---
abstract: 'We present the results of a 50 ks long Chandra observation of the dipping source XB 1916–053. During the observation two X-ray bursts occurred and the dips were not present at each orbital period. From the zero-order image we estimate the precise X-ray coordinates of the source with a 90% uncertainty of 0.6. In this work we focus on the spectral study of discrete absorption features, during the persistent emission, using the High Energy Transmission Grating Spectrometer on board the Chandra satellite. We detect, for the first time in the 1st-order spectra of XB 1916–053, absorption lines associated to , , and , and confirm the presence of the and absorption lines with a larger accuracy with respect to the previous XMM EPIC pn observation. Assuming that the line widths are due to a bulk motion or a turbulence associated to the coronal activity, we estimate that the lines are produced in a photoionized absorber distant from the neutron star $4 \times 10^{10}$ cm, near the disk edge.'
author:
- 'R. Iaria, T. Di Salvo, G. Lavagetto, N. R. Robba, L. Burderi'
title: 'Chandra Observation of the Persistent Emission from the Dipping Source XB 1916–053 '
---

Introduction
============

Low Mass X-ray Binaries (LMXBs) consist of a low mass star ($\le 1$ M$_\odot$) and a neutron star (NS), generally with a weak magnetic field ($B \le  10^{10}$ G). In these systems the X-ray source is powered by the accretion of mass overflowing the Roche lobe of the companion star and forming an accretion disk around the NS. Different inclinations of the line of sight with respect to the orbital plane can explain the different properties of the lightcurves of these systems. At low inclinations ($i \le 70^\circ$) eclipse and dips are not observed in the lightcurves, while they can be present at high inclination angles because the companion star and/or the outer accretion disk intercept the line of sight.

About 10 LMXBs are known to show periodic dips in their X-ray lightcurves. The dips recur at the orbital period of the system and are probably caused by a thicker region in the outer rim of the accretion disk, formed by the impact with the disk of the gas stream from the companion star (White & Swank, 1982). The dip intensities, lengths and shapes change from source to source, from cycle to cycle. For systems seen almost edge-on, X-ray emission is still visible due to the presence of an extended accretion disk corona (ADC, White & Holt 1982). The ADC can be periodically eclipsed by the companion star and have a radius between $10^9$ and $5 \times 10^{10}$ cm, with an appreciable fraction (from 10% up to 50 %) of the accretion disk radius (see Church & Balucinska-Church, 2001 for a review). Information on the emitting region in LMXBs can be obtained by studying the spectrum of these high inclination sources and its evolution during the dips. The energy spectrum of the dipping sources can be well described using two different scenarios. In the “absorbed plus unabsorbed” scenario (e.g., Parmar et al. 1986) the persistent (non-dipping) spectral shape was used to model the spectra during the dips, but is divided into two parts. One part is allowed to be absorbed, whereas the other one is not. The spectral evolution during dipping is well modelled by a large increase in the column density of the absorbed component, and a decrease of the normalization of the unabsorbed component. The latter component is generally attributed to electron scattering in the absorber. In the “progressive covering” scenario (e.g., Church & Balucinska-Church 1995), the X-ray emission is assumed to originate from a point-like blackbody, or disk-blackbody component (probably from the neutron star surface or the inner region of the accretion disc), together with a power law (probably from the extended ADC). These two components can be used to fit the spectrum both during the persistent emission and the dipping activity. Moreover, during the dipping the model well describes the spectral changes due to the partial and progressive covering of the power-law emission from an extended source. The absorption of the point-like component is allowed to vary independently from that of the extended component, and usually no partial covering is included because during the dipping activity it is fully covered. Both these approaches have been applied to XB 1916–053 (e.g., Yoshida et al. 1995; Church et al. 1997, respectively).

