NON-THERMAL X-RAYS AND INTERSTELLAR GAS TOWARD THE γ-RAY SUPERNOVA REMNANT RX J1713.7−3946: EVIDENCE FOR X-RAY ENHANCEMENT AROUND CO AND H i CLUMPS

The ¹²CO(J = 2–1) data at 230 GHz were taken with the NANTEN2 4 m telescope in the period from August to November in 2008, and were published in Papers I, II, and Maxted et al. (2012). The front end was a 4 K cooled double sideband receiver and a typical system temperature was ∼250 K in the single sideband including the atmosphere toward the zenith. The telescope had an angular resolution (FWHM) of 90'' at 230 GHz. We used an acoustic optical spectrometer having 2048 channels with a bandwidth of 390 km s⁻¹ and resolution per channel of 0.38 km s⁻¹. Observations were carried out in the on-the-fly mode with an integration time of 1.0 s or 2.0 s per grid, and provided a Nyquist-sampled 30'' grid data set. The ambient temperature load was employed for the intensity calibration. The absolute intensity scale was estimated by observing the Ori KL object [$5^{\mathrm{h}} 35^{\mathrm{m}} 14\buildrel{\mathrm{s}}\over{.}52; -5{^\circ } 22{^\prime }28{\buildrel{\prime\prime}\over{.}} 2$ (J2000)] (Schneider et al. 1998) and the main beam efficiency, η_mb, was estimated to be 0.83. The rms noise fluctuations with 1.0 s and 2.0 s integrations were better than 0.66 K and 0.51 K channel⁻¹, respectively. The pointing accuracy was estimated to be better than ∼15'' by observing Jupiter every 2 hr. The image was smoothed with a Gaussian kernel with FWHM of 60''. The observed area is shown in Figure 1. In addition, we used the ¹²CO(J = 1–0) data at 115 GHz taken with the NANTEN 4 m telescope, which were already published in Moriguchi et al. (2005) and Fukui et al. (2012). The angular resolution of the data was 26 (FWHM) and the velocity resolution and rms noise fluctuations were 0.65 km s⁻¹ and 0.3 K, respectively. Observations were carried out in the position-switching mode with a 2' grid spacing (for more detailed information, see Moriguchi et al. 2005 and Fukui et al. 2012). The ¹²CO(J = 2–1) data were used for comparison with the X-ray images, while the ¹²CO(J = 1–0) data was mainly used for estimating molecular mass from the integrated ¹²CO intensity W(¹²CO) (K km s⁻¹) by using a relationship, N(H₂) = X_CO · [W(¹²CO) (K km s⁻¹)] (cm⁻²), where an X_CO factor of 2.0 × 10²⁰ (cm⁻² (K km s⁻¹)⁻¹) was adopted (Bertsch et al. 1993).

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The 21 cm H i spectral data were taken from the Southern Galactic Plane Survey (McClure-Griffiths et al. 2005) with the Australia Telescope Compact Array combined with the 64 m Parkes Radio Telescope. The combined beam size and the grid spacing of the data set were 22 and 40'', respectively. The velocity resolution and typical rms noise fluctuations were 0.82 km s⁻¹ and 1.9 K, respectively. We applied the correction for the H i self-absorption by following the previous analysis of the cold H i gas without CO emission (see Section 3.3 of Paper II).

