On the Contribution of the 6.7 keV Line Emission of Algol Binary 1 [619013]

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On the Contribution of the 6.7 keV Line Emission of Algol Binary 1
System to the 6.7 keV Line E mission from the Galactic Ridge 2
*Ambrose C hukwudi EZE1, 2, Romanus N wachukwu Chijioke EZE2, 3& Sudum ESAENWI4 3
[anonimizat] ,Tel:+[anonimizat] ;[anonimizat] ,Tel:+[anonimizat] 4
;[anonimizat] , Tel:+[anonimizat] 5
1Department of Physical and Geosciences, Faculty of Natural and Applied Sciences, Godfrey 6
Okoye University, Ugwuomu -Nike, P.M.B. 01014, Enugu State. Nigeria. 7
2Department of Physics and Astronomy, Faculty of Physical Sciences, University of Nigeria, 8
Nsukka, 410001, Nigeria. 9
3Institute of Space and Astrono mical Science, Japan Aerospace Exploration Agency, 3 -1-1 10
Yoshinodai, Chou -ku, Sagamihara, Kanagawa 252 – 0222, Japan. 11
4NASRDA -Centre for Basic Space Science, Nsukka. 12
*Correspondence: [anonimizat] 13
Abstract: We carried out spectroscopic analysis of the extracted stellar flare of the Algol binary 14
system and resolved a strong 6.7 keV line emission. The 6.7 keV line emission of the Algol binary 15
system is similar to the 6.7 keV line of the Galactic Ridge X-ray Emission (GRXE). The equivalent 16
width (EW) compared favorably with the EW of the 6.7 keV emission line obtained from different 17
Galactic ridge regions . In the Galaxy, we have a reasonable number of Algol binary systems as 18
eclipsing strong coronal X -ray emitters and many other stars which are also strong X -ray emitters, 19

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and are characterized by frequent quiescent and super flaring phases as observed by Suzaku, and 1
these systems, could contribute to the 6.7 keV emission line from the Galactic ridge. 2
Keywords : Binaries, Algol -stellar flares, X -ray, Galactic Ridge X -ray Emission. 3
4
1. INTRODUCTION 5
Flare stars are variable stars that exhibit violent and sporadic flare activity. Flare stars have 6
strong magnetic fields in their coronae. The eruption in the ma gnetic field generates stellar flares 7
which are due to sudden, rapid and intense dramatic increase in the brightness of the stars that las t 8
for a few minutes. The stellar flares rise to the peak brightness during rapid rotation of these sta rs, 9
but decay to normal brightness in an exponential curve. Flare stars exhibit coronal activities of about 10
3 orders of magnitude more energetic than in the Sun [1]. The characteristic of the spectra of flare 11
stars is the presence of large areas of strong magnetic field i n the coronae which give rise to strong 12
emission lines. The presence of emission lines requires the existence of dense chromospheres, and 13
the heating of the dense chromospheres is associated with magnetic regions in the coronae. The 14
stellar flare emission measure depends on the temperature in the flare [2]. High temperature in the 15
flare result to large emission measure and, probably emission lines of abundant element in the 16
coronae. In X -ray band, the emission lines of the ionized elements fall in the 1.8 t o 40 Å when the 17
temperature in the flare is KT ≥ 1 keV, and this provides measurements of fluxes of iron (Fe) 18
emission lines [1]. Spots are also known to exist on the surface of flare stars; therefore, the phys ical 19
processes involved in the atmosphere of f lare stars are probably not distinct from those occurring in 20
the Sun. Flare stars are expected to flare only during those parts of their evolution when rotation is 21
“fast”. The faster the flare star rotates, the stronger are the magnetic fields generated by the dynamo 22