The improved sensitivity and spectral resolution of Chandra and XMM-Newton are allowing to observe narrow absorption features, from highly ionized ions (H-like and He-like), in a larger and larger number of X-ray binaries. These features were detected from the micro-quasars GRO J1655–40 (Ueda et al. 1998; Yamaoka et al. 2001) and GRS 1915+105 (Kotani et al. 2000; Lee et al. 2002). Recently the Chandra High-Energy Transmission Grating Spectrometer (HETGS) observations of the black hole candidate H 1743–322 (Miller et al. 2004) have revealed the presence of blue-shifted and absorption features suggesting the presence of a highly-ionized outflow. All LMXBs those exhibit narrow X-ray absorption features are all known dipping sources (see Table 5 of Boirin et al. 2004) except for GX 13+1. This source shows deep blue-shifted Fe absorption features in its HETGS spectrum, again indicative of outflowing material (Ueda et al. 2004).

More recent Diaz Trigo et al. (2005) has modelled the changes in both the X-ray continuum and the Fe absorption features during dipping of six bright LMXBs observed by XMM-Newton. They concluded that the dips are produced by an increase in the column density and a decrease in the ionization state of the highly ionized absorber. Moreover, outside of the dips, the absorption line properties do not vary strongly with orbital phase, this implies that the ionized plasma has a cylindrical geometry with a maximum column density close to the plane of the accretion disk. Since dipping sources are normal LMXBs viewed from close to the orbital plane this implies that ionized plasmas are a common feature of LMXBs. Similar results were obtained by Boirin et al. (2005) studying a XMM-Newton observation of XB 1323–619.

XB 1916–053 (4U 1916–05) is a dipping source with the shortest period of all dipping sources of 50 min (Walter et al., 1982), and it is also notable because of the difference of 1 % between the X-ray and optical periods (see Callanan et al., 1995 and references therein). Recently Retter et al. (2002) have favored the superhump model to explain this discrepancy. The superhump model invokes a precessing accretion disk which identifies the X-ray period as orbital. XB 1916–053 was observed with OSO-8 and Ginga above 10 keV. From the OSO-8 results of White & Swank (1982) it was clear that dipping persisted up to 20 keV. Using BeppoSAX data Church et al. (1998) showed that the spectrum of the dipping source extends above 100 keV.

Boirin et al. (2004) studied a 17 ks XMM-Newton observation using EPIC pn and RGS data in timing mode, the exposure time during the persistent emission was 10 ks. The authors detected, in the EPIC pn data, a K$_\alpha$ and a K$_\alpha$ absorption line centered at 6.65 and 6.95 keV, with upper limits on the line widths of 100 and 140 eV, respectively; moreover they marginally detected, in the RGS data between 0.5 and 2 keV, an absorption line centered at 1.48 keV with an upper limit of the corresponding line width of 41 eV and, finally, an absorption edge at 0.99 keV. The absorption line of 1.48 keV was associated to K$_\alpha$ and the absorption edge was associated to moderately ionized Ne and/or Fe. Using the ratio between the and column density they have estimated a ionization parameter log($\xi$) of 3.92 erg cm s$^{-1}$. From a combined analysis during and out of the dipping intervals they concluded that during the dipping activity the absorber is composed by cooler material.

In this work we present a spectral analysis of the persistent emission from XB 1916–053 in the 0.8–10 keV energy range using a 50 ks long Chandra observation. The observation covered entirely 16 orbital period however we noted that the dips were not present at all. We clearly detected, for the first time in the spectra of this source, the presence of the K$_\alpha$, K$_\alpha$, K$_\alpha$, and K$_\alpha$ absorption lines and confirmed the presence of the K$_\alpha$, and K$_\alpha$ absorption lines, although the better energy resolution of Chandra and the larger statistics allowed to well determine the widths of each line. We discuss that the absorption lines were produced in a photoionized absorber placed at the edge of the accretion disk, probably the same absorber producing the dips when it is less photoionized.

Observation
===========

XB 1916–053 was observed with the Chandra observatory on 2004 Aug 07 from 02:27:22 to 16:07:31 UT using the HETGS. The observation had a total integration time of 50 ks, and was performed in timed graded mode. The HETGS consists of two types of transmission gratings, the Medium Energy Grating (MEG) and the High Energy Grating (HEG). The HETGS affords high-resolution spectroscopy from 1.2 to 31 Å(0.4–10 keV) with a peak spectral resolution of $\lambda/\Delta
\lambda  \sim 1000$ at 12 Å  for HEG first order. The dispersed spectra were recorded with an array of six charge-coupled devices (CCDs) which are part of the Advanced CCD Imaging Spectrometer-S (Garmire et al., 2003)[^1]. We processed the event list using available software (FTOOLS v6.0.2 and CIAO v3.2 packages) and computed aspect-corrected exposure maps for each spectrum, allowing us to correct for effects from the effective area of the CCD spectrometer.

