On the origin of the Fe Kα emission line from intermediate polar EX Hyrae

New Astronomy 35 (2015) 84–87

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On the origin of the Fe Ka emission line from intermediate polar EX Hyrae S. Esaenwi a,b, R.N.C. Eze a,⇑ a b

Department of Physics and Astronomy, University of Nigeria, Nsukka, Enugu State 410001, Nigeria NASRDA-Centre for Basic Space Sciences, Nsukka, Enugu State 410001, Nigeria

h i g h l i g h t s  We resolved the 6.4, 6.7, and 7.0 keV lines.  The 6.4 keV line is created by reflection of hard X-rays from the N_H absorption.  The 6.7 and 7.0 keV lines are largely emitted through collisional excitation.

a r t i c l e

i n f o

Article history: Received 21 August 2014 Received in revised form 20 September 2014 Accepted 22 September 2014 Available online 30 September 2014 Communicated by E.P.J van den Heuvel Keywords: Stars: novae Cataclysmic variables X-rays: stars

a b s t r a c t We present spectral analysis of intermediate polar EX Hya observed with Suzaku satellite on 18-07-2007 for 91 kilo seconds (ks). We model the spectrum with an absorbed thermal bremsstrahlung and three Gaussian lines for the Fe Ka line complex. We resolved the 6.4, 6.7, and 7.0 keV lines, each corresponding to the neutral (or low-ionized), He-like, and H-like iron ions. The fluorescence line is due to irradiation of neutral (or low ionized) material (iron) by hard X-ray sources, as a collisional origin would lead to rapid ionization. The He-like and H-like iron ions are emissions created by collisional excitation and photoionization/excitation in the vicinity of the hot white dwarf (WD). We study the emission of these lines in EX Hya and found that the fluorescence line is likely created by reflection of hard X-rays from the N H absorption columns as our search for the reflection of hard X-rays on the surface of the WD yielded a null result. The 6.7 and 7.0 keV lines, however, are largely emitted through collisional excitation in the vicinity of the compact white dwarf shock heated by the shock front. We also discussed Fe Ka emission line as a diagnostic tool for high energy astrophysical sites. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction 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 P 3 hours (Chanmugam and Frank, 1987), except for EX Hya which has P orb 6 2 hours (Warner, 2003). IPs are known to emit harder X–ray spectra, compared to the polars, which is attributed to their high accretion rates (Aungwerojwit, 2011).

⇑ Corresponding author. Tel.: +234 8037791388. E-mail address: [email protected] (R.N.C. Eze). http://dx.doi.org/10.1016/j.newast.2014.09.009 1384-1076/Ó 2014 Elsevier B.V. All rights reserved.

Ex Hya is a bright X-ray source with a progressive occultation of a rotating accretion disk by the companion star (Hellier et al., 1987). It emits X-rays in soft range (0.7–2 keV) and hard range (3–10 keV) (Watson et al., 1978). It is an eclipsing binary with orbital period of 98 min, which was estimated from the recurrence time of the eclipse (Vogt et al., 1980). There is a controversy in the measurement of the mass of the WD of the primary, which ranges from (0.42–0.79) M (see, Cropper et al., 1998; Cropper et al., 1999; Cropper et al., 2002; Beuermann and Reinsch, 2008; Yuasa et al., 2010; Hayashi and Ishida, 2014). It has an accretion rate of 2:8  1015 gs1 (Suleimanov et al., 2005). The fluorescence (6.4 keV) line is usually emitted whenever the neutral (or low ionized) material (iron) is irradiated by a hard X-ray source, whereas the He-like (6.7 keV) and H-like (7.0 keV) lines are created by collisional excitation and photoionization in a hot plasma. However, in systems like EX Hya, collisional excitation dominates (see, Eze, 2014). The mCVs have been observed to emit Fe Ka line, which are resolved into fluorescence (6.4 keV), He-like