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[3]. This applies to the flare stars of higher mass, but in flare stars of low mass, rotation need n ot be 1
excessively fast in order to generate magnetic field since the convection zone can penetrate into th e 2
center of the stars. There are numer ous flare stars including T. Tau stars, RS CVn ( RS Canum 3
Venaticorum variables) systems, Algols, W UMa and NU UMa systems [1, 3, 4, 5] that have been 4
observed in the Milky Way Galaxy. 5
Algol (Beta Persei) is an eclipsing binary system in the constellation P erseus at Right 6
Ascension 03h 08m 10.1315s and Declination +40ș57' 20.33". The Algol binary system is about 28.46 7
pc away from the Earth at an orbital inclination of 81.4ș, and located about 92.8 light years from t he 8
Sun with an apparent magnitude of about -2.5. Algol (Beta Persei) was first detected in X -ray energy 9
region by Small Astronomy Satellite (SAS) 3 in October 1975 [6]. It was confirmed that Algol’s 10
stellar flares, observed with Sounding rocket flights, are strong coronal X -ray emitters, and the 11
Mass-transfer model; Roche Lobe overflow or stellar wind mechanisms explain the X -ray emission 12
from Algol [7]. The Algol binary system consists of a primary early -type main -sequence star (Algol 13
A; B -star, e.g. B8V) and a Roche Lobe filling secondary sub -giant star (Algol B; K -star, e.g. K2IV). 14
The X -ray emission from the Algol binary system arises mostly from the corona of the K -star since 15
B-star is X -ray dark [8]. The two stars (K – and B – stars) are tidally locked and rapidly rotating and 16
this causes the co nvection zone of the K -star to generate magnetic energy that is dissipated at the 17
stellar surface in the form of X -rays [9,10]. The presence of elemental abundance (such as; S, Si, Al, 18
Mg, Ne, Fe, C, N, O) in the convection zone (chromospheres/coronal) dur ing the quiescent and peak 19
flared phases of the Algol binary system have been obtained and confirmed by GIGA (Japanese for 20
‘galaxy’ X -ray) and ASCA (Advanced Satellite for Cosmology and Astrophysics) satellites [11,12]. 21
ROSAT (Röntgensatellit) observation revealed that the X -ray luminosity of the Algol binary system 22
during the flared and quiescent phases are 32102 ergs-1 and 31107.0 ergs-1 [13, 14]. The B -star has 23

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a mass of 3.7 M ʘ and a radius of 2.9 R ʘ, whereas the mass of the K -star is 0.81 M ʘ with a radius of 1
3.5 R ʘ, and the binary separation between the two stars is about 14.14 R ʘ (Mʘ = 1.988 × 1030 kg, R ʘ 2
= 6.957×105 m; [15]). The massive B -star is in the main sequence phase, whereas the less massive 3
sub-giant K -star is still on the evolutionary stage and this can give rise to an Algol Paradox. In the 4
stellar evolution model, as the K -star fills its Roche Lobe, most of its mass is transferred onto the B – 5
star in the main sequence phase as both stars ar e tidally locked with an orbital period of 2.87 days. 6
During Algol binary eclipsing, the B -star occults major fractions of the X -ray emission from the K – 7
star. Tidal forces synchronize rotational and orbital periods of this close binary system. Stellar 8
flaring activities in the Algol binary system occur as a result of chromospheric and coronal activities; 9
chromospheric evaporation. This generates magnetic activities and the charged particles produced, 10
during magnetic activities, transport the released magnet ic energy into the corona [13] which 11
manifest as stellar flares. The light -curve of the Algol binary system shows a definite and significant 12
stellar flare activity as K -star comes out of the eclipse contributing 85% of X -ray emission whereas 13
B-star contrib utes 15% of the X -ray emission [16]. The observed X -ray stellar flares from Algol 14
begin with a magnetic instability that eventually leads to magnetic reconnection in tangled magnetic 15
fields in the corona. In the reconnection region, heating, particle accel eration, and some bulk mass 16
acceleration take place and the energized particle travels along closed magnetic fields toward the 17
stellar surface (of B -star) where accretions are formed. As the material/particles reach denser layers 18
in the chromospheres, the y deposit their energy by collisions, heating the ambient plasma 19
explosively to millions of temperature [17]. The ensuing over pressure drives the hot plasma into 20
coronal loops, at which point the “X -ray stellar flares” manifests with extreme luminosities and 21
radiative energies with often time scales in excess of one hour. This suggests that stellar flares f rom 22
Algol belong to a class of “2 -ribbons”or arcade stellar flares which exhibit long rise time with large 23
magnetic field structure and complex loop arc ades [18]. 24