The brightness of the source required additional efforts to mitigate “photon pileup” effects. A 512 row “subarray” (with the first row = 1) was applied during the observation reducing the CCD frame time to 1.7 s. Pileup distorts the count spectrum because detected events overlap and their deposited charges are collected into single, apparently more energetic, events. Moreover, many events ($\sim 90
\%$) are lost as the grades of the piled up events overlap those of highly energetic background particles and are thus rejected by the on board software. We, therefore, ignored the zeroth-order events in our spectral analysis. On the other hand, the grating spectra were not, or only moderately (less than 10 %), affected by pileup. In this work we analysed the 1st-order HEG and MEG spectra; since a 512 row subarray was applied the 1st-order HEG and MEG energy range were shrinked to 1–10 keV and 0.8–7 keV, respectively.

To determine the zero-point position in the image as precisely as possible, we estimated the mean crossing point of the zeroth-order readout trace and the tracks of the dispersed HEG and MEG arms. We obtained the following coordinates: R.A.=$19^h18^m47^s.871$, Dec.=$-05^{\circ}  14\arcmin  17\arcsec.09$ (J2000.0, with a 90% uncertainty circle of the absolute position of 0.6[^2]). We compared the Chandra position of XB 1916–053 referred to B1950 to the coordinates of the optical counterpart previously reported (Liu et al., 2001 and references therein) finding an angular separation of 8.7. Moreover we considered the coordinates of XB 1916–053 reported by the the NASA HEASARCH tool “Coordinate Converter”[^3] and compared those to the Chandra position achieved by ourself. The coordinates obtained with the tool were R.A.= $19^h18^m47^s.78$, Dec.=-05$^{\circ}$1411.2 (referred to J2000.0), having an angular separation from the Chandra coordinates of 6. Unfortunately it was not possible a comparison with the previous XMM observation because, in that case, the data of XB 1916–053 were taken by all the EPIC cameras in timing mode (see Boirin et al., 2004). In Fig. \[fig0\] we reported a region of sky around the Chandra zero-order image of the XB 1916–053 using the coordinate system referred to B1950 in order to compare the Chandra position, the “Coordinate Converter” tool position, and the position of the optical counterpart (Liu et al., 2001).

In Fig. \[fig1\] we showed the 20 s bin time lightcurve taking into account only the events in the positive first-order HEG. The mean count rate in the persistent state was 6 count s$^{-1}$. During the observation two bursts were observed, the count rate at their peaks was a factor three larger than during the persistent emission. Moreover we observed only four dips (see Fig. \[fig1\]) which did not show a regular periodicity as observed in the previous XMM-Newton observation (Boirin et al., 2004); in fact, while the second observed dip occurred $\sim 50$ min after the first one, as aspected for this source, the other dips occurred after temporal intervals two times larger ($\sim 1.7$ h), indicating that the central region of XB 1916–053 was not occulted at every orbital passage.

Spectral Analysis of the Persistent Emission
============================================

We selected the 1st-order spectra from the HETGS data, excluding the bursts and the dips, with a total exposure time of the persistent emission of 42.3 ks. Data were extracted from regions around the grating arms; to avoid overlapping between HEG and MEG data, we used a region size of 25 and 33 pixels for the HEG and MEG, respectively, along the cross-dispersion direction. The background spectra were computed, as usual, by extracting data above and below the dispersed flux. The contribution from the background is $0.4  \%$ of the total count rate. We used the standard CIAO tools to create detector response files (Davis 2001) for the HEG -1 (MEG -1) and HEG +1 (MEG +1) order (background-subtracted) spectra. After verifying that the negative and positive orders were compatible with each other in the whole energy range we coadded them using the script [ *add\_grating\_spectra*]{} in the CIAO software, obtaining the 1st-order MEG spectrum and the 1st-order HEG spectrum. Finally we rebinned the resulting 1st-order MEG and 1st-order HEG spectra to 0.015 Å and 0.0075 Å, respectively. It is worth to note that the absolute wavelength accuracy, connected to the error of 0.6 associated to the absolute source position, is $\pm  0.006 {\rm \AA}$ and $\pm
0.011 {\rm \AA}$ for HEG and MEG, respectively.