We used Suzaku archive data of RX J1713.7−3946 taken from the Data Archives and Transmission System (DARTS at ISAS/JAXA). The observations performed 15 pointings toward the main features and two OFF pointings of RX J1713.7−3946 and were published by Takahashi et al. (2008) and Tanaka et al. (2008) except for the four pointings observed in 2010 February. Previous and current observations are summarized in Table 1, and the field of view (FoV) of each observation is shown in Figure 1. Active detector systems aboard the Suzaku satellite were the X-ray Imaging Spectrometer (XIS; Koyama et al. 2007) and the Hard X-ray Detector (Takahashi et al. 2007). The XIS consists of four CCD cameras placed at the foci of X-Ray Telescopes (XRTs; Serlemitsos et al. 2007). We analyzed only XIS data in the present paper. The spaced-row charge injection (SCI; Nakajima et al. 2008; Uchiyama et al. 2009) was used in the latter four pointings (see also Table 1). Unfortunately, XIS 2 was on closed access since 2006 November 9, possibly owing to micrometeorite damage. XIS 0 showed an anomaly in Segment A on 2009 June 23. Thus, for the data in the latter four pointings we used XIS 0 (except for Segment A), XIS 1, and XIS 3. We used "cleaned event files" processed and screened by versions 2.0 or 2.4 of the Suzaku pipeline depending on observation dates. First, we created the photon count images from the cleaned event files in the energy bands 1–5 keV and 5–10 keV. Here, we subtracted the non-X-ray background (NXB) using xisnxbgen, which estimates NXB count rate based on night Earth observation data. Then, we corrected for XRT vignetting effects by simulating flat field images with xissim (Ishisaki et al. 2007). Additionally, we masked the region of ⁵⁵Fe calibration sources in the energy band 5–10 keV. Finally, we smoothed the images by using a Gaussian kernel with a FWHM of 45''. We performed data reduction with the version 6.11 of the HEAsoft tools.

Figure 2 shows mosaic images of RX J1713.7−3946, which were constructed by using the data from XIS 0+1+2+3. Figures 2(a) and (b) show the soft-band (1–5 keV) and hard-band (5–10 keV) images, respectively. The unit for the images is 10⁻⁴ counts s⁻¹ pixel⁻¹, and the pixel size is ∼167. We find that the soft- and hard-band images are very similar to each other, which was already discussed in a previous study by Tanaka et al. (2008). Figure 2 shows the western rim clearly as well as several peaks of ∼10×10⁻⁴ counts s⁻¹ pixel⁻¹ in the northern rim and inside the SNR. In the soft-band image, thick white circles indicate the locations of the two bright point-like sources toward the inner part of the SNR. The left one is associated with a Wolf–Rayet star CD−39 11212B (Pfeffermann & Aschenbach 1996), which corresponds to two X-ray point sources cataloged (1WGA J1714.4−3945 and EXO 1710−396; see also Table 2). The other one is thought to be a neutron star because of its X-ray spectral characteristics (Lazendic et al. 2003), which is cataloged as an X-ray point source (1WGA J1713.4−3949) and a pulsar (PSR J1713−3949). We also showed the modified color scale image in the energy band 1–5 keV, which enhances the regions of the low-photon counts ∼7×10⁻⁴ counts s⁻¹ pixel⁻¹. In addition to the localized peaks in X-rays, we find that diffuse X-ray emission is extended inside the SNR. In order to estimate the level of these background X-rays in the 1–5 keV X-ray image, we show two histograms of the X-ray counts in Figure 13 (see Appendix B); one is for the whole region observed with Suzaku (Figure 1) and the other is for the nine circles of 6 arcmin diameter without significant peaks inside the SNR (Figure 13). In the histogram inside the SNR, we find a peak at ∼3.86×10⁻⁴ counts s⁻¹ pixel⁻¹ and identify this level as the background within the SNR. On the other hand, we consider that a primary peak at ∼1.16×10⁻⁴ counts s⁻¹ pixel⁻¹ for the whole region indicates the background level outside the SNR. In Figure 2(c) we plotted the positions of the X-ray point sources, pulsars, and Wolf–Rayet stars (Table 2) in order to test whether the X-ray distribution is influenced by these point sources. We see no excess toward the point sources in Figure 2(c) except for the two bright point-like sources marked in Figure 2(a), and consider that X-ray features inside the SNR are not due to the point sources but are intrinsic to the SNR.

