Cyclotron Observations of Binary X-Ray Pulsars

Transcription

1 Progress of Theoretical Physics Supplement No. 169, Cyclotron Observations of Binary X-Ray Pulsars Tatehiro Mihara, 1, ) Yukikatsu Terada, 1 Motoki Nakajima, 2 Motoko Suzuki, 1 Teruaki Enoto, 3 Kazuo Makishima, 1,3 Fumiaki Nagase, 4 Keisuke Sudoh 5 and the Suzaku Cyclotron Team 1 RIKEN, Wako , Japan 2 Department of Physics, Nihon University, Tokyo , Japan 3 Department of Physics, The University of Tokyo, Tokyo , Japan 4 ISAS/JAXA, JAXA-ISAS, Sagamihara , Japan 5 Department of Physics, Rikkyo University, Tokyo , Japan Suzaku has observed some cyclotron X-ray pulsars in the SWG (Science working group) phase. Suzaku detected the cyclotron feature in the lowestluminosityfrom A In Her X-1, Suzaku measured new data points in the year-long trend of cyclotron resonance energy, and searched for the second harmonic structure. The RXTE observations of X gave the second example on changing the resonance energy and indicated that the energy ratio of the 2nd and 1st cyclotron energies is apart from two. 1. Introduction Dip-like features in the smooth hard X-ray (10 60 kev) spectra in the binary X-ray pulsars are interpreted as the cyclotron resonance scattering features (CRSF), which is produced by the gyro motion of the electrons in the magnetic fields. When the rotation radius becomes as small as the atom (Bohr radius), the orbital radius and the electron kinetic energy are quantized to so-called Laundau s levels. They are equally separated levels as E a (n+1/2+s),n=1, 2, 3,,wheres = ±1/2 is the electron spin. The difference of the neighboring levels is E a (kev) = 11.6 B(10 12 gauss). The CRSF of binary X-ray pulsars is only the direct method for measuring the surface magnetic field strength. The measured strong magnetic field is consistent with the following view. The accretion matter (plasma) gets away from the disc plane at some radius (Alfven radius), and goes along the magnetic field line onto the magnetic poles. The poles are different from the rotational poles and the hot spots appear and disappear with the rotation, which makes X-ray pulses (or modulation). To date, 14 binary pulsars are known to have CRSFs, and 3 possible ones (Table I). The history of CRSF began with its discovery from Her X-1 at 40 kev, showing the existence of a highly magnetized neutron star. 1) The second stage was to increase the number of examples, which lead to show that the CRSF is common among X-ray binary pulsars and the magnetic-field distribution is centered at Gauss, which is the same as the radio pulsars. 2) 5) The third stage is to investigate the CRSF itself in relation to the geometry of accretion. One is the higher harmonics, and those up to fifth were discovered in 4U ), 8) Another is change of the resonance energy. ) mihara@crab.riken.jp

2 192 T. Mihara et al. Table I. The X-ray binary pulsars with CRSF. Object E a [kev] Instruments 4U , ) HEAO , ) SAX ) RXTE 4U , 38 11) Ginga 12) SAX 4U ) Ginga Vela X-1 24, 52 14) Mir 15) RXTE X , 16) Ginga 49, 74 17) Integral Cep X ) Ginga Cen X ) SAX XPer 29 20) RXTE MXB ) RXTE XTEJ ) RXTE 4U ) SAX GX ) Ginga Her X ) Balloon A , ), 24) HEXE 25) Suzaku Integral RXTE GS ? 2) Ginga OAO ? 26) SAX LMC X-4 21? 100? 2) Ginga 27) SAX Fig. 1. Change of the CRSF energy with luminosity in 4U taken from Nakajima ) The dashed line is the prediction by the height model (E 0 =18keV). The first evidence of the change of the cyclotron energy was observed from 4U in 1991 with Ginga. 3) The double CRSF with fundamental energy of 11 kev in 1990 changed to the single CRSF centered at 17 kev in This change was related to the change of the luminosity. The height model was proposed and it also gave a good explanation to the continuous changes observed with RXTE successively in 1999 in Fig. 1. 9) The data also showed a saturation of resonance energy at high luminosity end, whose mechanism remains unknown. Now, three questions arise and we carried out Suzaku and RXTE observations. 1. Does CRSF energy change in the low luminosity? 2. Is the break point always at erg s 1? 3. Is saturation at high luminosity common? 2. A with Suzaku The transient X-ray pulsar A had a flare in 2005 June and again in one-orbit later in August. Suzaku observed it on 2005 September 14 to detect the CRSF in the decaying phase. 25) The luminosity was erg s 1, which was 1/10 of the peak in the August flare and 1/100 of the normal flares. Still, Suzaku detected the CRSF at 45.5 kev, which was the same energy as in the higher luminosities in Fig. 2. The feature was in absorption, and not in emission as predicted in low luminosities. 28) The resonance energy was constant within two orders of luminosity

