara, XZ Sgr, X Gru, V505, TV Cas, etc.) like the Algol (Be ta Persei) and other 6
flare stars could, along with CVs, hSSs, ABs, and ASBs, account for the total luminosity of 6.7 keV 7
emission line from the Galactic ridge. 8
ACKNOWLEDGEMENT 9
The authors acknowledge the Suzaku team for providing data and any relevant fil es used in 10
the analysis presented here. This research made use of data obtained from Data Archives and 11
Transmission System (DARTS), provided by Center for Science -satellite Operation and Data 12
Archives (C -SODA) at ISAS/JAXA. We are also very grateful to the Nigerian TETFund for the 13
TETFund National Research Grant support which was used to provide the computer and relevant 14
softwares used in this research work. 15
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Table: Eclipsing Algol binary system Spectral parameters 21

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Spectral Parameter Valu e Unit
KT 3.61 0.12 keV
Fcount 0.18±0.01 10-3 Photons s-1 cm-2
E6.7 6.660 0.003 keV
EW 6.7 510.18±0.50 eV
1
2
3
4
5 KT = Continuum temperature, F count = flux count, E 6.7 = Energy center of the 6.7 keV , EW 6.7 =
equivalent width of 6.7 keV emission line.
Fig. 1 : Background subtracted light -curve of the eclipsing Algol’s
stellar flares as a function of time during observation .

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1
2
3
4 Fig. 2 : The upper panel shows spectrum of the eclipsing Algol binary system, while the
data and the best -fit model are shown by crosses and solid lines respectively, and the peak of
the spectrum is 6.7 keV line as represented with dotted lines from the energy axis (black; XIS
FI and red; XIS BI) . The ratio of the data to the best -fit model is shown by crosses in the lower
panel.