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Figure 3 shows four overlays of the ¹²CO(J = 2–1) distribution and X-ray images in the two energy bands 1–5 keV (Figures 3(a) and (b)) and 5–10 keV (Figures 3(c) and (d)), respectively. A V_LSR range of CO from −20.2 to −9.1 km s⁻¹ is shown in Figures 3(a) and (c) and that from −9.1 to 1.8 km s⁻¹ in Figures 3(b) and (d). These velocity ranges correspond to that of the interacting molecular gas (Fukui et al. 2012). Figure 3 indicates that CO and X-rays show a good correlation at a pc scale as already noted by Moriguchi et al. (2005). It is remarkable that most of the X-ray features are found toward CO clumps. The most outstanding X-rays are seen in the west of the shell, where the strongest CO emission is located (Figures 3(b) and (d)). The second brightest X-rays are in the north of the shell, where CO emission is also distributed (Figures 3(a) and (c)). The southern part of the CO emission appears to delineate the southern rim of the SNR (Figures 3(b) and (d)), while the eastern shell with weak X-rays only has a few small CO features (Figures 3(b) and (d)).

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The CO distribution is highly clumpy. In order to make a detailed comparison with the X-rays, we cataloged CO clumps in the ¹²CO(J = 1–0) data in Figure 12 in Appendix A. We identified 22 CO clumps in total, which were selected by the following two criteria in the ¹²CO(J = 1–0) data; (1) the peak position is located within SNR boundary, (2) the peak brightness temperature is higher than 1 K, and (3) the total clump surface area, defined as the region surrounded by the contour at half of the maximum integrated intensity, is larger than a three-beam area. We give their observed parameters in ¹²CO(J = 1–0) and ¹²CO(J = 2–1) in Table 3. Thirteen of them are identified either by Fukui et al. (2003) or Moriguchi et al. (2005). The rest of the clumps are newly identified in the present work. Most of the CO clumps have a single velocity component of line width ∼3–5 km s⁻¹. Only clump O (Moriguchi et al. 2005) has two velocity components of ∼7.5 km s⁻¹ separation, and is divided into two clumps O and O_b. Five of them, D_W, G_E, O_b, O_SW, and Z, have molecular mass higher than 50 M_☉ (see also Table 3), and four of them, C_E, Q_W, Z_NW, and Z_NE, have molecular mass less than 50 M_☉. We focus hereafter on the 18 CO clumps that have molecular mass greater than 50 M_☉ as shown in Figure 3, so that the derivation of the physical parameters is ensured for a quantitative comparison with the X-rays (Section 2.3). Except for the five clumps, C, I, L, O_SW, and Z, that are located inside the SNR boundary, most of the CO clumps (A, B, D, D_W, E, G, G_E, O, O_b, Q, R, U, and W) are distributed on the outer boundary of the SNR shell.