To fit the continuum we used the rebinned spectra in the 0.8–7 keV and 1–10 keV for first-order MEG and first-order HEG, respectively. We well fitted the continuum using an absorbed power law and adding, in the MEG data, a systematic edge at around 2.07 keV with a negative optical depth of $\sim  -0.2$ to take in account of an instrumental artifact (see Miller et al. 2002, and references therein). We obtained a $\chi^2$(d.o.f.) of 2325(2395). We found an equivalent hydrogen column of N$_H  = 0.44 \times  10^{22}$ cm$^{-2}$, a photon index of 1.5, and a power-law normalization of 0.11. In Fig. \[fig2\] we reported the data and the residuals with respect to the continuum described above. The presence of several absorption features was clearly evident in the residuals. The corresponding absorbed flux and the unabsorbed luminosity assuming a distance to the source of 9.3 kpc were $\sim  7.3 \times  10^{-10}$ ergs cm$^{-2}$ s$^{-1}$ and $\sim 7.5 \times  10^{36}$ ergs s$^{-1}$ in the 0.6–10 keV, respectively. We noted that the luminosity was a factor 1.7 larger than during the XMM observation (see Tab. 1 in Boirin et al., 2004). To have a confirm of this result we analysed the RXTE ASM lightcurve. We observed that during the XMM observation the ASM count rate was 1.12 C/s, while during the Chandra observation the ASM count rate was 1.82 C/s with an increase of intensity of almost 1.63, similar to the value obtained from the spectral analysis. In Fig. \[fig2b\] we reported the ASM lightcurve of XB 1916–053, the dashed vertical lines indicated the start time of the XMM and Chandra observations, the solid horizontal line indicated the level of the ASM count rate during the XMM observation, and, finally, the dotted horizontal line indicated the level of the ASM count rate during the Chandra observation.

To resolve the absorption features we fixed the continuum and added a Gaussian line with negative normalization for each feature. We used the 1st-order MEG spectrum to resolve the absorption features below 3 keV and the 1st-order HEG spectrum to resolve those in the 6–7 keV energy band. We detected four absorption lines below 3 keV, these were centered at 1.021, 1.471, 2.004, and 2.617 keV and corresponded to K$_\alpha$, K$_\alpha$, K$_\alpha$, and K$_\alpha$, respectively; the equivalent widths were -2.13, -1.18, -2.82, and -3.56 eV, respectively. In Fig. \[fig3\] we showed four expanded views of the residuals with respect to the continuum in the narrow energy ranges around the centroids of each of the absorbed features. In the 6–7 keV energy range we detected two absorption lines centered at 6.693 and 6.966 keV, corresponding to K$_\alpha$, and K$_\alpha$; the equivalent widths were -12.7, and -29.9 eV, respectively. In Fig. \[fig4\] we showed the residuals with respect to the continuum in the 6.4–7.1 keV energy range, in Table \[tab1\] we reported the parameters of the continuum and of each line. We noted that the line energies did not fit with the lab energies. This was not due to a physical effect but to a systematic error associated to the uncertainty of 0.6  of the source position which also identifies the zero-point of the dispersion arms. Considering the systematic error associated to the absolute wavelength accuracy (i.e. $\pm  0.006 {\rm  \AA}$ and $\pm 0.011  {\rm \AA}$ for HEG and MEG, respectively) the line energies are compatible with the lab energies.

