MAXI Observation of Crab Pulsar during the Gamma-ray Flare in September 2010

1. MAXI Observation of Crab Pulsar during the Gamma-ray Flare in September 2010 Mikio Morii, Nobuyuki Kawai, Ryuichi Usui, Kosuke Sugimori (Tokyo Tech), Mutsumi Sugizaki, Tatehiro Mihara, Takayuki Yamamoto, Masaru Matsuoka (RIKEN), and the MAXI team morii@hp.phys.titech.ac.jp http://maxi.riken.jp The Crab nebula is the standard candle of high energy X-ray and gamma-ray astronomy. The X-ray and gamma-ray fluxes and spectra have been believed to be stable for a long time scale of years. Surprisingly, on 2010 Sep. 19-20, AGILE and FERMI detected the GeV flare, in which the GeV flux enhanced about factor two . On the other hand, the X-ray flux change was not reported by RXTE-ASM, Swift-BAT and INTEGRAL-ISGRI. MAXI/GSC was monitoring the crab nebula with a time resolution of 0.2 ms during the GeV flare, then MAXI/GSC can check the changes of the pulse fraction and the pulse profile during the flare duration. The preliminary results of the analysis is shown here. There were no significant changes of pulse profile and pulse fraction during the GeV flare (Sep.19 -20). The pulse profile on Sep. 21-23 was marginally different from the template pulse profile (preliminary). The timing calibration and pulse profile analysis are still going on. Introduction Analysis Timing Calibration Surprisingly, the Crab flared on Sep. 19 – 20, 2010 in GeV gamma-ray range (AGILE and FERMI; ATEL #2855, #2861). Swift/BAT(15 – 50 keV) and RXTE-ASM (2 – 10 keV) did not detect the flux change (ATEL #2856, #2858). INTEGRAL observed the Crab nebula (Sep. 12 – 19 12h, partially covering the duration of gamma-ray flare), but did not detect the significant flux increase in 20 – 400 keV (ATEL #2856). Swift/XRT and RXTE/PCA follow-up observations on Sep. 22 and Sep. 24 did not detect any changes of flux, spectrum and pulse profile (ATEL #2866, #2872). Swfit/XRT also reported no evidence for active AGN near Crab (ATEL #2868). Radio timing observation of the Crab pulsar showed no evidence for a glitch, and no changes of pulsed flux and pulse shape (ATEL #2889). GSC(Gas Slit Camera) on MAXI [1] was monitoring the Crab nebula with a time resolution of 0.2 ms and detected the pulsation. The preliminary analysis of the pulse profile and pulse fraction is presented. We selected a circular region around the Crab nebula with a radius of 1.5 deg (~= FWHM of MAXI/GSC). The time of events was barycentric-corrected by mxbarycen. Energy range of 2 – 10 keV was used. The events were folded using the radio ephemeris provided at Jodrell Bank[2]. Pulse period and phase were compared with the radio ephemeris. For the detailed analysis of the pulsation, we must check the validity of event timing of MAXI system. The pulse phase of the Crab pulsar is used for the timing calibration. We confirmed that the history of pulse frequency of the Crab pulsar agrees well with the radio ephemeris. We also confirmed the pulse phase constancy during September 2010, although there is a constant phase deviation from the radio ephemeris due to the constant deviation of absolute time. Therefore, the timing calibration is satisfactory at least for analysis of pulse profile and pulse fraction. Pulse Profile and Pulse Fraction The figure 1 shows pulse profiles of the Crab pulsar. The top panel corresponds to the span of the GeV Gamma-ray flare. The pulse profiles were fitted by a template pulse profile obtained by MAXI/GSC from 2009-08-15 to 2009-08-31. The pulse profile data were fitted with a model of a x (template) + b. The fitting results are shown in the figure 2, where the shape of the template is shown in red histograms. The reduced chi2 and the probability are shown in the right of figure 2. The pulse fractions (PF) are calculated by PF = Σ a x (template) / Σ( a x (template) + b ). The time history of the pulse fraction and the reduced chi2 are shown in the figure 3. Fig. 3 Sep Oct Aug Jul Reduced Chi2 (dof) , Probability Fig. 1 Fig. 2 counts/bin, 64bin/period 0.97(62), P = 0.54 Pulse Fraction 1.33(62), P = 0.043 Gamma-ray Flare Reduced Chi2 (dof=62) Summary 1.25 (62), P = 0.091 We did not detect the change of pulse profile and pulse fraction during the GeV flare (Sep. 19-20). The pulse profile on Sep. 21-23 was marginally different from the template (preliminary). Further timing calibration and more detailed pulse profile analysis are necessary. 1.14(62), P = 0.22 References [1] Matsuoka, M. et al. (2009) PASJ, 61, 999 [2] Jodrell Bank: http://www.jb.man.ac.uk/research/pulsar/crab.html