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(6.7 keV) and H-like (7.0 keV) lines (Ezuka and Ishida, 1999; Mukai et al., 2003; Hellier and Mukai, 2004; Yuasa et al., 2010). Ezuka and Ishida (1999) also suggested that the reflection of hard X-rays from the white dwarf surface makes a significant contribution to the observed Fe Ka fluorescence line. However, the origin of these lines in mCVs are yet to be completely addressed. Eze (2014) discussed in details previous observations of the Fe Ka line in the seyfert galaxies, quasars and other galaxies. The Fe Ka line in systems like EX Hya, were observed to be similar and contributes to Fe Ka 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 and Sazonov, 2007; Revnivtsev et al., 2009) (see, Eze et al., 2012). This makes the study of the Fe Ka 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 Ka line in EX Hya using spectral analysis of Suzaku data of the source. In Section 2, we discuss the observation of the source with Suzaku satellite, in Section 3, we present our data analysis and in Section 4, we discuss our results and conclusions. 2. Observation The observation of EX Hya was done using Suzaku satellite on 18-07-2007 for 91 ks (ObsID 402001010). Suzaku is a joint Japanese–US mission, which was developed by the Japanese Institute of Space and Aeronautical Science (ISAS), a branch of the Japan Aerospace Agency (JAXA), in conjuction with National Aeronautics and Space Administrations Goddard Space Flight Center (NASA/GSFC) and many other institutions. It was launched with the M–V launch vehicle from JAXA’s Uchinoura Space Center (USC) on July 10, 2005, to replace the ASTRO-E which failed to orbit during its launch on February 10, 2000. The details of Suzaku instrumentation can be found in (Mitsuda et al., 2007) and references therein. The satellite’s primary mission is to obtain high spectral resolution data of astrophysical objects in the energy range of 0.2–700 keV (soft X-rays to c-rays), for guest investigator(s) through the sponsorship of a NASA Guest Observer Program. After one year from the end of the period of observation, the collected data are placed in the Suzaku Public Archive for public usage. The EX Hya data used in this paper was retrieved from the Suzaku Public Archive.

service since November 9, 2006 due to operational fault, therefore, no data was obtained from it. XIS1 is a back-illuminated (BI) chip and its spectrum is referred to as BI. The analysis of the HXD PIN detector was done by downloading the non- X-ray tuned PIN background files 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 Ka 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-tonoise 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 Ka 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 Fig. 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.

EX Hya

-3 0 3 10-2

1 10-1 10

10

-1

7.0

-2

6.5

-3

Intensity (s keV-1)

6.0

XIS FI XIS BI HXD PIN

-3

0

3 10

-4

χ

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 off set the corner calibrations to extract events for the XIS detectors. The extracted events were used to produce the source spectrum. The same radius with no apparent sources and offset from both the source and corner calibrations was used extract the background event and hence produce XIS background spectrum. The response files, which includes the redistribution matrix file (RMF) and the ancillary response file (ARF) were generated for the XIS detectors using the FTOOLS, xisrmfgen and xismarfgen commands respectively. The XIS 0, 2 and 3 are front-illuminated (FI) chips whose features are similar, so their spectra were merged and referred to as FI. XIS 2 had been out of

10-1

3. Data analyzes and results

3

5

10 Energy (keV)

40

Fig. 1. The spectrum of EX Hya. In the upper panel, the data and best-fit model are shown by crosses and solid lines, respectively. Each spectral component is represented by dotted lines. In the lower panel, the ratio of the data to the bestfit model is shown by crosses. The inset in the upper panel is an enlarged view for the Fe Ka complex lines.

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EX HYA

Unit

N fH N pH C kT F cont

0:78  0:05

1022 cm2

105  15

1022 cm2

0:41  0:03 10  1 22:5  0:1

keV

E6:4 F 6:4

6:41  0:01 3:5  0:2

EW6.4 E6:7 F 6:7

28þ2 3 6:66  0:01 3:2  0:3

EW6.7

36þ5 2

E7:0 F 7:0

6:95  0:01 1:2  0:3

EW7.0

109þ5 3

103 photons s1 cm2 keV 105 photons s1 cm2 eV keV 105 photons s1 cm2 eV

512.0

s

105 photons s1 cm2 eV

10

keV

0

We searched for the origin of Fe Ka 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 Fig. 2.