Discussion
==========

We have analyzed a 42 ks Chandra observation of the persistent emission from XB 1916–053. The position of the zeroth-order image of the source provides improved X-ray coordinates for XB 1916–053 (R.A.= $19^h18^m47^s.871$, DEC=-05$^{\circ}$1417.09), with an angular separation of 8.7  to the optical counterpart (see Liu et al., 2000) and of 6 to the X-ray position reported by the “Coordinate Converter” tool (see section 2) . We performed a spectral analysis of the persistent emission using the 1st-order MEG and HEG spectra. The continuum emission is well fitted by an absorbed power law with photon index 1.5. The equivalent hydrogen column density of the absorbing matter was to $  0.44  \times  10^{22}$ cm$^{-2}$, this value is the same obtained by Boirin et al. (2004) analyzing the XMM RGS spectra of XB 1916–053. Also we note that the unabsorbed luminosity, in the 0.6–10 keV energy range, is a factor 1.7 larger than the previous XMM observation, this conclusion was furtherly supported by our analysis of the RXTE ASM lightcurve. Another interesting detection is that the dips are not observed at each orbital period, this could be explained considering that the larger flux from the source during our observation could largerly photoionize the matter of the absorber at the disk edge.

We clearly detected the presence of the (H-like), (H-like), (H-like), (H-like), (He-like), and (H-like) absorption lines. The and absorption lines were already observed by Boirin et al. (2004) using the XMM EPIC pn observation, in that case the line widths had upper limits of $<100$ and $<140$ eV, respectively. Thanks to the higher spectral resolution of Chandra HEG, to an observation four times longer and to a brightness of the source two times larger, we found that the line widths are $<13$ and between 0.1 and 21 eV for and , respectively. Moreover Boirin et al. (2004) marginally detected, in the RGS data, a absorption line with an upper limit on the line width of 41 eV. Also in this case, thanks to higher statistics, we found a more stringent upper limit on the line width of 1.3 eV. Furthermore we noted that Boirin et al. (2004) detected in the RGS spectra an absorption edge at $0.99 \pm  0.02$ keV during the persistent emission of the source. The larger statistics of our observation allowed us to well fitted the absorption edge near 1 keV using an absorption line at 1.02 keV associated to . Finally we noted that the energy of the absorption edge was between 0.87 and 0.97 keV in the dipping energy spectra of XB 1916–053 observed with XMM RGS (Boirin et al., 2004), we think that this discrete feature could be an absorption line associated to , this possibility does not change the scenario proposed by Boirin et al. (2004) suggesting that during the dips the absorbing matter is less photoionized.

Since both and absorption features were detected, some physical parameters of the plasma responsible for the lines could be estimated. The column density of each ion could be estimated from the EW of the corresponding absorption line, using the relation quoted e.g., by Lee et al. (2002, see also references therein) linking the two quantities, which is valid if the line is unsaturated and on the linear part of the curve of growth, which was verified in the case of XB 1916-053. The ratio between and column densities could then be used to estimate the photo-ionization parameter, $\xi$, using the calculations of Kallman & Bautista (2001). Following this approach, we derived column densities of $1.5 \times 10^{17}$ cm$^{-2}$ and $6.6 \times 10^{17}$ cm$^{-2}$ for and , respectively. We found $\xi \sim  10^{4.15}$ erg cm s$^{-1}$, that was slightly larger than the ionization parameter obtained from XMM data by Boirin et al. (2004) of $\xi_{XMM}  = 10^{3.92}$ erg cm s$^{-1}$. We noted that $\xi$ was a factor 1.7 larger than $\xi_{XMM}$, the same factor obtained comparing the unabsorbed luminosity during our observation and the XMM observation (see above). Moreover the ionization parameter associated to the H-like ions of Ne, Mg, Si, and S should be $\sim 10^3$, that was lower than that associated to and absorption lines. It is worth to note that both Boirin et al. (2004) and ourself inferred the ionization parameters $\xi$ basing on the photoionized model by Kallman & Bautista (2001) which assumed an ionization continuum consisting of a power law with $\Gamma=1$, but Diaz Trigo et al. (2005) have found a lower value of the ionization parameter of log$(\xi)=3.05 \pm 0.04$, using a photon index of $\Gamma=1.87 $ (obtained fitting the XMM data), and assuming a cutoff energy of 80 keV as obtained by Church et al. (1998) using BeppoSAX data.