Table 3. Properties of CO Clumps

	12CO(J = 1–0)							12CO(J = 2–1)
Name	αJ2000	δJ2000	$T_{\rm R^\ast }$	Vpeak	ΔVLSR	Mass		αJ2000	δJ2000	$T_{\rm R^\ast }$	Vpeak	ΔVLSR
	(h m s)	(° ' '')	(K)	(km s−1)	(km s−1)	(M☉)		(h m s)	(° ' '')	(K)	(km s−1)	(km s−1)
(1)	(2)	(3)	(4)	(5)	(6)	(7)		(8)	(9)	(10)	(11)	(12)
A........	17 11 35.9	−39 59 01.8	8.5	−10.3	4.8	686		17 11 38.4	−39 58 46.9	6.6	−10.0	4.5
B........	17 12 26.5	−40 06 06.3	4.2	−8.0	4.6	190		17 12 26.8	−40 05 55.5	3.3	−8.1	4.5
C........	17 12 25.9	−39 56 04.4	9.4	−12.0	3.8	397		17 12 27.0	−39 54 58.0	7.5	−11.9	4.6
CE.......	17 13 01.3	−39 53 35.2	1.1	−9.1	1.6	10		17 12 57.4	−39 53 42.3	3.7	−8.8	1.4
D........	17 11 28.0	−39 30 37.6	4.0	−11.1	4.8	292		17 11 32.5	−39 30 03.9	3.3	−9.3	4.8
DW......	17 11 01.3	−39 34 17.6	2.3	2.4	3.3	137		17 11 34.3	−39 32 31.2	3.1	−1.1	6.0
E........	17 11 29.1	−39 50 38.5	2.0	−6.1	7.2	159		17 11 55.4	−39 51 07.2	2.0	−6.0	5.0
G........	17 10 55.6	−39 45 55.2	3.3	−10.8	8.0	307		17 10 56.3	−39 45 20.6	2.8	−11.5	4.8
GE.......	17 11 27.1	−39 47 49.6	5.4	−12.8	2.6	168		17 11 21.0	−39 47 24.4	4.3	−12.3	2.7
I.........	17 12 08.2	−39 43 43.3	1.8	−9.9	5.4	103		17 12 16.6	−39 43 22.8	1.3	−10.4	5.9
L........	17 12 25.8	−39 28 53.4	4.0	−12.0	5.7	370		17 12 30.2	−39 28 14.6	3.2	−11.7	6.0
O........	17 13 46.7	−39 27 49.8	1.1	−6.4	4.9	61		17 13 46.0	−39 26 28.6	1.2	−4.6	4.7
Ob......	17 13 46.7	−39 27 49.8	1.9	1.1	3.4	80		17 13 46.0	−39 26 28.6	1.9	1.0	3.8
OSW....	17 13 24.8	−39 37 08.6	2.8	−1.6	2.0	60		17 13 08.9	−39 36 43.2	3.5	−1.3	1.3
Q........	17 15 13.4	−39 25 06.2	2.9	−2.8	3.2	108		17 15 11.7	−39 26 47.4	2.8	−2.2	2.8
QW......	17 14 49.3	−39 31 35.1	3.0	−14.3	2.4	46		17 14 53.0	−39 31 30.7	3.7	−14.1	1.8
R........	17 15 39.9	−39 38 34.6	4.1	−3.3	2.4	67		17 15 32.0	−39 39 28.5	3.2	−3.1	2.2
U........	17 14 34.2	−40 06 27.0	3.7	−4.8	1.3	58		17 14 13.4	−40 06 25.2	3.0	−4.6	1.3
W.......	17 13 42.8	−40 16 40.7	5.0	−5.1	3.0	402		17 13 30.0	−40 15 03.6	3.3	−4.9	3.4
Z........	17 14 18.7	−39 56 55.1	2.6	−20.1	2.7	72		17 13 53.3	−39 54 46.8	2.9	−19.8	2.9
ZNW....	17 13 45.0	−39 52 14.9	3.0	−19.8	2.6	36		17 13 45.6	−39 51 09.2	5.1	−19.8	1.8
ZNE.....	17 14 57.4	−39 49 59.3	2.2	−20.0	3.3	31		17 14 53.3	−39 49 25.7	3.6	−19.6	2.5

Notes. Column 1: clump name. Columns 2–7 and 8–12: observed properties of the ¹²CO(J = 1–0, 2–1) spectra obtained at the peak positions of the CO clumps. Columns 2–3: position of the peak CO intensity. Column 4: peak radiation temperature $T_{\rm R^{\ast }}$. Column 5: V_peak derived from a single Gaussian fitting. Column 6: FWHM line width ▵V_LSR. Column 7: total mass of the clumps derived by using the relation between the molecular hydrogen column density N(H₂) and the ¹²CO(J = 1–0) intensity W(¹²CO), N(H₂) = 2.0 × 10²⁰[W(¹²CO) (K km s⁻¹)] (cm⁻²) (Bertsch et al. 1993). See also the text for more details. Columns 8–12: the observed properties same with Columns 2–6 for the ¹²CO(J = 2–1) spectra. The properties of A–C, D, E, G, I–O, Q, and R–W derived from ¹²CO(J = 1–0) are shown by Moriguchi et al. (2005) and Sano et al. (2010).

