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Analysis of Cometary X-ray Spectra
S. Z.Khalaf and M. I.Jaleel
1 INTRODUCTION
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-rays have been observed emanating from the Sun since the
1940s by the U.S. Naval Research Laboratory Blossom ex- periment on board [1]. On June 12, 1962 it detected the first
X-rays from other celestial sources (Scorpius X-1) [2]. It is the
first X-ray source found in that constellation.
X-ray and Extreme Ultraviolet (EUV) emission is usually as-
sociated with high temperature environments. The discovery that comets are bright emitters in this spectral regime was therefore a big surprise [3], and is in strong contrast to our understanding that comets are dirty snowballs surrounded by a gaseous coma with a temperature of approximately 50 K. After the first discovery by ROSAT of the X-ray emission from Comet Hyakutake, a search through the observatory’s archives proved that in fact all comets have total visual magnitude mv
< 12.0 at a distance 2 AU emitted X-rays. The Emission was highly variable in time, and many of the observed comets dis- played a characteristic crescent shape [4].Comets emit X-rays via the process of solar wind charge exchange (SWCX). Gas in the coma of the comet donates one or more electrons into an excited energy level of a highly-charged ion of the solar wind. In the subsequent relaxation of the ion, a UV or X-ray photon is emitted [5].
In the early of (1996) lisse et al. have been discovered a new
phenomenon the first Soft X-ray emission from comet
C/Hyakutake by Rontgen satellite (ROSAT) and she suggested that the emission could be explained by thermal bremsstrah- lung associated with hot electrons, due to solar wind interac-
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• S. Z. Khalaf is currently lecturer Assist Prof. has PhD in Astronomy Dep. InBaghdadUniversity,Iraq,PH-+9647707819819.
E-mail:salman.zkhalaf@yahoo.com
• M.I.Jaleel, Baghdad University, Science College, Astronomy Department, Iraq –
Baghdad.
tion effects [3]. The same event has been recorded by extreme ultraviolet explorer (EUVE) in (1997) [6]. Another discovered that Hale-Bopp comet emission X-ray by Krasnopolsky et al.in (1997) [7].In the same year in (January 1, 1997) Cravens pro- posed an alternate emission mechanism, the solar wind con- tains a large number of heavy ion species with a range of charge state. These ions will readily charge transfer with com- etary neutrals, producing ions which can be highly excited and consequently emit photons in the extreme ultraviolet and X-ray part of spectrum [5].
Shapiro et al. in (1999) also explained the X-ray emission
from comets by bremsstrahlung of electron produced by lower hybrid waves. The direct measurements of electron energy distributions in comet Halley from the Vega and Giotto space- craft preclude bremsstrahlung at a level exceeding equal 1% of the observed X-ray emission [8]. Vladimir et al. in (2000) they observed comet Hale –Bopp post perihelion in November 1997 and studied observing data on three comets Encke, Mueller (C/1993 A1), Borrelly from EUVE archive [9].In the same year Green wood et al. (2000) they measurement of charge ex- change and X-ray emission cross section for solar wind – comet interactions and they have been gotten results are com- pared to cross section values used in recent comet models. The importance of applying accurate cross sections, including double charge exchange, to obtain absolute line-emission in- tensities was emphasized [10].Krasnopolsky in (2001) He de- termined a peak X-ray volume emission rate in comet Hya- kutake during the Roentgen satellite (ROSAT) observation. He found that scaling of the electron fluxes observed in comet Halley from the Vega probe to the known properties of the solar wind electrons agrees with theory of electron accelera- tion by lower hybrid waves in comets [11,12]. Russo, Fernan- dez, Mumma et al. in (2001,2003,20034) studies have shown the spectra of comets from the infrared to the X-ray regions, provided a wealth of data on the molecular composition of
