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A&A
Volume 549, January 2013
Article Number A23
Number of page(s) 4
Section Stellar structure and evolution
DOI https://doi.org/10.1051/0004-6361/201220275
Published online 10 December 2012

© ESO, 2012

1. Introduction

Magnetars are a peculiar class of neutron stars. Most of the about 20 known magnetars are characterized by strong dipolar magnetic fields (~1014−1015 Gauss) that are  ~10−1000 times higher than the average value in radio pulsars, near or even above the quantum electrodynamic field strength,  G (Harding & Lai 2006), although with two exceptions (Rea et al. 2010, 2012). They have bright X-ray luminosities (Lx ~ 1032−1036 erg s-1) from 0.1–300 keV, longer rotation periods than most ordinary radio pulsars (~2–12 s), and very high period derivatives (~10-13–10-11 s s-1). For more details see recent reviews on magnetars by Mereghetti (2008) and Rea & Esposito (2011).

The most successful model for explaining the X-ray emission from these objects invokes the decay and instability of their magnetic fields (Duncan & Thompson 1992; Thompson & Duncan 1993, 1995). The dichotomy between magnetars and ordinary pulsars may indicate different progenitors (Thompson & Duncan 1996). This scenario for birthing a magnetar postulates a very rapidly spinning proto-neutron star at birth, which would then have a high rotational energy. This excessive energy was searched in the supernova remnants (SNRs) surrounding magnetars. However, those SNRs show no excess in X-rays relative to those around normal pulsars (Vink & Kuiper 2006).

With this paper we aimed at testing whether the additional energy at birth could have gone in TeV emission.

To date, no magnetar has been detected at energies above 1 MeV. Although recently, the H.E.S.S. collaboration presented their discovery of extended TeV γ-ray emission toward the magnetar SGR 1806-201, it is doubtful that the emission is driven by the magnetar itself (Rowell et al. 2011). The Fermi-LAT Collaboration presented upper limits for 13 magnetars after 17 months of sky survey observations between 0.1 and 100 GeV (Abdo et al. 2010). Şaşmaz Muş & Göğüş (2010) studied specially the Fermi data of 4U 0142+61. Neither steady nor pulsed emission was found. In this work we present a search for the emission at very high energies (VHE; 200 GeV–50 TeV) from the two magnetars 4U 0142+61 and 1E 2259+586 with the MAGIC telescopes. These sources have been also observed by the VERITAS Collaboration and corresponding upper limits above an energy of 400 GeV have been presented in Guenette et al. (2009). The present MAGIC observations of these two magnetars extend the spectrum to lower energies, 200 GeV.

2. The observed magnetars

The source 4U 0142+61 is located at α2000, , at a distance of  ~3.6 kpc. With an X-ray luminosity of LX ~ 1 × 1035 erg   s-1 it is one of the most X-ray luminous magnetars known (McGill Pulsar Group 2012). This makes it a good target to search for persistent VHE emission. Long term spin period variations (P ~ 8.7 s) were discovered during observations with EXOSAT (Israel et al. 1994), leading to the measurement of the period derivative Ṗ ~ 2 × 10-12 s s-1, and consequently of the very strong magnetic field B ~ 1.3 × 1014   G (McGill Pulsar Group 2012). The bright 1–10 keV emission coming from 4U 0142+61 has been observed by many X-ray satellites (White et al. 1987; Israel et al. 1999; Patel et al. 2003; Rea et al. 2007a,b) revealing an X-ray spectrum typical of an Anomalous X-ray Pulsar (AXP), best described by an absorbed blackbody plus a power law (NH ~ 1022   cm-2,kT ~ 0.4   keV and Γ ~ 3.62). A very strong hard X-ray emission has been reported by INTEGRAL up to 250 keV, with a spectrum well modeled with a steep power-law with a photon index of  ~1 (Kuiper et al. 2006). At the time of data taking with the MAGIC telescope, there were only COMPTEL upper limits in the MeV range suggesting a spectral break in the hard X-ray emission of this object. The upper limits, however, do not put strong constraints on the HE or VHE gamma-ray emission of the object, especially given the high systematic uncertainty of the background subtraction in the data COMPTEL analysis (Schönfelder 2004). Recently, the upper limits derived by the Fermi-LAT Collaboration (Abdo et al. 2010) and by Şaşmaz Muş & Göğüş (2010) point to a cutoff in the MeV band.