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Finally, we compare the cold H i gas without CO with the X-rays in the southeast-rim of the SNR (hereafter SE-rim; Paper II). The cold H i gas has a density around 100 cm⁻³ and is likely interacting with the shock in a similar way to CO. Figure 4 shows an enlarged view of the SE-rim overlaid with the H i proton column density contours. The integration range is −20.0 to −11.0 km s⁻¹. We apply the H i self-absorption by following the analysis in Paper II. The lowest contour level and the contour interval in the H i proton column density are 2.0 × 10²¹ cm⁻² and 0.1 × 10²¹ cm⁻², respectively. It is remarkable that the H i distribution corrected for the self-absorption is complementary to the X-ray peaks in the low-photon-count region in the SE-rim.

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In this section we make a detailed comparison of spatial distributions between CO/H i clumps and X-rays in images, radial and azimuthal distributions (Figures 5, 7, and 8).

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In the left and middle panels of each row of Figure 5, we show the images of the CO integrated intensity overlaid on the distributions of the soft (1–5 keV, left) and hard (5–10 keV, middle) X-rays. The crosses in each image indicate the center of gravity of the CO clumps listed in Table 4, which is somewhat different from the peak position in Table 3. Each arrow indicates the direction of the center of the SNR. The dashed white circles represent radii 006 and 012 of the center of gravity. We see a trend that the X-rays are enhanced toward the CO, while the CO peak generally shows offsets from the X-ray peak.

In Figure 5's right panels, we plot radial profiles of the CO integrated intensity and X-ray counts averaged at each radius for the 1–5 keV and 5–10 keV bands. In order to characterize quantitatively the radial distribution, we first identify the peak in the 1–5 keV radial distribution. In addition, we defined the separation from the center of gravity of the CO clump to the X-ray peak in the radial distribution. Ten of the 17 clumps have peaks of the X-rays and positive X-ray slopes inside the peak in Figure 5. Clump W has no peak but also shows a clear positive X-ray slope. On the other hand, four of the CO clumps show negative slope and the other one shows a nearly flat slope. The radial distributions of the X-rays are generally smooth and monotonic so that linear approximation is reasonable in estimating the intensity gradients. Only the CO clumps O+O_b show a complicated non-monotonic radial distribution of the X-rays, which may be due to the blending of the two velocity components. We made least-squares fit to the X-rays by a straight line for simplicity for the 10 clumps within the peak, and for clump W and the other six clumps within a radius of 006. The values of the slope are listed in Table 5, and are shown as a histogram in Figure 6. Twelve of the 16 clumps (75%) show positive slopes in the X-rays, indicating that the X-rays show a decrease toward the CO clump and are brightened in the surroundings of the CO clump. For the remaining four with negative or flat slopes, except for clump Z, we also find clear relative enhancement of the X-rays in the surroundings of the CO clumps (in Figure 5, left and middle panels). Clump Z has no clear X-ray depression toward the center, while enhanced X-rays are seen in its surroundings (Figure 5, left and middle panels in the last low). We conclude that the enhanced X-rays around the CO clumps are a general trend among the CO clumps, as is consistent with the previous result on clump C (Paper I). We note that the averaged behavior is that the CO clumps have a radius of 004 ± 001 and the X-rays are distributed with a separation 007 ± 003 from the center of each clump (see also Table 4).

In addition, we made a similar analysis on the cold H i in the SE-rim. The H i distribution estimated from self-absorption well delineates the outer boundary of the X-rays (Figure 7), and their relative distributions are similar to the case of clump A. The H i column density distribution is fairly flat with no clear peak. Instead of using the H i peak, we draw a line passing through the center of the SNR and the soft X-ray peak, and define the H i column density peak on this line. The radial distributions are plotted centered on this H i peak. We find that the X-rays are clearly enhanced around the cold H i, which is a trend similar to the CO clumps.