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comet atmosphere.In (2005) they have been detected Charac- teristic X-ray emission lines from simulants of comet surfaces as they undergo collisions with highly charged ions (HCIs), It is found that the emission of characteristic K L, K M X-rays appears to proceed during positive charging of the surface by the HCI beam [13].D.Bodewits et al. in (2007) they presented results of the analysis of cometary X-ray spectra with an ex- tended version of our charge exchange emission model [4]. They have applied this model to the sample of 8 comets thus far observed with the Chandra X-ray observatory and ACIS spectrometer in the 300–1000 eV range. Their analysis showed that spectral differences can be ascribed to different solar wind states; furthermore they predict the existence of a fourth spec- tral class, associated with the cool, fast high latitude wind [16].In (2012) since the initial discovery of Cometary charge exchange emission, more than 20 comets have been observed with a variety of X-ray and UV observatories. They reviewed the possibilities and limitations of each of those in their contri- bution [4].They have analysis of simultaneous X-Ray and UV observations of comet C/2007 N3 (Lulin) taken on three days between January 2009 and March 2009 using the Swift obser- vatory. They compared and discussed the X-ray and UV mor- phology of the comet and showed that the peak of the come- tary X-ray emission is offset sunward of the UV peak emis- sion, assumed to be the nucleus, by approximately 35,000 km. They showed that the measured X-ray light curve can be very well explained by variations in the comet’s gas production rates, the observing geometry and variations in the solar wind flux [14]. Ewing et al., in 2013 they have been presented the detection of new Cometary X-ray emission lines in the 1-2 KeV range ,they have selected five comets from the Chandra sam- ple . They modeled the spectra with an extended version of our solar wind charge exchange emission model [15].
The first comet that has been taken in this research C/linear 1999 is a long period comet; Linear has been discov- ered on September, 27, 1999. The comet made its closest ap- proach to the Earth on July, 22, 2000 at a distance of 0.3724 AU (5571000Km). It came to perihelion on July 26, 2000 at a dis- tance of 0.765 AU from the sun [17]. This comet observed by Lisse with a parameter shown in table (1).
The second comet that has been taken C/ NEAT (2001) is a
long period comet discovered on August 24, 2001 by the Near-
Earth Asteroid Tracking program (NEAT). The comet ap- proached within 0.32 AU of the Earth on May 6, 2004. And on May 12, 2004 detects by Chandra telescope with a parameter as shown in table (1).
We have been taken FITS image from Chandra X-ray tele- scope. The Chandra X-ray Observatory is a space telescope; it is sensitive to X-ray sources 100 times fainter than any previ- ous X-ray telescope, enabled by the high angular resolution of
its mirrors. Chandra consists of two X-ray detecting devices the High Resolution Camera (HRC) and the Advanced X-ray Astrophysics Facility CCD Imaging Spectrometer (ACIS).There are two sets of CCDs on the device: 4 imaging chips (ACIS-I) and 6 spectrometer chips (ACIS-S).
Then we have been taken the total image and by using
SAO image ds9 software we have been analysis X-ray image
after that we get the data for comet and by using spread sheet we get the chart as shown in figure (1) for C/linear 1999 comet and figure (2) for C/NEAT 2001 comet these analysis leads to get the results as explained in the result and discussion.
TABLE 1
THE COORDINATES OF COMETS
Fig. 1. The total X-ray image of C/ Linear 1999 comet.
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Fig . 3. Relation between Photon count (s) and Energy (eV) for C/linear
1999 comet.
Fig. 2. The total X-ray image of C/ NEAT 2001 comet.
Figure (1) and (2) shows the total image of X-ray of Hart- ley and Tuttle comets. It has been shown that these images are divided into square arrays. The reason as explains above is the Chandra telescope consists of several sensors in a matrix each of these sensors receive energy within certain energies. When all the energies are collected from these sensors, we will have the spectrum of the comet.
When highly charged ions from the solar wind collide on a neutral gas in comet the result is emission one or more pho- tons in the range of X-ray energy these photon counted per second as a function of energy in eV is the spectrum of comet each peaks in these spectrum is a material that we have been found in these comet, which each materials have line emission in particular energy like a fingerprint [18].