The AXP 1E 2259+586 is located at embedded in the SNR CTB109. The source has a magnetic field of B ~ 0.59 × 1014   G and a distance of  ~4 kpc, making it a good candidate for MAGIC observations (McGill Pulsar Group 2012). RXTE measured the period (P ~ 7 s) and the period derivative (Ṗ ~ 0.5 × 10-12   s s-1) (Gavriil & Kaspi 2002). The X-ray spectrum is variable depending on the source emission state (Kaspi et al. 2003; Woods et al. 2004). After undergoing an outburst in 2002, the source returned into its possible quiescence state and the corresponding spectrum is best fitted by a blackbody plus a power law (NH ~ 1022   cm-2,kT ~ 0.4   keV and Γ ~ 3.75) (Zhu et al. 2008).

3. The MAGIC telescopes, analysis, and data

The MAGIC Collaboration operates two 17 m diameter imaging atmospheric Cherenkov telescopes on the Canary Island of La Palma. The data sets presented here were taken in 2008, i.e. before the second MAGIC telescope was operational (mono data), and in 2010 when both telescopes were already taking stereoscopic data. Details about the performance of MAGIC in mono and stereo mode can be found in Albert et al. (2008) and Aleksić et al. (2012). All data presented in this work were taken in the so-called wobble mode and were analyzed using the MARS analysis framework (Moralejo et al. 2009; Aleksić et al. 2012). The analyses presented here have an analysis threshold of 200 GeV. The upper limits were calculated using the Rolke algorithm (Rolke et al. 2005) with a confidence level (C.L.) of 95% assuming a Gaussian background and 30% of systematic uncertainty in the flux level. Since 1E 2259+586 is embedded in a SNR and may contain more than one emission region (see below) relevant parameters for the observations are the MAGIC field of view of 3.5° and the angular resolution of  ~0.07° above 300 GeV (Aleksić et al. 2012).

Table 1

Magnetar parameters taken from McGill Pulsar Group (2012), along with the MAGIC results presented here.

4U 0142+61 was observed for 25.41 h. After quality cuts 16.58 h of effective observation time remain. These mono data were taken between August and December 2008 covering a zenith angle range between 33° and 40.6°.

Data for 1E 2259+586 were taken in stereo mode wobbling around the sky position 0.12° away from the magnetar to have the shell of the SNR and the magnetar in the same field of view. Given the angular resolution of the MAGIC telescopes, these two possible TeV sources would be spatially separable with MAGIC. The region was observed between August and November 2010 for 14.33 h within a zenith angle range of 29°–43°. After quality cuts this amounted to 8.22 h of effective observation time.

4. Results

Neither source was detected by MAGIC. We computed the integral flux upper limits above 200 GeV with 95% C.L. assuming a differential energy spectral shape of a power law with an index of 2.6, similar to that of the Crab Nebula spectrum. The results are given in Table 1. A 25% change in the photon index yields a variation of about 7%. In Fig. 1 we show the corresponding test statistic (TS)2 map for 1E 2259+586. No excess was found at either the magnetar position nor at any location within the surrounding SNR. The TS map for 4U 0142+61 is not shown here, but shows the same flat behaviour. The upper limit for the extended SNR will be discussed elsewhere. The white contours represent the X-ray emission of the surrounding SNR detected with the XMM-Newton satellite (0.1–15 keV). We also searched for pulsations for both magnetars. For the pulsed analysis of 1E 2259+586 we used a timing solution valid at the epoch of the MAGIC observations, as derived by Içdem et al. (2012). We did not detect any significant pulsation at VHE energies. In the case of 4U 0142+61, we searched for pulsation using the ephemeris of Şaşmaz Muş & Göğüş (2010). We did not find any pulsation at VHE energies for this source either.

thumbnail Fig. 1

TS map of 1E 2259+586. The green cross represents the magnetar position. The white contours show the X-ray emission of the surrounding SNR CTB 109 detected with the XMM-Newton satellite. The color scale represents the TS value.

thumbnail Fig. 2

Spectral energy distributions of 4U 0142+61 a) and 1E 2259+586; b) from X-rays to TeV energies. In black the points and upper limits in the keV up to the GeV energy range are shown. The upper limits derived by the VERITAS Collaboration are shown in gray and the upper limits from this work are shown in red. See text for further details on the data and upper limits presented here.

