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A&A
Volume 509, January 2010
Article Number L8
Number of page(s) 4
Section Letters
DOI https://doi.org/10.1051/0004-6361/200913583
Published online 19 January 2010
A&A 509, L8 (2010)

LETTER TO THE EDITOR

The column density towards LMC X-1

M. Hanke1 - J. Wilms1 - M. A. Nowak2 - L. Barragán1 - N. S. Schulz2

1 - Dr. Karl Remeis-Observatory & ECAP, University of Erlangen-Nuremberg, Sternwartstr. 7, 96049 Bamberg, Germany
2 - MIT Kavli Institute for Astrophysics and Space Research, NE80-6077, 77 Mass. Ave., Cambridge, MA 02139, USA

Received 2 November 2009 / Accepted 17 December 2009

Abstract
We measure the neutral absorption towards the black hole X-ray binary system LMC  X-1 from six archival soft X-ray spectra obtained with the gratings and/or CCD detectors on Chandra, XMM-Newton, and Swift. Four spectral models for the soft continuum are investigated. While the powerlaw model may overestimate $N_{\rm H}$ considerably, the others give consistent results. Taking the lower metalicity of the Large Magellanic Cloud into account, we find equivalent hydrogen column densities of $N_{\rm H}$ = (1.0-1.3) $\times$ $10^{22}~\rm cm^{-2}$, with a systematic dependence on the orbital phase. This variation in the neutral absorption can nearly explain the orbital modulation of the soft X-ray flux recently detected with the All Sky Monitor (ASM) on the Rossi X-ray Timing Explorer (RXTE).

Key words: X-rays: individuals: LMC X-1 - X-rays: binaries - X-rays: ISM - galaxies: abundances

1 Introduction

The extragalactic X-ray sources in the Large Magellanic Cloud (LMC), our $\approx$48 kpc distant neighboring galaxy, were discovered in the late 1960s (Mark et al. 1969; Price et al. 1971). Because the density of stars is high, their optical identifications were uncertain for a long time. The X-ray binary system (XRB) LMC  X-1 is located ${\approx}0\hbox{$.\!\!^\circ$ }5$south-southeast of the 30 Doradus star-formation region, in the NGC 2078 (LMC N159F) nebula. Pakull (1980), Hutchings et al. (1983,1987), and Cowley et al. (1995) were able to identify the counterpart of LMC  X-1 with an $m_{V}{=}14\hbox{$.\!\!^{\rm m}$ }5$O7/8 giant (Hutchings et al. 1983; Negueruela & Coe 2002). This has allowed the placement of strong dynamical constraints on the compact object's mass. Orosz et al. (2009) have recently used optical spectra of this star - labeled as ``star #32'' by Cowley et al. (1978) and also often called Pakull's star - to confirm the black hole (BH) candidacy of LMC  X-1. They derive an orbital period of 3.909 d, which is consistent with the modulation of the soft X-ray flux of LMC  X-1 (Levine & Corbet 2006). Deriving an extinction of AV=2.28 $\pm$ 0.06 - much more than previously assumed - from the V-K color excess, Orosz et al. (2009) infer a BH mass of 10.9 $\pm$  $1.6~M_\odot$.

The persistent XRB LMC  X-1 is the only dynamically confirmed BH candidate that so far has only been found in the high/soft (thermal dominant) X-ray spectral state; that is, its X-ray spectrum can be described by a multi-temperature disk blackbody component plus a weak soft ( $\Gamma \gg 2$) power-law component (Ebisawa et al. 1989; Schlegel et al. 1994; Yao et al. 2005; Haardt et al. 2001; Wilms et al. 2001; Nowak et al. 2001; Cui et al. 2002). In comparison, LMC X-3 usually shows a similarly soft X-ray spectrum, but also (partial) transitions to the low/hard state (Wilms et al. 2001; Smith et al. 2007), while Cyg X-1 regularly transits between the low/hard and a soft-intermediate state and never reaches the thermal dominant state (Wilms et al. 2006). LMC  X-1 is therefore an ideal target for measuring the BH spin parameter a* from the soft X-ray continuum produced by the relativistic accretion disk. Gierlinski et al. (2001) constrained a* to be less than 0.998 from a 24 ks 0.7-10 keV ASCA-SIS spectrum. Gou et al. (2009) have recently reported a* = 0.90+0.04-0.09from 18 selected RXTE-PCA spectra with exposures between 5-11 ks and covering 2.5-20 keV. These authors fix the column density for the photoelectric absorption to $N_{\rm H}$ = 4.6 $\times$ $10^{21}~\rm cm^{-2}$as reported by Cui et al. (2002) based on low statistics.

