On the Origin of the Fe K Alpha Emission Line from Intermediate Polar EX Hydrae

By

S. Esaenwi, and R.N.C. Eze

Intermediate polars are sub-class of magnetic cataclysmic variables (mCVs), in which a white dwarf (WD) primary accretes matter from a low-mass main sequence star. The moderate magnetic field (B~10 MG) of the WD truncates the inner parts of the disk, hence the accretion flow streams down towards the magnetic poles and onto the WD's surface (Galis et al., 2008; Barlow et al., 2006).

The WD in IPs rotates asynchronously with the orbital period of the system, Porb ≠ Prot, with Prot >> 100s and Porb ≥ 3 hours (Chanmugam & Frank, 1987), except for EX Hya which has Porb ≤ 2 hours (Warner, 2003).

3

Hard X-rays/Fe kα

lines

Bremsstrahlung/collisional ionization Fe k

α lin

es/

conti

nuum

flu

x

Pre-shock region

shock region

Hard X-rays/Fe kα lines

IPs are known to emit harder X-ray spectra,

compared to the polars, which is attributed

to their high accretion rates (Aungwerojwit,

2011).

The mCVs have been observed to emit Fe K

line, which are resolved into fluorescence

(6.4 keV), He-like (6.7 keV) and H-like (7.0

keV) lines (Ezuka & Ishida, 1999; Mukai et

al., 2003; Hellier & Mukai, 2004; Yuasa et al.,

2010).

The Fe K line in systems like EX Hya, were observed to be similar and contributes to Fe K line of the Galactic X-ray emission (GRXE, Bleach et al. 1972; Worrall et al. 1982; Iwan et al. 1982; Koyama et al. 1996; Kaneda et al. 1997; Ebisawa et al. 2001; Tanaka 2002; Revnivtsev & Sazonov 2007; Revnivtsev et al. 2009)(see, Eze et al. 2012).

This makes the study of the Fe K line from such systems significant beyond simply providing a better understanding of these systems themselves.

In this paper we discuss the origin of the Fe K line in EX Hya using spectral analysis of Suzaku data of the source.

The observation of EX Hya was done using

Suzaku satellite on 18-07-2007 for 91 kilo

seconds (ObsID 402001010).

We analyzed our data using the version 2.0 of the standard Suzaku pipeline products and the analysis soft wares, HEASoft version 6.11 and XSPEC version 12.7. In the data reduction,

We discarded all data collected when the satellite was in the south Atlantic anomaly for both the XIS sensors and the HXD PIN detectors.

Also discarded, were the data obtained when the Earth elevation and Earth-day elevation angles were respectively less than 5 and 20 degrees. These were done to reduce the effects of contamination due to the background X-ray noise.

We used a 250" radius with no apparent

sources and offset the corner calibrations to

extract events for the XIS detectors.

The extracted events were used to produce

the source spectrum.

The analysis of the HXD PIN detector was

done by downloading the non-X-ray tuned

PIN background les and the appropriate

response matrix files for the observation, as

generated by the Suzaku team.

The HXD PIN data analyzed were those in the 12 -

40 keV energy band. The good time intervals

were merged using the mgtime FTOOLS to obtain

a common value for both the PIN background and

source event files.

The source and background spectra for the

source were extracted using the XSELECT filter

time file routine.

The dead time of the observed spectra was

corrected using the hxddtcor command of the

Suzaku FTOOLS.

Exposure time for observation for the derived background spectrum was increased by a factor of 10 to take care of the event rate in the PIN background, which was made 10 times higher than the real background for suppression of the Poisson errors, in accordance with the standard Suzaku analysis procedure.

XSPEC version 12.7 was used for the spectral fitting.

The spectrum was modeled using the absorbed thermal bremsstrahlung model with three Gaussian lines for the Fe K emission line complex.

We assumed two types of absorption by full-

covering and partial covering matter. Our fittings

cover 3-10 keV for the XIS BI, 3-12 keV for the

XIS FI and 15-40 keV for the HXD PIN.

We ignored the energy range below 3 keV in the

XIS FI and BI detector to avoid intrinsic

absorption (multi-column absorption and ionized

absorption of the pre-shock gas) which is known

to affect data at this energy range, and energies

above 10 keV were ignored for XIS BI because

the instrument background is higher compared

to the XIS FI detectors.

We also ignored the energy range above 40 keV

in the HXD PIN detector in order to obtain high

signal-to-noise ratio signals.

The cross-normalization factor for the PIN and

XIS data was fixed at the typical value of 1 for

XIS and at 1.16 and 1.18 for HXD PIN, for XIS

nominal and HXD-nominal respectively.

The Fe K emission line complex, which resolves

to the low ionized or neutral (6.41 keV), the He-

like (6.70 keV) and the H-like (7.00 keV) iron

lines were resolved clearly as shown in Figure

1.The best fitting parameters of EX Hya for a thermal bremsstrahlung model with a partial-covering matter and full-covering matter are shown in Table 1.

We searched for the origin of Fe K fluorescence line in our source by adding the XSPEC reflection model to the absorbed thermal bremsstrahlung model. The spectral parameters are presented in Table 2.

We generated an XIS light curve of the source after background subtraction. Both the BI and the FI data were combined.

The energy range used is 0.1 - 12.0 keV, while the bin size is 512.0 s.

The light curve of EX Hya shows no stellar flaring activity at the time of observation as shown Figure 2.

PARAMETER VALUE UNIT

NHf 0.78 ± 0.05 1022 cm-2

NHp 105 ± 15 1022 cm-2

c 0.41 ± 0.03 keV

(KT) 10 ± 1 KeV

(Fcounts) 22.5 ± 0.1 photons s-1cm-2

E6.4 6.41 ± 0.01 KeV

E6.7 6.66±0.01 KeV

E7.0 6.95±0.01 KeV

F6.4 3.5 ± 0.2 10-5 photon s-1 cm-1

F6.7 3.2±0.3 10-5 photon s-1 cm-1

F7.0 1.2±0.3 10-5 photon s-1 cm-1

EW6.4 28 +2, -3 eV

EW6.7 36 +5, -2 eV

EW7.0 109 +5, -3 eV

The origin of hard X-rays from EX Hya

On the Origin of the Fe K Emission Line

Fe K Emission Line as a Diagnostic Tool for

High Energy Astrophysical Sites

We acknowledge the Suzaku team for providing

data and some relevant files used in the

analysis of this work. This research made use

of data obtained from Data Archives and

Transmission System (DARTS), provided by

Center for Science-satellite Operation and

Data Archives (C-SODA) at ISAS/JAXA.

THANK

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