4. Discussions and conclusion 4.1. The origin of hard X-rays from EX Hya Hard X-rays from IP, such as EX Hya is usually emitted at the shock front region where there is shocking heating of the surrounding plasma to a higher temperature of the order of 10 keV. EX Hya has a moderate magnetic field (B 10 MG) of the WD, which truncates the inner parts of the disk, hence the accreting materials are magnetically channeled to the accretion column at the WD poles. This is followed by thermal bremsstrahlung cooling by free electrons with kT of the order of 10 keV and above (Cropper, 1990; Warner, 2003). The emission of hard X-rays is assumed to be through the post-shock region, which is below the shock front created from the impacting accretion column. The emitted soft X-rays from the system are believed to be from the absorption and reprocessing of the hard X-rays in the plasma surrounding the WD. We obtained a strong bremsstrahlung continuum with a temperature of 10 keV coupled with the strong Fe Ka line at neutral or low-ionized (6.41 keV), He-like (6.70 keV), and H-like (7.00 keV) signifying that the hard X-rays from EX hya are thermal in origin.

Table 2 The reduced v2 (R v2 ), full v2 (F v2 ) and degree of freedom (dof) for the absorbed bremsstrahlung and an absorbed bremsstrahlung plus XSPEC reflect model, F–test probability and reflection normalization.

1

Bin time:

Count/sec 20

Value

15

Parameter

25

Table 1 Best Fitting Parameters of EX Hya by the Thermal Bremsstrahlung Model.

Parameter

Value

BM, (R v2 )/(F v2 )/dof BRM, (R v2 )/(F v2 )/dof FP RN

1.45/3338.2/2300 1.45/3336.2/2296 0.85 –

Parameters are the absorbed bremsstrahlung model (BM),an absorbed bremsstrahlung plus XSPEC reflect model (BRM),F–test probability of null result for the BRM (FP)and reflection normalization (RN).

5×104

105 Time (s)

Start Time 14299 21:48:40:414

1.5×105

2×105

Stop Time 14302 10:32:24:414

Fig. 2. Light curve of EX Hya. showing no sign of stellar flaring activity in the source at the time of observation.

4.2. On the origin of the Fe Ka emission line We searched for reflection of hard X-rays from the surface of the WD in the EX Hya by adding the XSPEC reflection model to the absorbed thermal bremsstrahlung model. Unfortunately, we were unable to detect reflection of hard X-rays from the surface of the WD (see Table 2), implying that the observed Fe Ka fluorescent emission line may be a result of reflection of hard X-rays from the N H full and partial absorbing columns. Eze (2014) found that the reflection of hard X-rays in the N H absorption columns dominates in three hard X-ray emitting symbiotic stars leading to the emission of Fe Ka fluorescent line, where as in SS73 17, the emission of the Fe Ka fluorescent line is due to a combination of reflection of hard X-rays from the surface of the WD and the N H absorption columns. The reflection–induced Fe Ka Fluorescent emission line could also come from the cold gas on or near the surface of the WD or pre-shock region (see, Hayashi et al., 2011). Photoionization (and excitation) and collisional ionization/excitation are dominant processes in most of the high energy astrophysical sites such as EX Hya system, other hard X-ray emitting mCVs, hard X-ray emitting symbiotic stars (hSSs) and AGN. The emission of He-like and H-like iron lines is a continuously ongoing process in such plasma (see e.g., Eze, 2014). However, one of the processes dominates in a given source and in the EX Hya, the dominant process may be the collisional ionization/excitation. This conclusion is based on the assumption that the hard X-rays from the source are thermal in origin. 4.3. Fe Ka emission line as a diagnostic tool for high energy astrophysical sites The Fe Ka line can be used to deduce the values of some physical parameters such as temperatures, densities, excitation conditions, ionization balance, elemental abundances and electric and magnetic field distributions in high energy astrophysical sites. Our current knowledge of the physical processes going on in high energy astrophysical sites such as mCVs, hard X-ray emitting symbiotic stars, X-ray binaries and AGN is mainly from the X-ray spectra of the source, which has the Fe Ka line as a main feature. The iron abundance in these sources is far more than any other heavy metal, hence, the iron fluorescent emission line is a hallmark of the accretion driven X-ray sources (Piro, 1993). We can use the Fe Ka

S. Esaenwi, R.N.C. Eze / New Astronomy 35 (2015) 84–87

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