An appropriate description of the absorption is, however, indispensable for modeling the soft X-ray continuum and likewise for modeling the visual extinction, hence the derivation of the system parameters from the dereddened optical spectrum of the companion star. In this Letter, we therefore aim to accurately describe the column density towards LMC  X-1. We describe the data in Sect. 2 and present the methods and our analysis in Sect. 3. We summarize and discuss our results in Sect. 4.

2 Observations and data reduction

Table 1:   Log of recent soft X-ray observations of LMC X-1 with good S/N. (Instruments not considered here are in parenthesis.)

\begin{figure} \par\includegraphics[width=8.8cm,clip]{13583fg1} \end{figure} Figure 1:

Flux-corrected spectra of LMC  X-1 from the six observations, shifted in flux according to the labels with respect to C1 for visual clarity. The gray data have been ignored because of calibration issues. The models shown here for illustrative purposes are also broadened by the instrumental response.

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We study the spectra from all six recent observations with instruments providing soft X-ray spectra (Table 1).

The Chandra observation C1 was performed using the HETGS (Canizares et al. 2005) and with the detector CCDs operated in timed exposure mode. The $\pm$first order HEG and MEG spectra, as well as the corresponding response matrices, were taken from the Chandra Transmission Grating Catalog archive TGCat[*].

All instruments of XMM-Newton (Jansen et al. 2001) were active during the first (shorter) XMM observation X1. The EPIC-pn camera (Strüder et al. 2001) was operated in timing mode. Its data are therefore not affected by photon pile-up (Wilms et al. 2003). The same is true for data from the Reflection Grating Spectrometers (RGS; den Herder et al. 2001) due to their dispersion of the photons, but not for data from the MOS cameras (Turner et al. 2001), which were operated in full frame imaging mode. For this reason, we only use the EPIC-pn spectrum and the first and second order spectra of RGS 1 and 2. For the second (longer) XMM observation X2, however, no pn-data are available. The data were reduced with the Science Analysis Software, XMMSAS, v. 7.1, following standard procedures, i.e., applying the SAS tasks epchain, emchain, rgsproc, evselect, rmfgen, and arfgen to produce spectra and response matrices.

Swift's X-ray telescope (XRT; Burrows et al. 2005) was operated in photon counting (PC) mode during the first Swift observation S1, which resulted in pile-up. For S2 and S3, the windowed timing (WT) mode was used. After reprocessing the data to apply the newest calibration, spectra were extracted using standard FTOOLS, handled via xselect. For the PC mode observation S1, we extract an annulus to exclude the region affected by pile-up, only yielding a low-quality spectrum. Ancillary response files were created with xrtmkarf, and suitable response files for each observation were obtained from the CALDB. The WT mode spectra are not calibrated as well as the PC mode one around the Si edge (Fig. 1), so we exclude their 1.5-2 keV data.

All spectral analysis was performed with the Interactive Spectral Interpretation System (ISIS; Noble & Nowak 2008; Noble et al. 2006; Houck & Denicola 2000)[*].

Table 2:   Comparison of elemental abundances (by number) in the Galactic ISM and in the LMC as $\epsilon(X)=12+\log_{10}(X/{\rm H})$.

Table 3:   Column density in units of $10^{22}~\rm cm^{-2}$for the six observations and the sine fit, obtained with different continuum models.

3 Analysis

\begin{figure} \par\includegraphics[width=12cm,clip]{13583fg2} \end{figure} Figure 2:

Correlation of the column density $N_{\rm H}$ and the photon index $\Gamma $, derived with the powerlaw model ( left) and the simpl model ( right) for observation X1. The contours show the 68%, 90%, and 99% confidence regions for two parameters of interest (i.e.,  $\Delta \chi ^2=2.30$, 4.61, and 9.21).

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An overview of previous $N_{\rm H}$ measurements for LMC  X-1 is given by Orosz et al. (2009, Table 2). We caution, however, that only <12% of the hydrogen column density towards the LMC, $N_{\rm H}$ = 4 $\times$ $10^{21}~\rm cm^{-2}$(measured in the LAB 21 cm survey; Kalberla et al. 2005; Bajaja et al. 2005), is of Galactic origin[*], while the largest part is detected at $v_{\rm LSR}$ = 200-300 km s-1 and thus is probably local to the LMC (Richter et al. 1987). As the absorption in the 0.5-10 keV band is mostly caused by metals (Wilms et al. 2000) and the LMC has a much lower metallicity than our Galaxy, we compile both abundance sets in Table 2. The LMC abundances are henceforth used throughout our analysis.

