ap-6-11.dvi Acta Polytechnica Vol. 51 No. 6/2011 Monitoring PSR B1509–58 with RXTE: Spectral analysis 1996–2010 E. Litzinger, K. Pottschmidt, J. Wilms, S. Suchy, R. E. Rothschild, I. Kreykenbohm Abstract We present an analysis of the X-ray spectra of the young, Crab-like pulsar PSR B1509–58 (pulse period P ∼ 151ms) observed byRXTE over 14 years since the beginning of themission in 1996. The uniformdataset is especially well suited for studying the stability of the spectral parameters over time as well as for determining pulse phase resolved spectral parameters with high significance. The phase averaged spectra as well as the resolved spectra can be well described by an absorbed power law. Keywords: pulsars: individual (PSR B1509–58) — stars: neutron — X-rays: stars. 1 Introduction ThepulsarPSRB1509–58wasdiscovered inEinstein X-Ray Observatory data from 1979 and 1980 [5]. The pulsar is associatedwith the supernova remnant G320.4–1.2 (MSH 15–52) in the constellation Circi- nus. PSRB1509–58hasbeen establishedasoneofonly a few known Crab-like sources, i.e., a young pulsar powering a synchrotron nebula [6]. PSR B1509–58’s nebula is considerably larger, its surface brightness is lower and the pulse period of P ∼ 151ms is slower than that of the Crab. Due to a very high spin-down rate of Ṗ ∼ 1.5 × 10−12 s s−1, however, the characteristic age P/2Ṗ of PSR B1509–58 is ∼ 1.6 × 103yr (e.g., [7]), i.e., comparable to that of the Crab (∼ 1.3×103yr). In the following we analyse the phase averaged spectra (Sec. 2) and the phase resolved spectra (Sec. 5) from calculated ephemerides (Sec. 3) for the three major calibration epochs 3–5 for the RXTE. Pulse profiles for the epochs are presented in Sec. 4. A short summary of the results and the implications for further spectral analysis with RXTE are given in Sec. 6 2 Phase averaged spectra From approximately monthly monitoring observa- tions of PSR B1509–58 time averaged PCA (PCU2 top layer, [1,2]) and HEXTE (cluster A and B, [4]) spectra were created by averaging individual mon- itoring spectra over major instrument calibration epochs, i.e. betweenMJD50188and51259(epoch3), MJD 51259 and 51677 (epoch 4), and from MJD 51677 onward (epoch 5). Figure 1 shows the epoch averaged counts spectra for epoch 5. Some spectral parameters and their uncertainties for the three fits are given in Table 1. No systematic uncertainties have been added to the spectra. Fig. 1: (a) PCU2 top layer and HEXTE counts spectra obtained by accumulating all suitable monitoring spectra of epoch 5 and best simultaneous fit model (absorbed power law with an iron line), (b) Best fit residuals 38 Acta Polytechnica Vol. 51 No. 6/2011 Table 1: Best fit parameters for the phase averaged PCU2 spectra of calibration epoch 3–5 Parameter Epoch 3 Epoch 4 Epoch 5 Γ 2.022±0.001 2.021±0.001 2.026±0.001 AΓ [ 10−2keV−1cm−2 s−1 ] 7.48±0.84 7.48±1.01 7.33± 0.12 NH [ 1022cm−2 ] 0.37±0.01 0.39±0.02 0.58± 0.02 EFe [keV] 6.65 +0.01 −0.16 6.50 +0.16 −0.03 6.50± 0.03 cHEXTE 0.65±0.05 0.76±0.04 0.85± 0.02 χ2red/dof 1.09/48 0.58/42 2.16/94 F4−10keV [ 10−11ergcm−2s−1 ] 10.25 10.34 9.85 Funabs4−10keV [ 10−11ergcm−2s−1 ] 10.57 10.57 9.93 F10−20keV [ 10−11ergcm−2s−1 ] 7.76 7.76 7.20 F20−200keV [ 10−11ergcm−2s−1 ] 25.07 25.15 22.36 The spectra are modeled by an absorbed power law. In addition we found clear indications of a narrow iron Kα line which is included by a Gaus- sian component added to the power law. Be- cause of the long monitoring time of epoch 5 (ten years) systematic features are visible in the spectra. For the PCA there are strong residu- als around 5keV depending on the three xenon L edges. Around 9.3keV a broad negative residual is visible. We model it with an additional neg- ative Gaussian line at this energy with a flux of −3 × 10−5cm−2 s−1. We speculate that this resid- ual might be related to imperfect modeling of the copper Kα emission line at 8.04keV. Also events from the americium calibration source of the PCA at 33keV are visible. They were also modeled with a negative Gaussian line with a flux of −1.4 × 10−4cm−2 s−1. 