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In early 80s, GRXE was unresolved X -rays emission along the galactic plane [19, 20], with 1
energy spectra (2 -10 keV) as basic properties. These spectra are characterized by many intense 2
emission lines from highly ionized ions of the abundant e lements (C, N, O, Ne, Mg, Si, Sr, Ar, Ca, 3
and Fe). Two hypotheses; “Diffuse source” and “Point source” scenarios were posited for the origin 4
of GRXE. The “Diffuse source” scenario proposed that GRXE is diffuse in nature, suggesting that 5
GRXE is generated w hen hot plasma fills the interstellar medium over the large scales [21, 22], of 6
the Milky Way Galaxy with Supernova explosions as a probable candidate sources of this hot 7
plasma. This hypothesis failed in explaining the origin of GRXE in that; energy injec tion (that is, 8
heating and replenishment of plasma) and confinement on the Galactic plane [23] is not possible. 9
The “Point source scenario” proposed that GRXE is discrete or “point -like” in nature, suggesting 10
that GRXE is the integrated emission of faint g alactic point sources [24, 25]. GRXE have a strong 11
iron emission line in 6 – 7 keV energy range [26]. These emission lines were resolved into; 6.4 keV 12
(Neutral or low -ionized) line emission, 6.7 keV (He -like) line emission, and 7.0 keV (H -like) line 13
emission [27]. The 6.7 keV and 7.0 keV emission lines are thermal emissions with broadening 14
spectra while 6.4 keV line emission is a narrow thermal emission consistent with the fluorescence of 15
cool matter. The 6.4 keV originates from the reflection of incident X -rays by cold gas [28], whereas 16
the 6.7 keV and 7.0 keV GRXE are as a result of photo ionization/collisional excitation in the 17
vicinities of the white dwarfs associated with the numerous discrete/faint coronally X -ray sources 18
(discrete X -ray stars). 19
GRXE traces the stellar mass population distribution in the Galaxy. Numerous point sources, 20
including RS CVn syst em, dwarf novae, and magnetic cataclysmic variables (mCVs) explain the 21
origin of GRXE [29], and each population source contributes to different X -ray energy regimes. 22
Magnetic CVs are the most probable candidate sources due to their spectral resemblance to the 23

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GRXE and iron K -lines. The major contributors to the Fe Kα -line of the GRXE are magnetic CVs 1
(mainly intermediate polars; IPs) because these population sources exhibit low luminosities, high 2
volume densities, and hard spectra shape [26, 28, 30]. The co mbination of IPs and active binaries 3
(ABs) will explain most of the Fe Kα -line emission in the Galactic ridge [28, 31]. Is there an 4
additional galactic source whose spectral component could contribute to the total luminosity of the 5
Fe Kα -line of the GRXE? Galactic sources reside very close to the galactic plane and these galactic 6
sources might be high mass X -ray binaries rather than CVs [26], and other yet to be identified 7
galactic binary systems that can contribute to the total luminosity of Fe Kα -line of the GRXE. 8
In this work, we resolved a strong 6.7 keV line emission from the extracted stellar flare of Algol a nd 9
found that the EW compares favorably with the EW of the 6.7 keV line from the Galactic ridge. We 10
are, therefore, suggesting that a collection of stellar flares in our galaxy, which emit such 6.7 keV 11
line could account for the total luminosity of the 6.7 keV line from the GRXE. 12
2. DATA ACQUISITION 13
We retrieved the observed Algol stellar flare data used in this research work from Suzaku 14
data Publi c Archive. The observation of Algol’s stellar flares was performed with X -ray Imaging 15
Spectrometer (XIS) at the focal planes of the X -ray Ray Telescope (XRT) on board the Suzaku 16
Satellite on March 8th – 10th, 2007 with a net exposure time of about 512 kilo seconds and 17
observation identification (Obs Id; 401093010). The XIS contains three functional sets of X -ray 18
Charge Coupled Device (CCD) camera systems (XIS 0, 1, and 3). XIS 0 and 3 have front – 19
illuminated (FI) CCDs; while XIS 1 has a back -illuminated (BI) CCD. Details of Suzaku Satellite, 20
the XRT and XIS of Suzaku satellite are found in [32, 33, 34] respectively. 21
22
23