As for all previous observations (see Sect. 1), the X-ray spectra of LMC  X-1 investigated here are very soft (see Fig. 1), but a hard (albeit very steep) component in addition to a thermal one is nonetheless needed to describe the data, except for S2 and S3. The powerlaw model, however, becomes unphysically strong at low energies (e.g., Done et al. 2002; Shrader & Titarchuk 1998). A steep photon index $\Gamma $ $\gg$ 2 (e.g., $\Gamma $ = 3.7 $\pm$ 0.1 as measured for X1, which has the best high-energy coverage due to the EPIC-pn spectrum) is compensated for in spectral fits by an incorrectly strong absorption (e.g., Yao et al. 2005; Suchy et al. 2008; Gou et al. 2009). In contrast, the empirical convolution model simpl (Steiner et al. 2009) has an intrinsic low-energy cut-off when convolving an input spectrum modeled by, e.g., diskbb or kerrbb[*]. Figure 2 shows that the (well known) correlation between $N_{\rm H}$ and $\Gamma $ vanishes when simpl is used instead of powerlaw. Although an even steeper photon index was found using simpl, the value of $N_{\rm H}$ is lower and is more narrowly constrained.

Because the derived absorption might depend on the shape of the continuum, we investigated different models, namely empirical ones - such as diskbb + powerlaw, simpl(diskbb), and simpl(kerrbb) (Li et al. 2005) - and the physical Comptonization model eqpair (Coppi 2000)[*]. These models typically describe our data equally well. In all fits, the disk has a temperature between 0.65 and 1.1 keV. The other parameters, too, are similar to previously obtained values. Table 3 and Fig. 3 show our results for the column density (assuming the LMC abundances given in Table 2) as a function of orbital phase  $\phi_{\rm orb}$for each of the six observations and all four of the aforementioned continuum models. In all cases where a steep power law substantially contributes to the model, the diskbb + powerlaw model gives a much higher $N_{\rm H}$ than the other models, because of the systematic error of the powerlaw model. We therefore ignore these values. The other models, however, are quite consistent with one another: their agreement on $N_{\rm H}$ is within <8 $\times$ $10^{20}~\rm cm^{-2}$, which is therefore an upper limit of the systematic error due to the choice of the continuum. Using the LMC abundances (Table 2), we find column densities in the range of (1.0-1.3) $\times$  $10^{22}~\rm cm^{-2}$.

\begin{figure} \par\includegraphics[width=8.8cm,clip]{13583fg3} \end{figure} Figure 3:

$N_{\rm H}$as a function of orbital phase $\phi _{\rm orb}^{\rm (T3)}$(see Table 1) using various continuum models. Note the different scale for the diskbb + powerlaw model, which may predict an unreliably large $N_{\rm H}$ (see text). The gray lines fit the results with sine curves.

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We detect a modulation of $N_{\rm H}$ with orbital phase. The observations X1 and X2 close to $\phi_{\rm orb}$ $\approx$ 0, when the BH is behind the donor star, require a systematically higher $N_{\rm H}$ than S3, C1, and S2 close to $\phi_{\rm orb}$ $\approx$ 0.5. To quantify this modulation by its mean and amplitude (Table 3), we fit sine curves to the six measurements for each continuum model (see Fig. 3), although we are aware that they do not describe the data very well and also predict the strongest absorption at $\phi_{\rm orb}$ = 0.15-0.17, which is not expected.

Finally, we find marginal evidence of ionized absorption in the high-resolution spectra (Fig. 4), but a detailed study of these features is beyond the scope of this paper.

\begin{figure} \par\includegraphics[width=8.5cm,clip]{13583fg4} \end{figure} Figure 4:

The Ne-edge in observation X2. The first order RGS spectra (black) reveal absorption lines of Ne IX at 13.45 Å and probably also Ne II at 13.62 Å, but the quality of the spectrum does not allow for a detailed study of the ionized absorber.

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4 Summary and discussion

The elements with the largest contribution to the photoabsorption in the soft X-ray band are significantly less abundant in the LMC than in the Galaxy (Table 2). Because of the lower metallicity, simply using radio-measured $N_{\rm H}$ values in an absorption model without adopting the LMC abundances will not allow for a correct description of the physical situation. Specifically for LMC X-1, the equivalent hydrogen column density inferred from this X-ray absorption study - taking the proper LMC abundances into account - is actually much higher than the H-column resolved by the LAB survey (at a half-power beam-width of 0 $\hbox{$.\!\!^\circ$ }$6; Kalberla et al. 2005), which is likely caused by additional material in the environment of LMC X-1 and in the system itself. This result was not obtained in earlier X-ray absorption measurements, as erroneously applying Galactic abundances resulted in lower $N_{\rm H}$ values.