3 Pulse period ephemeris The pulse phase resolved analysis for the PCA is based on high time-resolution GoodXenon event mode data, filtered for PCU2 top layer events. Ephemerides forPSRB1509–58were calculated from the pulse frequencies of each observation. The ref- erence epoch was set to t0(MJD) = 52921.0, the averaged time of the monitoring (see Figure 2, Ta- ble 2). With this result barycentered pulse phase- and energy resolved source count rates (pha2 files) were created using a modified version of the FTOOL fasebin [3]. Fig. 2: Frequencies of each observation, calculated by epoch folding the barycentered GoodXenon lightcurve, vs. time with the best fit of a polynomial of quartic grade. A linear decline is visible Table 2: Values for the pulse frequency and its first three derivatives ν [ s−1 ] ν̇ [ 10−11s−2 ] ν̈ [ 10−21s−3 ] ... ν [ 10−29s−4 ] 6.61032±1e−6 −6.6965±0.002 1.18± 0.24 1.77±0.36 39 Acta Polytechnica Vol. 51 No. 6/2011 4 Pulse profiles Pulse profiles in the energy range 3–43keV for the three major epochs are shown in Figure 3. The peak was centered to phase 1.0 by shifting the individ- ual pulse profiles and adding them up. The decline in rate between epochs is an instrumental effect and is accounted for in the calibration of the PCU2 top layer. The bigger errorbars in epoch 4 are due to a shorter duration and therefore less observations (30 in epoch 3, 13 in epoch 4, 213 in epoch 5). A clear division in peak Φ = 0.88 − 1.25 and off-peak Φ=0.44−0.75 is possible. Fig. 3: Pulse profiles for the energy range 3–43keV of PSR B1509–58 for the three major epochs 3, 4 and 5 5 Phase resolved spectra All pulsed (i.e., peak minus off-peak) and unpulsed (regular background) spectrawere summed to obtain averaged spectra for epochs 3–5, respectively. For the averagedphase resolved spectra, as for the phase averaged spectra before, an absorbed power law was fitted (energy range 3–20keV). For epoch 5 it showed that the residuals for the pulsed emission improved by including a cutoff (see Figures 4 and 5). The pulsed spectra showed no indication for an iron line at 6.4keV. The line is part of the unpulsed spectra andhence of thePWNofPSRB1509–58. This is also the explanation for the different values of the NH. In thepulsed emissionwe see thebeamof thepulsar and the galactic extinction. In the unpulsed emission we see the surroundingPWNwithout the pulsar and the galactic extinction. Therefore the value for the latter is smaller than for the pulsed emission. The best fit parameters for epoch 5 are shown in Table 3. Fig. 4: PCU2 top layer counts spectra of the pulsed emis- sion of epoch 5. Residuals are shown for an absorbed power law and after including a cutoff Fig. 5: The same as Figure 4 but for the unpulsed emis- sion Table 3: Best fit parameters for the pulsed and unpulsed emission for epoch 5 Parameter E5 pulsed E5 pulsed E5 unpulsed Γ 1.37±0.01 1.27±0.01 2.27±0.01 Ecut [keV] 116±8 NH [ 1022cm−2 ] 2.40±0.1 1.97±0.11 1.34±0.02 EFe [keV] 6.57 +0.01 −0.07 F4−10keV [ 10−11ergcm−2s−1 ] 4.28±0.02 4.29±0.02 7.99±0.01 Funabs4−10keV [ 10−11ergcm−2s−1 ] 4.57±0.02 4.52±0.02 8.30±0.01 F10−20keV [ 10−11ergcm−2s−1 ] 5.56±0.02 5.57±0.02 4.90±0.01 χ2red 1.09 0.91 1.26 40 Acta Polytechnica Vol. 51 No. 6/2011 6 Summary and conclusions We couldwell describe the spectra with an absorbed power lawwithaGaussian line for thephaseaveraged and the unpulsed spectra and without the Gaussian for the pulsed spectra. For the pulsed spectrum of epoch 5 a cutoff improves the fit, while for the other epochs and theunpulsed emission it hasno effect. No significant changesbetween the values of the different epochs for the averaged, pulsed and unpulsed emis- sion were seen. Long observations show systematic effects from the instruments and are therefore good for describing the calibration effects. As forthcoming work we intend to add HEXTE spectra to the epoch averaged phase resolved analysis. References [1] Jahoda, K., Swank, J. H., Giles, A. B., et al.: Proc. EUV, X-Ray, and Gamma-Ray Instrumen- tation for Astronomy VII, 1996, Vol. 2808, 59. [2] Jahoda,K.,Markwardt,C.B., Radeva,Y., et al.: ApJS, 163, 401, 2006. [3] Kreykenbohm, I., Coburn, W., Wilms, J., et al.: A&A 395, 129, 2002. [4] Rothschild, R. E., Blanco, P. R., Gruber, D. E., et al.: ApJ, 496, 538, 1998. [5] Seward,F.D.,Harnden, Jr.F.R.: ApJ, 256,L45, 1982. [6] Seward, F. D., Harnden, Jr. F. R., Szymko- wiak, A., et al.: ApJ, 281, 650, 1984. [7] Zhang, L., Cheng, K. S.: A&A, 363, 575, 2000. E. Litzinger Dr. Remeis-Observatory/ECAP,FAU Bamberg, Germany K. Pottschmidt CRESST/NASA-GSFC Greenbelt, MD, USA UMBC Baltimore, MD, USA J. Wilms Dr. Remeis-Observatory/ECAP,FAU Bamberg, Germany S. Suchy CASS/UCSD La Jolla, CA, USA R. E. Rothschild CASS/UCSD La Jolla, CA, USA I. Kreykenbohm Dr. Remeis-Observatory/ECAP,FAU Bamberg, Germany 41