We computed the FWHMs of the absorptions lines in units of km s$^{-1}$, these values were reported in Table \[tab1\] and were plotted in Fig. \[fig5\] in which we note that the values of the FWHMs are compatible with a velocity of 650 km s$^{-1}$ (dashed horizontal line). We investigated the nature of the line widths. Initially we assumed that the line widths were produced by thermal broadening and we estimated the plasma temperature using the relation $kT= 511 m_I/m_e  (\sigma/E)^2$ keV, where $kT$ is the temperature of the plasma, $m_I$ and $m_e$ are the mass of the ion and electron, respectively, and $\sigma$ and $E$ are the width and the centroid of the absorption line in keV. We found that the , , , and absorption lines should be produced in a region with a temperature between 20 and 40 keV, while, we found an upper limit of $\sim  200$ keV to the temperature of the region where the and absorption line should be produced. The interpretation of the line as thermally broadened was not consistent with the interpretation of the iron line ratios as diagnostics of ionization parameter. If the temperature was really 20 keV or greater, then all the elements would be fully stripped, with the possible exception of iron. A more probable scenario was that the line widths were broadened by some bulk motion or supersonic turbulence with a velocity around to 650 km s$^{-1}$ as indicated by the FWHMs. XB 1916–053 has an extended corona around the compact object (Church et al., 1998), assuming that the mechanism generating the turbulence or bulk motion was due to the presence of the extended corona we can achieve some informations about where the absorption lines were produced. Coronal models tend to have turbulent velocities which are locally proportional to the virial or rotational velocity (Woods et al., 1996). At $10^9$ cm (the coronal radius, see Narita et al., 2003) the virial velocity is 4400 km/s, considering a neutron star of $1.4 M_{\odot}$. It is very difficult to construct plausible dynamical models in which the matter moves at 10% of the virial velocity, then these lines should be produced at much larger distances, near to $4 \times 10^{10}$ cm, i.e. near the disk edge. According to this scenario we concluded that the absorbing matter was located at the same distance from the neutron star of the bulge which was likely responsible for the dips themselves. In the hypothesis that the thickness of the absorbing region was much less than $4
\times 10^{10}$ cm, its distance from the source, we could estimate a constraint on the thickness $d$ of the absorber using $d  < L /\xi N$ (Reynolds & Fabian 1995), where $L$ is the unabsorbed luminosity, $N$ is the equivalent hydrogen column density of the photoionized matter, and $\xi$ measures the corresponding ionization parameter. Assuming the cosmic abundance for the iron and a population of with respect to neutral iron of 0.5 we found $d < 8 \times 10^4$ km.

Conclusion
==========

We studied the persistent emission of XB 1916–053 using a 42 ks Chandra observation. We improved the position of the source, the new coordinates are R.A.= $19^h18^m47^s.871$ and Dec.=-05$^{\circ}$1417.09 (J2000.0) with an uncertainty circle of the absolute position of 0.6.

We detected the , , , and absorption lines centered at 1.021, 1.471, 2.004, and 2.617 keV, respectively. These lines were never observed before in XB 1916–053. Of all the X-ray binaries exhibiting absorption lines only Cir X–1 shows the same wide series of absorption lines, although in that case P-Cygni profiles were evident (Brandt & Schulz, 2000).

We confirmed the presence of the Ly$_\alpha$ and Ly$_\alpha$ absorption lines at 6.69 and 6.96 keV already observed by XMM-Newton. From the study of equivalent widths of the two lines we inferred a ionization parameter log($\xi$) of 4.15 that was a factor 1.7 larger than during the previous XMM observation. The unabsorbed luminosity in the 0.6–10 keV energy range, $7.5 \times
10^{36}$ erg s$^{-1}$, was also larger of the same factor and we verified this result studying the RXTE ASM lightcurve of XB 1916–053. The increase of the ionization parameter and of the luminosity of the same quantity indicated that these two lines were produced in the same region. We estimated that the absorption line widths could be compatible with a broadening caused by bulk motion or turbulence connected to the coronal activity, finding from the broadening of the absorption lines that these were produced at a distance from the neutron star of $4 \times 10^{10}$ cm, i.e. near the disk edge and at the same radius of the absorber which causes the dipping when the corresponding ionization parameter decreases.