3 Cyclotron of X-Ray Pulsars 193 and can be considered as that at the neutron star surface. The bending of the resonance energy would be in L x > erg s 1, if it would exist. 3. X with RXT E X had a flare in 2004 December, which was only 7 months before Suzaku was launched. Continuous observations were performed with RXTE. X had a very deep CRSF in ) and the high energy observations had been waited for years. In fact, the second and third CRSF were detected from this flare. 17) X has also become the second source to show the bending in the Lx-Ea relation 30) as shown in Fig. 2. The flare of X did not show a saturation in the higher luminosity end, and it can be understood as the luminosity did not reach the saturating point. A remarkable point is that the bending occurred at erg s 1, which is 6 times higher than 4U This would be understood in two ways. One is to assume the Fig. 2. Luminosity Resonance energy relations for various sources. Taken from Naka- higher magnentic field requires higher break-point luminosity in such a way as jima ) E a L 1/2 X empirically. The other is that X-ray might be beamed and it appeared 6 times stronger than the uniform radiation, while the break point is actually the same. Our height model 3) deduces more than 3 times flux to sideways than to upwards, and the 6 times would be possible. There is a fact that the pulse fraction of X is small, which means either we are looking from close to the rotational axis, or the magnetic axis is co-aligned to the rotational axis. If the geometry is the latter case and we are looking sideways, the beaming is possible in X Another interesting result from this observation is that the energy ratio of the 2nd and 1st cyclotron energies is larger than 2.0 significantly, and it increases with luminosity. 30) A possible explanation is that the 2nd harmonic represents magnetic field of deeper place than that of the 1st, since it has smaller (1/10) cross section. 4. Her X-1 with Suzaku Suzaku observed Her X-1 on 2005 October 5 and 2006 March 29 for the purpose of the energy and timing calibrations. 31), 32) Staubert ) and references therein investigated the change of the cyclotron energy of Her X-1 over 17 years. They proposed a stepwise change between 1991 (Ginga) and 1993 (BATSE) infig.3. A theory explains it as the expansion of the magnetic fields in the neutron star

4 194 T. Mihara et al. 46 Year [kev] Position Line Cyclotron Mir HEXE HEAO-1 Balloon HEXE GRIS Ginga BATSE BeppoSAX RXTE Integral Suzaku MJD Fig. 3. Historical changes of the cyclotron resonance energy of Her X-1. Taken from Fig. 1 in Staubert ) The red Suzaku points were obtained by using the same fitting model. The thicker red bars of Suzaku are statistical ones and the thinner ones represent statistical and systematic ones. (See the online edition for the color version of this figure.) atmosphere by the accretion stream. 33) Our Suzaku data, also plotted in Fig. 3, are consistent with the recent RXTE and Integral points although they have still large systematic errors. We also searched for the second harmonic of Her X-1, an indication of which was reported with BeppoSAX in the pulse-phase resolved spectrum. 5) We also found a possible dip feature at 70 kev in the spectrum of (descending peak off peak). 31) It is, however, in low significance and needs to be studied further. References 1) J. Trümper et al., Astrophys. J. 219 (1978), ) T. Mihara, Ph.D. Thesis, Tokyo Univ. (1995). 3) T. Mihara et al., Adv. Space Research 22 (1998), ) W. Coburn, Ph.D. Thesis, Univ. of California, San Diego (2001). 5) T. di Salvo et al., Nucl. Phys. B (Proc. Suppl.) 132 (2004), ) D. dal Fiume et al., Astron. Astrophys. Lett. 329 (1998), 41. 7) A. Santangelo et al., Astrophys. J. 523 (1999), 85. 8) W. A. Heindl et al., Astrophys. J. 521 (1999), 49. 9) M. Nakajima et al., Astrophys. J. 646 (2006), ) W. A. Wheaton et al., Nature 282 (1979), ) K. Makishima et al., Proc. 28th Yamada Conf., Frontiers of X-ray Astronomy (1992), p ) G. Cusumano et al., Astron. Astrophys. Lett. 338 (1998), ) G. W. Clark et al., Astrophys. J. 353 (1990), ) E. Kendziorra et al., The Compton Observatory Science Workshop (1992), p ) I. Kreykenbohm et al., Astron. Astrophys. 395 (2002), ) K. Makishima et al., Astrophys. J. 365 (1990), ) I. Kreykenbohm et al., Astron. Astrophys. Lett. 433 (2005), ) T. Mihara et al., Astrophys. J. 379 (1991), ) A. Santangelo et al., Astrophys. J. 340 (1998), ) W. Coburn et al., Astrophys. J. 552 (2001), ) V. A. McBride et al., Astron. Astrophys. 451 (2006), 267.

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