Note in Figure (3) there are six clear peaks each vertex represents a specific substance. These substances are Carbon (C) at energy 250 eV Kα1, oxygen (O) at energy 550 eV Kα1, Neon (Ne) at energy 850 eV Kα1, Magnesium (Mg) at energy
1350 eV, Silicon (Si) at energy 1750 eV Kα1, Sulfur (S) at energy
2250 eV Kβ1.The observed energy and standard for material as
shown in table (3) .
TABLE 2
THE RESULTS OF THE MATERIALS FOUND IN TW O COMETS
Fig. 4. Relation between Photon count (s) and Energy (eV) for C/NEAT
2001 comet.
Figure (4) demonstrates the spectrum of C/NEAT 2001 comet. The materials that have been found are Carbon (C) at energy 250 eV Kα1, oxygen (O) at energy 550 eV Kα1, Silicon (Si) at energy 1750 eV Kα1, and Phosphorus (P) at energy 2150 eV Kβ1,. The materials of C/NEAT 2001 comet and there standards energy shown in table (3).
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From table (3), we can recognize that there is a difference
TABLE 3
OBSERVED AND STANDARD ENERGY OF MATERIALS
between the observed and standard energy. This difference is attributed that these materials do not exist free, but they are united with the others.
Depend on the mechanism to get the X-ray spectrum the transition of electrons between main and sub shells of at- oms, each of these transition denoted by the specific symbols. Usually only the K-lines are useful in X-ray diffraction. There are several lines in the K-set. The strongest are Kα1, Kα2, and Kβ1.from the results we have been gotten transition type Kα1 and Kβ1.
We have been noted in the spectrum for two comets used in this research, that both of them contain of Carbon with high abundance depend on the photon counts, and low ener- gy. So when the energy increases we have been gotten heavier elements with lower abundance.
The dominant chemical elements found in comet are (H,C,O,N,S,Si) forming the different components [19]. When we have been analyzed the spectrum of comet, we found an- other elements appeared like, Neon and Magnesium in C/linear comet. From these results noted that the abundance of these materials is few depend on the value of the photon counts.
Comet composed mostly of mixture of water ice, dust, and compounds of carbon and silicates, that had been found in these two comets as shown in table (2). Water consists of 70% to 90% of the comet; it means that the Hydrogen percent is very high in spite of the percentage of the Hydrogen atom doesn’t appear in results because the hydrogen atom has one proton and one electron. So we can calculate exactly what the energy of electron has in each shell. But hydrogen is also the least energetic element. Even the most energetic line hydrogen emits (when an electron drops down from the second shell to the first) has only enough energy to be an ultraviolet photon. So hydrogen atoms do not emit X- rays. But another element have more electron, so it can be emitted photons in the range of X-ray like Carbon has six electrons but Carbon lines are at low energy of X-ray as shown in figure (3 , 4).
Another element appear the in results is the Oxygen
(Z=8), so it line’s energy greater than Carbon line. This is be- cause the Oxgyen have more electrons, therefore increasing the atomic number of the materials leads to increase the amount of the observed energy. This is demonstrate in our results that started from Carbon (Z=6), energy (250 eV) and ending with Sulfur has (Z=16), energy (2250 eV) as shown in figure (4).
The ten most common elements in the Milky Way Galaxy it’s not depend on the atomic number so hydrogen is the most abundant element in the known Universe, although hydrogen has the atomic number equal one. These ten element as shown in reference [21] most of the material that we found in comets correspond to the most abundant material in the universe, according to the rankings of materials, except phosphor, which appeared in C/NEAT 2001 as showed in figure (4).
4. CONCLUSIONS
The conclusion could be drawn as follows:
1. Both of comets contain Carbon because it is the fourth
most abundant element in the universe.
2. The strongest line emission is Kα1 and Kβ1.
3. We have been gotten emission spectrum as a result of
the interaction between solar wind and Cometary ion
tail.
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