Identification of CXOU J171405.7−381031 as a New Magnetar with XMM-Newton

Abstract

1. Introduction

Anomalous X-ray Pulsars (AXPs) have been distinguished from other types of pulsars by their peculiarity, such as a long spin period of a neutron star (⁠|$P =$| 2–12 s) and moderately bright X-ray emission with |$L_{\rm X} \approx$| 10|$^{33\hbox{--}35}\ $|erg s|$^{-1}$| that is in general much greater than the spin-down luminosity of the neutron star (⁠|$\dot{E} \approx$| 10|$^{31\hbox{--}34}\ $|erg s|$^{-1}$|⁠).¹ Since there is no evidence of mass accretion, it has been thought that the X-ray emission is replenished with magnetic energy of the neutron star. As a matter of fact, the magnetic field strength, estimated on the basis of the dipole radiation framework, of AXPs with known |$\dot P$| is all in excess of the critical magnetic field |$B_{\rm c} =$| 4.4 |$\times$| 10|$^{13}\ $|G, above which the differential energy of the neighboring Landau levels exceeds the rest-mass energy of an electron. The AXPs, together with Soft Gamma-ray Repeaters (SGRs), are now classified as “magnetars” (Duncan & Thompson 1992; Thompson & Duncan 1995, 1996). As of 2010 June 9, 10 AXPs and 9 SGRs were known.¹ It is, however, still unclear that magnetars are endowed with such extraordinary energy, compared with the other ordinary pulsars. One of the difficulties lies in the fact that only a few magnetars are found to be associated with host supernova remnants (SNRs), which provide us with information on their progenitors, independent age estimations, and so on. Obviously we need more magnetar samples with the SNR association.

CXOU J171405.7|$-$|381031 was discovered during the course of identifying Galactic TeV sources. Aharonian et al. (2006) discovered the TeV source HESS J1713|$-$|381 with the atmospheric Cherenkov telescope H.E.S.S., and indicated its association with the supernova remnant (SNR) CTB 37B. Using Chandra data, Aharonian et al. (2008) identified the point source CXOU J171405.7|$-$|381031 in the radio shell of CTB 37B (Kassim et al. 1991). Its location is slightly offset (⁠|$\approx$| 1|$^{\prime}$|⁠) from the peak of the H.E.S.S. brightness contour (see figure 2 of Nakamura et al. 2009). Nakamura et al. (2009) observed CTB 37B with Suzaku (Mitsuda et al. 2007), and found that its spectrum is well represented by a power law with a photon index of 3.0|$\ \pm\ $|0.2. The hydrogen column density to CXOU J171405.7|$-$|381031 (⁠|$\approx$|4 |$\times$| 10|$^{22}\ $|cm|$^{-2}$|⁠) is consistent with that of diffuse thermal emission of CTB 37B, which strengthens the association of CXOU J171405.7|$-$|381031 to the SNR. The distance to this SNR is estimated to be 10.2|$\ \pm\ $|3.5 kpc (Caswell et al. 1975), and we cite this value here. These facts lead us to conclude that CXOU J171405.7|$-$|381031 is probably a new AXP, although the limited time resolution of the XIS (8 s: Koyama et al. 2007) on board Suzaku in the full-frame mode and the ACIS (3.24 s: Garmire et al. 2000) on board Chandra in the imaging mode precludes them from detecting pulsation. Halpern and Gotthelf (2010) finally discovered, using the Chandra cc mode observation data, that CXOU J171405.7|$-$|381031 pulsated at a period of 3.82305|$\ \pm\ $|0.00002 s on 2009 January 25, which is well within the range of the AXP pulse period. The pulse shape is sinusoidal with a pulse fraction of 31%. They also detected an excess emission above the power-law spectrum above |$\sim$|6 keV, which is one of the common features among AXPs.

To further strengthen an identification of CXOU J171405.7|$-$|381031 with an AXP, it is important to measure |$\dot P$|⁠. The known |$\dot P$| of AXPs exceeds 10|$^{-12}\ $|s s|$^{-1}$|⁠,¹ which is systematically larger than those of the other rotation-powered pulsars. Furthermore, under the dipole radiation assumption, we are able to estimate the strength of the magnetic field, which is important for seeing if CXOU J171405.7|$-$|381031 is a magnetar. In order to evaluate |$\dot P$|⁠, we carried out an observation of CTB 37B with XMM-Newton (Jansen et al. 2001). Monitoring the X-ray flux is also important, since most magnetars show X-ray time variability. Halpern and Gotthelf (2010) showed that its flux changed between Suzaku and Chandra observations, although Suzaku’s low spatial resolution prevented us from concluding that this source showed the time variability, since there could be contamination of diffuse nonthermal X-rays, which is quite common in young SNRs (Bamba et al. 2005).