We are sincerely grateful to the anonymous referee for the useful suggestions given to improve this work. This work was partially supported by the Italian Space Agency (ASI) and the Ministero della Istruzione, della Universitá e della Ricerca (MIUR).

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[lc]{}

Continuum &\

$N_{\rm H}$ $\rm (\times 10^{22}\;cm^{-2})$ & $0.4448^{+0.0090}_{-0.0087}$\
photon index & $1.4957^{+0.0099}_{-0.0095}$\
N$_{po}$ & $0.1108^{+0.0014}_{-0.0013}$\
&\
K$_\alpha$ &\

E$$ (keV) & $1.02056^{+0.00073}_{-0.00043}$\
$\sigma$ (eV) & $1.40^{+0.71}_{-0.48}$\
I$$ ($\times 10^{-4}$ cm$^{-2}$ s$^{-1}$) & $-2.24^{+0.55}_{-0.63}$\
EW$$ (eV) & $-2.13 \pm 0.57$\
FWHM$$ (km s$^{-1}$) & $970^{+490}_{-330}$\
&\
K$_\alpha$ &\

E$$ (keV) & $1.47116 ^{+0.00046}_{-0.00045}$\
$\sigma$ (eV) & $<1.3$\
I$$ ($\times 10^{-5}$ cm$^{-2}$ s$^{-1}$) & $-7.4^{+1.8}_{-2.0}$\
EW$$ (eV) & $-1.18^{+0.29}_{-0.32}$\
FWHM$$ (km s$^{-1}$) & $< 620$\
&\
K$_\alpha$ &\

E$$ (keV) & $2.00352  ^{+0.00071}_{-0.00072}$\
$\sigma$ (eV) & $1.79^{+1.03}_{-0.84}$\
I$$ ($\times 10^{-4}$ cm$^{-2}$ s$^{-1}$) & $-1.13^{+0.20}_{-0.21}$\
EW$$ (eV) & $-2.82 \pm 0.53$\
FWHM$$ (km s$^{-1}$) & $630^{+360}_{-300}$\
&\
K$_\alpha$ &\

E$$ (keV) & $2.61653^{+0.00472}_{-0.00082}$\
$\sigma$ (eV) & $2.76^{+158.03}_{-0.28}$\
I$$ ($\times 10^{-5}$ cm$^{-2}$ s$^{-1}$) & $-9.6^{+3.6}_{-1.9}$\
EW$$ (eV) & $-3.56^{+1.32}_{-0.69}$\
FWHM$$ (km s$^{-1}$) & $740^{+42000}_{-100}$\
&\
K$_{\alpha}$ &\

E$$ (keV) & $6.6925^{+0.0088}_{-0.0057}$\
$\sigma$ (eV) & $<13$\
I$$ ($\times 10^{-5}$ cm$^{-2}$ s$^{-1}$) & $-8.3^{+3.7}_{-3.5}$\
EW$$ (eV) & $-12.7 \pm 5.6$\
FWHM$$ (km s$^{-1}$) & $<1400$\
&\
K$_{\alpha}$ &\

E$$ (keV) & $6.9558^{+0.0062}_{-0.0066}$\
$\sigma$ (eV) & $9.7^{+21.0}_{-9.6}$\
I$$ ($\times 10^{-4}$ cm$^{-2}$ s$^{-1}$) & $-1.83^{+0.46}_{-0.58}$\
EW$$ (eV) & $-29.9^{+7.6}_{-9.4}$\
FWHM$$ (km s$^{-1}$) & $980^{+2100}_{-970}$\
\[tab1\]

\

[^1]: See http://asc.harvard.edu/cdo/about\_chandra for more details.

[^2]: See http://cxc.harvard.edu/cal/ASPECT/celmon/ for more details.

[^3]: See http://heasarch.gsfc.nasa.gov/cgi-bin/Tools/convcoord/convcoord.pl