The top three panels of Figure 8 show the same XIS mosaic images (1–5 keV) as in Figure 2(c). The lines connecting the CO/H i clump positions (shown in Figures 5, 7, and Table 4) and the center of the SNR (l, b) = (3473, −05) or (α_J2000, δ_J2000) = (17^h13^m34^s, −39°48'17'') are taken as the origins of the azimuth angle, which is measured counterclockwise. The azimuthal angular distribution of the X-rays around each CO/H i clump is estimated with respect to the direction of the center of the SNR. We measured the azimuthal angular extent of the X-rays in each clump in the 1–5 keV band image by adopting the background level inside the SNR as the threshold (Section 3.1). The results are shown in Figure 8 and Table 4. Figure 9's upper panel shows the range of angles and the lower panel shows a histogram of the peak angle. The five clumps inside the SNR, clumps C, E, I, L, and O_SW, are fully surrounded by the X-rays, which show a peak toward the azimuthal angle −180° to −120° (clumps C, I, and O_SW) or +60° to +120° (clumps E and L). These distributions indicate that the clumps inside the SNR are surrounded by the enhanced X-rays, whereas those on the border of the SNR have enhanced X-rays only toward the center of the SNR. In Figure 9's lower panel we see a trend that the X-rays are enhanced at an azimuthal angle around 0, while scattering is large, ±60° (clumps A, D, D_W, G, G_E, O+O_b, Q, R, U, W, Z, and SE-rim). The histogram was fitted by a Gaussian function from −120° to 120°, and we find that the best-fit parameters of center and sigma are 14° ± 5° and 51° ± 5° degrees, respectively.

We then test if the X-ray absorption by the ISM affects the X-ray images. The X-ray counts in 1–5 keV are generally about eight times higher than in 5–10 keV and the distributions in the two bands are fairly similar to each other (Figures 5 and 7). The ratio eight is consistent with an X-ray photon index of ∼2.3 typical to the SNR (e.g., Tanaka et al. 2008). Only clump L shows the largest difference from eight by a factor of more than two in the plot, indicating a softer spectrum. The similarity of the two bands suggests that X-ray absorption is not significant, because the X-rays should show a much harder spectrum if absorption is significant. The optical depth due to the ISM at the X-rays is proportional to the (−8/3)th power of the energy and is expressed as follows (Longair 1994);

$$\begin{eqnarray} &&\tau _{\mathrm{x}} = 2\times 10^{-22}\phantom{0}N_{\mathrm{H}}\phantom{0}\mathrm{(cm^{-2})} \cdot \varepsilon ^{-8/3} \phantom{0}\mathrm{(keV)}, \end{eqnarray} \tag{ 1 }$$

where N_H (cm⁻²) is the ISM column density and ε (keV) is X-ray photon energy. The maximum ISM column density in the SNR is estimated to be 1×10²² cm⁻² toward the brightest CO peak, peak C (e.g., Sano et al. 2010). Even for this column density absorption optical depths are 2, 0.3, 0.03, and 0.004 at 1 keV, 2 keV, 5 keV, and 10 keV, respectively, as calculated by Equation (1), and absorption does not likely affect the X-ray distribution significantly. We made a test on the X-ray absorption toward clump C, where the X-rays show depression, which may be due to absorption. The X-ray intensity ratio toward clump C is ∼8.6 between the 1–5 keV and 5–10 keV bands as observed by Suzaku (H. Sano et al. 2014, in preparation). By using Xspec we calculated the X-ray intensity integrated over the two energy bands for three different values of absorbing column density (i.e., 0.3, 1, and 3×10²² cm⁻²) for an X-ray photon index of 2.3, and found that the intensity ratios between the two bands are ∼11.1, 7.0, and 3.5, respectively. Column density slightly less than 1×10²² cm⁻² fits the X-ray observations reasonably well as is consistent with the CO observations.