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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 S uzaku pipeline products 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 radi us 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 spectra, 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 Redistribution 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 spectro meter Ancillary matrix 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 performed 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 (hydroge n 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
22
23

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3. RESULTS/ANALYSIS 1
The Table below shows the best -fit spectr al parameters and the error in each parameter are 2
estimated at the 90% confidence ranges. Fig. 1 shows the background subtracted light -curve of the 3
Algol binary system. Fig. 2 shows the resolved 6.7 keV emission line of stellar flares of Algol binary 4
syste m. The 6.7 keV emission line corresponds to the peak of the spectrum curve of both XIS FI and 5
XIS BI. 6
3.1 THE EQUIVALENT WIDTHS (EWs) OF THE 6.7 keV LINE EMISSION OF THE 7
ALGOL STELLAR FLARES AND THE GRXE 8
The research work of [27] analyzed the spectral data of iron emission lines on the Galactic 9
ridge observed at eight different regions with Suzaku satellite. They obtained ~ 6.67 keV emission 10
line energy (on each region) at equivalent width range; ~ 300 eV – 980 eV, and argued that the ~6.7 11
keV emission line was clearly found in the Galactic ridge, indicating that thin thermal hot plasmas 12
with equivalent width range; ~ 300 eV – 980 eV are located along the Galactic ridge. This observed 13
center energies of the ~6.7 keV emission line from [27] are c onsistent with the theoretical value of 14
Fe line in Collision Ionization Equilibrium (CIE) plasma (6.680 keV; [35]). In 2012, [30] carried ou t 15
a broadband spectral analysis of the GRXE at different twelve (12) Galactic bulge/ridge regions 16
observed with Suza ku satellite for total exposure of one mega seconds, and obtained a 17
composite/cumulative center energy spectrum at 6.68 keV with EW =14
17 322
 eV, and the stellar 18
population point sources; cataclysmic variables (CVs) and active binaries (ABs) are the major 19
contributors of the GRXE. Hence, the 6.7 keV line emission equivalent width (EW = 510.18 eV) 20
from Algol’s stellar flares compared favorably with the 6.7 keV emission line equivalent widths 21
obtained from the Galactic ridge. 22

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1
4. DISCUSSION OF R ESULTS 2
The 6.7 keV emission line is produced as a result of photo ionization/collisional excitation in 3
the hot plasma . The light -curve of the Algol binary system shows a typical signature of a stellar 4
flare. The Bremstrahlung model with a Gaussian line for the 6.7 keV emission line gave a 5
statistically acceptable fit (see Table 1). We resolved prominent 6.7 keV emission line, with an 6
equivalent width (EW= 510.18 eV), from stellar flares of Algol binary system which compares 7
favorably with the equivalent wid ths of the 6.7 keV emission line obtained from different Galactic 8
ridge regions. Therefore, the Algol binary system might be among the probable galactic sources 9
whose stellar flares contribute to the 6.7 keV GRXE. 10
4.1 CONTRIBUTION OF STELLAR FLARES FROM ALGOL BINARY SYSTEM TO 11
THE 6.7 keV EMISSION LINE FROM THE GALACTIC RIDGE 12
13
Our Galaxy hosts luminous point sources such as low mass accreting White Dwarfs; 14
magnetic & non -magnetic CVs, active binaries, and coro nally active late -type stars with intrinsic X – 15
ray luminosity range; 1030-33 ergs-1 which are plausible candidates for the origin of the 6.7 keV 16
emission line [36], but these point sources have not completely explained the total luminosity of 6. 7 17
keV emissi on line. X-ray flaring stars and X -ray binaries (e.g. Algol) whose luminosity range; 1030-18
33 ergs-1 could also be probable candidate sources that can contribute to the total luminosity of 6.7 19
keV emission line. 20
In 2009, [29] resolved about 80% of the Gala ctic ridge sources (473) detected by Chandra 21
Observatory in energy range; 0.5 -7 keV into point/discrete sources (accreting White dwarfs of 22