In addition, we presented the first evidence that the column density varies in the range (1.0-1.3) $\times$ $10^{22}~\rm cm^{-2}$. A modulation with orbital phase is strongly suggested and would be consistent with absorption in the stellar wind of the donor giant. Orosz et al. (2009) assume that the orbital modulation of the X-ray flux is mostly caused by Thomson scattering in the stellar wind since they find similar amplitudes[*] in all three RXTE-ASM energy bands, namely $A_{\rm A(1.5{-}3~keV)}$ = 7.2 $\pm$ $1.0\%$, $A_{\rm B(3{-}5~keV)}$ = 7.7 $\pm$ $1.1\%$, and $A_{\rm C(5{-}12~keV)}$ = 3.8 $\pm$ $2.9\%$. From a modulation in $N_{\rm H}$ with a full amplitude of 3 $\times$ $10^{21}~\rm cm^{-2}$, $A_{\rm A}$ = $7.7{-}6.9\%$, $A_{\rm B}$ = $1.6{-}2.7\%$, and $A_{\rm C}$ = $0.4{-}1.7\%$ are expected, depending on the assumptions about the ASM response; i.e., the variation seen with the ASM is almost consistent with the suggested neutral absorption. The phase of the current sine fit, however, is not. More soft X-ray observations covering more phases are clearly needed, because the structure of the stellar wind might be more complex than a sine curve. With the three 50 ks Chandra observations that we have gained for AO 11, we will be able to better constrain the modulation.

Acknowledgements
We thank N. Przybilla and M. F. Nieva for providing the LMC abundances. M.H. and J.W. acknowledge funding from the Bundesministerium für Wirtschaft und Technologie through the Deutsches Zentrum für Luft- und Raumfahrt under contract 50OR0701. We thank the MIT Kavli Institute and the ISSI (Bern) for their hospitality during the preparation of this work.

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Footnotes

... TGCat[*]
See http://tgcat.mit.edu
...)[*]
See http://space.mit.edu/cxc/isis/
... origin[*]
See http://www.astro.uni-bonn.de/ webaiub/english/tools_labsearch.php?alpha=05+39+38.7&beta=-69+44+36
...kerrbb[*]
As a convolution model that relies upon a spectral model outside of the energy range spanned by the noticed data, simpl must be evaluated on a suitably extended grid.
...(Coppi 2000)[*]
For X1, the $N_{\rm H}$ derived with diskbb + compTT (Titarchuk 1994) is also consistent with the one from, e.g., simpl(diskbb).
... amplitudes[*]
The fractional full amplitude is here $A=\rm (max{-}min)/mean$.
Copyright ESO 2010

All Tables

Table 1:   Log of recent soft X-ray observations of LMC X-1 with good S/N. (Instruments not considered here are in parenthesis.)

Table 2:   Comparison of elemental abundances (by number) in the Galactic ISM and in the LMC as $\epsilon(X)=12+\log_{10}(X/{\rm H})$.

Table 3:   Column density in units of $10^{22}~\rm cm^{-2}$for the six observations and the sine fit, obtained with different continuum models.

All Figures

  \begin{figure} \par\includegraphics[width=8.8cm,clip]{13583fg1} \end{figure} Figure 1:

Flux-corrected spectra of LMC  X-1 from the six observations, shifted in flux according to the labels with respect to C1 for visual clarity. The gray data have been ignored because of calibration issues. The models shown here for illustrative purposes are also broadened by the instrumental response.

Open with DEXTER
In the text

  \begin{figure} \par\includegraphics[width=12cm,clip]{13583fg2} \end{figure} Figure 2:

Correlation of the column density $N_{\rm H}$ and the photon index $\Gamma $, derived with the powerlaw model ( left) and the simpl model ( right) for observation X1. The contours show the 68%, 90%, and 99% confidence regions for two parameters of interest (i.e.,  $\Delta \chi ^2=2.30$, 4.61, and 9.21).

Open with DEXTER
In the text

  \begin{figure} \par\includegraphics[width=8.8cm,clip]{13583fg3} \end{figure} Figure 3:

$N_{\rm H}$as a function of orbital phase $\phi _{\rm orb}^{\rm (T3)}$(see Table 1) using various continuum models. Note the different scale for the diskbb + powerlaw model, which may predict an unreliably large $N_{\rm H}$ (see text). The gray lines fit the results with sine curves.

Open with DEXTER
In the text

  \begin{figure} \par\includegraphics[width=8.5cm,clip]{13583fg4} \end{figure} Figure 4:

The Ne-edge in observation X2. The first order RGS spectra (black) reveal absorption lines of Ne IX at 13.45 Å and probably also Ne II at 13.62 Å, but the quality of the spectrum does not allow for a detailed study of the ionized absorber.

Open with DEXTER
In the text

Copyright ESO 2010

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