Since neither source experienced an outburst in X-rays during our observing intervals, we can compare our upper limits with data taken with different instruments during different quiescent epochs. In Figs. 2a,b we present the 0.1 keV–3 TeV multi-band spectral energy distribution (SED) of 4U 0142+61 (1E 2259+586), respectively. For both sources the corresponding differential and integral upper limits derived in this work are shown (red lines in Fig. 2). In the case of 4U 0142+61, the 0.1–200 keV data are from XMM-Newton-PN and INTEGRAL-ISGRI (Rea et al. 2007a; den Hartog et al. 2008; Gonzalez et al. 2010) plotted together with the 2σ COMPTEL upper limits (den Hartog et al. 2006; Kuiper et al. 2006). For 1E 2259+586 we show data points from XMM-Newton-PN (Woods et al. 2004) together with COMPTEL upper limits (Kuiper et al. 2006). The upper limits provided by the Fermi-LAT Collaboration were calculated for three different energy ranges (Abdo et al. 2010). For the overall energy bin from 0.1–10 GeV a photon index of 2.5 was assumed. A cutoff is mimicked by splitting this energy bin into two parts with photon indices of 1.5 and 3.5, respectively. The assumed slopes are indicated in Fig. 2. The results derived by the VERITAS Collaboration on the two sources are also shown for comparison (gray dashed lines). They correspond to 99% C.L. integral flux upper limits of 8.68 × 10-13   cm-2 s-1 for 4U 0142+61 and 2.49 × 10-12   cm-2 s-1 for 1E 2259+586 by assuming a power-law with a photon index of 2.5 above 400 GeV (Guenette et al. 2009). The upper limits for both sources are compatible with a break in the power law at  ~1 MeV. However, the SED lacks any measurements above hard X-rays, what gives complete freedom under the corresponding instrumental sensitivity.

Cheng & Zhang (2001) presented a model for the VHE radiation from magnetars. They predicted emission of γ-rays in the GeV band coming from the outer gap for the two sources we studied. This model has been recently revised by Tong et al. (2011), who updated the observational parameters to calculate the γ-ray radiation properties of all AXPs and SGRs using the models by Zhang & Cheng (1997) and Cheng & Zhang (2001). The scenario by Tong et al. (2011) predicts that 4U 0142+61 should have been detected by Fermi-LAT, although they explain the lack of a detection by Fermi-LAT (Abdo et al. 2010; Şaşmaz Muş & Göğüş 2010) by invoking accretion. For 1E 2259+586 the model does not predict GeV emission. We note that although none of the current models predict TeV range emission for either magnetar, the existence of diffuse emission around 1E 2259+586 could lead to the appearance of an extra component in the SED besides any magnetospheric emission.

Using the MAGIC telescopes we studied for the first time the possibility of magnetars to be a new TeV source class on the examples of 4U 0142+61 and 1E 2259+586. This exploratory work led to a non-detection of the VHE gamma-ray emission from either of them. This result indicates that magnetars are probably not VHE emitters during their quiescent state, as expected from the various theoretical models. However, the possibility of magnetars being VHE emitters during flaring episodes cannot be ruled out because of the lack of VHE observations during these high-activity periods. Consequently, our future searches for VHE emission of magnetars will be performed during outbursts3.

1

Originally, magnetars were divided into two categories: soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs; Woods & Thompson 2006).

2

Our test statistic is (Li & Ma 1983, Eq. (17)), applied on a smoothed and modeled background estimation. Its null hypothesis distribution mostly resembles a Gaussian function, but in general can have a somewhat different shape or width.

3

In order to provide fast reactions to such events in the future, MAGIC has installed an alert system, which receives alerts provided by several satellites and points the telescopes to the flaring source automatically, as it is also done for observations of Gamma Ray Bursts.

Acknowledgments

We would like to thank the Instituto de Astrofísica de Canarias for the excellent working conditions at the Observatorio del Roque de los Muchachos in La Palma. The support of the German BMBF and MPG, the Italian INFN, the Swiss National Fund SNF, and the Spanish MICINN is gratefully acknowledged. This work was also supported by the CPAN CSD2007-00042 and MultiDark CSD2009-00064 projects of the Spanish Consolider-Ingenio 2010 programme, by grant DO02-353 of the Bulgarian NSF, by grant 127740 of the Academy of Finland, by the DFG Cluster of Excellence “Origin and Structure of the Universe”, by the DFG Collaborative Research Centers SFB823/C4 and SFB876/C3, and by the Polish MNiSzW grant 745/N-HESS-MAGIC/2010/0.

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All Tables

Table 1

Magnetar parameters taken from McGill Pulsar Group (2012), along with the MAGIC results presented here.

In the text

All Figures

thumbnail Fig. 1

TS map of 1E 2259+586. The green cross represents the magnetar position. The white contours show the X-ray emission of the surrounding SNR CTB 109 detected with the XMM-Newton satellite. The color scale represents the TS value.
In the text
thumbnail Fig. 2

Spectral energy distributions of 4U 0142+61 a) and 1E 2259+586; b) from X-rays to TeV energies. In black the points and upper limits in the keV up to the GeV energy range are shown. The upper limits derived by the VERITAS Collaboration are shown in gray and the upper limits from this work are shown in red. See text for further details on the data and upper limits presented here.
In the text

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