In section 2, we describe how observation and data screening have been carried out. In section 3, we present the results of our timing and spectral analysis. We have clearly detected a pulsation from the source. Comparing our result with the Chandra pulse period (Halpern & Gotthelf 2010), we detected |$\dot P$| significantly. We describe our calculation of the characteristic age and the magnetic field strength in section 4, and argue that CXOU J171405.7|$-$|381031 should be regarded as a new magnetar.

2. Observation and Data Reduction

The XMM-Newton observation of the SNR CTB 37B was carried out from 2010 March 17 13:16 (UT) to March 18 23:06 (UT). Our primary objective is timing analysis to determine the physical parameter of CXOU J171405.7|$-$|381031. Hence, we concentrate on data taken with the EPIC pn (Strüder et al. 2001), whose time resolution is 73.4 ms, which is much better than that of EPIC MOS (2.6 s: Turner et al. 2001). We conducted a data analysis with SAS ver. 9.0.0. We have first checked the background flare using the cleaned event file pipe-line-processed with the CCF dated on 2010 April 30 in the data package. We extracted a light curve in the 10–12 keV band, and produced a GTI file that excluded time intervals with a 10–12 counting rate of |$\gt $|0.35 c s|$^{-1}$|⁠. We then revised the event file, applying this GTI file. As a result, the effective exposure time became 40.264 ks. Using the new event file thus processed, we constructed a pn image in the 1–10 keV band, as shown in figure 1. CXOU J171405.7|$-$|381031 is clearly detected at |$\ell =$| 348|$^\circ$|40|$^{\prime}$|51|$^{\prime\prime}\!\!\!.$|087, |$b =$| 0|$^\circ$|22|$^{\prime}$|15|$^{\prime\prime}\!\!\!.$|820, which are consistent with those from Chandra (Aharonian et al. 2008). The nonthermal diffuse emission extending to the south of CXOU J171405.7|$-$|381031 (Nakamura et al. 2009) is also detected. The small green circle with a radius of 30|$^{\prime\prime}$| is the extraction region of the source photons, whereas the other dashed circle with a radius of 2|$^{\prime}$| is that for the background extraction. The intensity of CXOU J171405.7|$-$|381031 became 0.264|$\ \pm\ $|0.003 c s|$^{-1}$| in the 1–10 keV band after subtracting the background.

Fig. 1

XMM-Newton pn image of the CTB37B region in the 1–10 keV band. The image was smoothed with a Gaussian with |$\sigma =$| 3 pixels. The small green circle with a radius of 30|$^{\prime\prime}$| is the region for extraction of the source photons, whereas the other circle (dashed, |$r =$| 2|$^{\prime}$|⁠) is the region for background extraction.

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Figure 2 shows the background-subtracted spectrum together with the best-fit model and the fit residuals. We can see deeply absorbed and hard emission. We have fitted an absorbed power-law model to the data. The metal composition of Anders and Grevesse (1989) is adopted as the solar abundance for the absorbing material. The fit is accepted with a |$\chi^{2}/$|d.o.f. of 372.34|$/$|341. The best-fit photon index, the hydrogen column density, and the observed and intrinsic fluxes in the 2.0–10.0 keV band are 3.45|$^{+0.09}_{-0.08}$|⁠, 3.95|$^{+0.15}_{-0.14} \times$| 10|$^{22}\ $|cm|$^{-2}$|⁠, (1.51|$\ \pm\ $|0.03) |$\times$| 10|$^{-12}\ $|erg cm|$^{-2}$|s|$^{-1}$|⁠, and (2.68|$\ \pm\ $|0.09) |$\times$| 10|$^{-12}\ $|erg cm|$^{-2}$|s|$^{-1}$|⁠, respectively (the errors represent the single-parameter 90% confidence limit). We can see positive residuals above |$\sim$|5 keV, which is probably the hard tail that is common among magnetars (Muno et al. 2007; Naik et al. 2008; Nakagawa et al. 2009; Enoto et al. 2010). A detailed spectral analysis will be presented in a forthcoming paper.

Fig. 2

XMM-Newton pn spectra of CXOU J171405.7|$-$|381031 and the two background regions. The background spectra were corrected for the aperture size.

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3. Timing Analysis

Since there is nearly no source flux below |$\sim$| 1 keV, we carried out a timing analysis in the 1–10 keV band. After a barycentric correction to the event file, we created a light curve with the minimum time resolution (73.4 ms), and first made a power spectrum in the frequency range below 0.5 Hz. The result is shown in figure 3a. A highly significant peak (⁠|$\sim$|100|$\sigma$|⁠) appears at 0.2614 Hz. We then carried out an epoch folding analysis near this frequency. The resultant periodogram is shown in figure 3b. The rotational period was obtained to be |$P =$| 3.825352|$\ \pm\ $|0.000004 s. No other period except for the harmonics was significant. Compared to the period obtained from the Chandra cc mode data (Halpern & Gotthelf 2010), 3.82305|$\ \pm\ $|0.00002 s, the period became longer by 0.00230|$\ \pm\ $|0.00002 s. The time, 416.264676 d, has elapsed since the Chandra observation (beginning at 2009 January 25 06:55:08); thereby the average period derivative is obtained to be |$\dot P =$| 6.40|$\ \pm\ $|0.05 |$\times$| 10|$^{-11}\ $|s s|$^{-1}$|⁠. Figure 3c shows the light curve folded at the best spin period. The pulse profile is similar to that obtained by Halpern and Gotthelf (2010), including the pulse fraction.