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luminosity; L 2-10 keV ~1031-32 ergs-1, and binary stars with strong coronal activity; coronally active 1
stars, of luminosity; L 2-10 keV < 1031 ergs-1) of strong Fe emission line at 6.7 keV, but the study did 2
not consider if stellar flares from other flaring stars and X -ray binaries (e.g. Algol) with intrinsic X – 3
ray luminosity range; 1031-33 ergs-1 are capable of gener ating various Fe emission lines in the Galactic 4
ridge. S tellar flares from RS Canum Venaticorum variables (RS CVn’s), Algols, young stellar 5
objects and other flare stars with luminosity range; 31106.1 ergs-1 to 33108.4 ergs-1 and 6
temperature distribution (KT= a few keV to ~ 10 keV) is capable of generating the various emission 7
lines required for the GRXE [37]. The research work by [38] analyzed the X -ray spectra of Algol, 8
observed with XMM (X -ray Multi -Mirror Mission) -Newton, in both the quiescent and the flaring 9
phases and resolved 6.7 keV emission line. The equivalent width of the 6.7 keV emission line was 10
not reported, though they argued that the hot temperature component [ K710)42( ] of the stellar 11
flares of Al gol have higher Fe abundance at 6.7 keV when compared to the cool temperature 12
component ( K6104.6 ). The photometric and spectroscopic analysis of 2002 X -ray point sources 13
by [39] reported that the thermal source group A (hard spectrum; White Dwarfs such as magnetic 14
and non -magnetic CVs, pre -CV, and Symbiotic stars) and group B (soft and broad spectrum; X -ray 15
active stars in flares) are the major contributors of Fe Kα -line emission of the Galactic ridge. These 16
stellar population sources show st rong Fe Kα -line at 6.7 keV and 7.0 keV. The mean equivalent 17
width of 6.7 keV is; 268ିଽଽାଶସସ eV for A and 413ିଶଵ଺ାଶ଺ଷ eV for B, and their fractional contribution to the 18
Fe Kα -line emission GRXE was estimated; 2:1, but [39] did not explain the mechanism that 19
generates the 6.7 keV emission line. The research work on stellar flares of four hard X -ray emitting 20
Symbiotic Star s (hSSs) and nineteen magnetic cataclysmic variables by [31, 40] attributed the 21
mechanism of the broadening spectra of Fe Kα -line of the GRXE (6.7 keV and 7.0 keV) to be as a 22
result of photo ionization/collisional excitation whereas the 6.4 keV line emissi on originates from 23