Fig. 3

(a) Power spectrum of CXOU J171405.7|$-$|381031 in the 1–10 keV band. A highly significant peak is detected at a frequency of 0.2614 Hz. (b) Periodogram around 0.2614 Hz. The pulsation period was obtained to be 3.825352|$\,\pm\,$|0.000004 s. (c) The light curve folded at this period.

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4. Discussion

From a timing analysis of the XMM-Newton pn data, we obtained |$\dot P =$| 6.40|$\ \pm\ $|0.05 |$\times$| 10|$^{-11}\ $|s s|$^{-1}$|⁠. Together with |$P =$| 3.825352 s, the spin-down luminosity (⁠|$\dot E =$| 3.9 |$\times$| 10|$^{46}\dot P P^{-3}\ $|erg s|$^{-1}$|⁠), the characteristic age [|$t_{\rm c} = P/$|(2|$\dot P$|⁠) s], and the dipole surface magnetic field (⁠|$B_{\rm s} =$| 3.2 |$\times$| 10|$^{19}\sqrt{P\dot P}\ $|G) can be derived to be 4.5 |$\times$| 10|$^{34}\ $|erg s|$^{-1}$|⁠, 9.5 |$\times$| 10|$^{2}\ $|yr, and 5.0 |$\times$| 10|$^{14}\ $|G, respectively. All of these estimated parameters are within the range of the known AXPs, and we thus conclude that CXOU J171405.7|$-$|381031 is a new magnetar, together with the large photon index of its spectrum. The ratio of the 2–10 keV luminosity (⁠|$L_{\rm X}$|⁠) to |$\dot E$| is 0.4, which is much smaller than that of typical magnetars. PSR J1846|$-$|0258 is a radio pulsar with |$B_{\rm s}$| larger than the critical magnetic field, which shows an |$L_{\rm X}/\dot E$| of 0.2 (Helfand et al. 2003) including its pulsar wind nebula (PWN), and should be a key source to connect magnetars and conventional radio pulsars. CXOU J171405.7|$-$|381031 thus closely resembles PSR J1846|$-$|0258 and seems to be a new key source between magnetars and radio pulsars.

Note that this source is the youngest AXP and the second-youngest magnetar so far, in the next place of SGR 1806|$-$|20 (⁠|$t_{\rm c} =$| 0.22 kyr: Mereghetti et al. 2005). Another important point is that this magnetar is associated with a young SNR. Nakamura et al. (2009) obtained that the ionization age of the thermal plasma associated with CTB 37B is 650|$^{+2500}_{-300}\ $|yr. A possible association of CTB 37B with the historical SNR SN 393 has long been discussed (Clark & Stephenson 1975; Stephenson & Green 2002). The characteristic age we obtained is consistent with that in these discussions. Vink and Bamba (2009) discovered a pulsar wind nebula around the second-youngest AXP, 1E 1547.0|$-$|5408 (⁠|$t_{\rm c} =$| 1.4 kyr: Camilo et al. 2007). CXOU J171405.7|$-$|381031 is now a good target for searching for a PWN, which will be a further study with better spatial resolution. This will lead to a better understanding of young magnetars.

Some magnetars show X-ray flares and long-term variability, and hence we investigated whether CXOU J171405.7|$-$|381031 has X-ray time variability. The absorbed 2–10 keV flux was (1.1|$\ \pm\ $|0.2) |$\times$| 10|$^{-12}\ $|erg cm|$^{-2}$|s|$^{-1}$| on 2007 February 2 by Chandra (Nakamura et al. 2009), 1.8 |$\times$| 10|$^{-12}\ $|erg cm|$^{-2}$|s|$^{-1}$| on 2009 January 25 by Chandra (Halpern & Gotthelf 2010), and (1.51|$\ \pm\ $|0.03) |$\times$| 10|$^{-12}\ $|erg cm|$^{-2}$|s|$^{-1}$| on 2010 March 17–18 by XMM-Newton by this work. We thus concluded that CXOU J171405.7|$-$|381031 showed significant time variability during the period of these years.

We acknowledge Jacco Vink for his fruitful discussions. This work was supported in part by a Grant-in-Aid for Scientific Research of the Ministry of Education, Culture, Sports, Science and Technology (No. 22684012, A.B.).

References