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the reflection of hard X -ray. Cataclysmic variables and active binaries contribute immensely to the 1
Fe Kα -line of the GRXE but the contributions of the hSSs are not negligible [41]. The contribution 2
of stellar flare from coronally -active star and binaries (ASBs; EW = 800 eV) and cataclysmic 3
variables (CVs; magnetic and non -magnetic, EW =170 eV) to the thermal GRXE have been 4
estimated [42]. He reported that ASBs and CVs contributes 78±12% and 16±4% of the thermal 5
GRXE in energy range; 2 -10 keV whereas in energy range; 6 -10 keV, ASBs and CVs contributes 6
62±10% and 21±5% of the thermal GRXE respectively while the contribution of bright X -ray 7
binaries (XRBs) was negligible on the cause of the research. The author concludes that ~80% of the 8
6.7 keV- and 7.0 keV emission lines are attributed to the X -ray emission (from the stellar flares) of 9
these two population sources in ratio; 3:1 split between ASBs and CVs, but the author did not 10
specifically quantify the contribution of these sources to the 6.7 keV emission line. Further argument 11
by [42] suggested that cumulative X -ray emission from bright Galactic X -ray binaries (XRBs) 12
together with X -ray emission from relatively young galactic source population might contribute 13
~20% of the thermal GRXE, and in view of this, Algol is an X -ray binary system. 14
[36] were of the view that we need point sources with large equivalent widths to explain the 15
total luminosity of 6.7 keV Fe characteristic emission line from the Galactic ridge. The equivalent 16
width of 6.7 keV emission line emitted in the Galactic plane and ridge are 540±200 eV and 350±40 17
eV [22, 43]. About ~80% of the 6.7 keV and 7.0 keV emission lines are from the X -ray emission 18
(from the stellar flares) of Coronally -active stars and binaries (ASBs) and c ataclysmic variables [42]. 19
In this present work, we discovered that t he equivalent width of the Algol (Beta Persei), 510.18 eV 20
compares favorably with the equivalent width of the 6.7 keV GRXE in the range; 300 eV – 980 eV 21
depending on the galactic position s [29], and this implies that a collection of sources like Algol 22

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binaries are probable X -ray sources that may explain the total luminosity of the 6.7 keV emission 1
line from the GRXE. 2
In order to determine the actual contribution of the 6.7 keV line emissi on of stars that flares 3
to that of the GRXE, the procedure by [25, 31, 41, 42, 44] should be followed where the stellar 4
density in the galaxy is taken into consideration in determining the total luminosity of the 6.7 keV 5
line of the stars to be compared wi th that of the GRXE. However, this is beyond the scope of this 6
present work, but Eze et al., in preparation will adequately address the issue. 7
5. CONCLUSION 8
9
We analyzed Algol’s (Beta Persei) stellar flare data observed with Suzaku. The light -curve of 10
the Algol binary system shows a typical signature of a stellar flare. We resolved 6.7 keV emission 11
line of stellar flares of Algol binary system. The 6.7 keV emission line of stellar flares of Algol is 12
similar to the 6.7 keV emission lines from the Galactic r idge in different regions. 13
We observed that the equivalent width (EW) of the 6.7 keV emission line of the Algol binary 14
system, 510 eV, compares favorably with the EW of the 6.7 keV emission lines from the Galactic 15
ridge. We are of the view that collection of similar systems (short period Algols; U Cep, TW Dra, 16
RZ Cas, δ Lib, RW Ara, XZ Sgr, X Gru, V505, TV Cas, etc.) like the Algol (Beta Persei) and other 17
flare stars could, along with CVs, hSSs, ABs, and ASBs, account for the total luminosity of 6.7 keV 18
emission line from the Galactic ridge. 19
20
ACKNOWLEDGEMENT 21
The authors acknowledge the Suzaku team for providing data and any relevant files used in the 22
analysis presented here. This research made use of data obtained from Data Archives and 23

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Transmission System (DARTS), provided by Center for Science -satellite Operation and Data 1
Archives (C -SODA) at ISAS/JAXA. We are also very grateful to the Nigerian TETFund for the 2
TETFund National Research Grant support which was used to provide the computer and relevant 3
softw ares used in this research work. 4
5
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5
Table: Eclipsing Algol binary system Spectra l parameters 6
Spectral Parameter Value 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
7
8
9
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 .

17

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 repre sented with dotted
lines from the energy axis (black; XIS FI and red; XIS BI) . The ratio of the data to th e best -fit model is shown by
crosses in the lower panel.