Hitomi X-ray studies of Giant Radio Pulses from the Crab pulsar

Abstract

To search for giant X-ray pulses correlated with the giant radio pulses (GRPs) from the Crab pulsar, we performed a simultaneous observation of the Crab pulsar with the X-ray satellite Hitomi in the 2 - 300 keV band and the Kashima NICT radio observatory in the 1.4 - 1.7 GHz band with a net exposure of about 2 ks on 25 March 2016, just before the loss of the Hitomi mission. The timing performance of the Hitomi instruments was confirmed to meet the timing requirement and about 1,000 and 100 GRPs were simultaneously observed at the main and inter-pulse phases, respectively, and we found no apparent correlation between the giant radio pulses and the X-ray emission in either the main or inter-pulse phases. All variations are within the 2 sigma fluctuations of the X-ray fluxes at the pulse peaks, and the 3 sigma upper limits of variations of main- or inter-pulse GRPs are 22% or 80% of the peak flux in a 0.20 phase width, respectively, in the 2 - 300 keV band. The values become 25% or 110% for main or inter-pulse GRPs, respectively, when the phase width is restricted into the 0.03 phase. Among the upper limits from the Hitomi satellite, those in the 4.5-10 keV and the 70-300 keV are obtained for the first time, and those in other bands are consistent with previous reports. Numerically, the upper limits of main- and inter-pulse GRPs in the 0.20 phase width are about (2.4 and 9.3) ×10^-11 erg cm^-2, respectively. No significant variability in pulse profiles implies that the GRPs originated from a local place within the magnetosphere and the number of photon-emitting particles temporally increases. However, the results do not statistically rule out variations correlated with the GRPs, because the possible X-ray enhancement may appear due to a > 0.02% brightening of the pulse-peak flux under such conditions.

Keywords: Giant radio pulses; X-rays:stars; pulsar:individual:B0531+21; radio continuum:stars.

Fig. 1.

In the main panel GRP candidates (S/N>5.5) are shown in the (TDB, φ) plane, where two clusters in φ are of main pulse and interpulse GRPs (see text). Scatterd points show the remaining noise contribution. The threshold S/N=5.5 corresponds to the minimum pulse energy 2.2 kJy μs. The strongest main pulse occurred at 13:07:25.645TDB had the peak S/N~ 659. It spreaded over ~40μs interval having the total pulse energy 358 kJy μs.

Fig. 2.

Light curve of the Crab with from Hitomi SXS, HXI within region a, HXI within region b, and SGD-1, from top to bottom panels, respectively. The black croses represent the entire clened events of the Crab with Hitomi, and red shows the same but within the simultaneous intervals with radio observatories.

Fig. 3.

Comparison of Crab pulse profiles between the NORMAL and the GRP cycles, which were shown in blue and red, respectively, and the green croses in the each bottom panel show the difference between them. The left and right panels were the plots of the MP-GRPs and IP-GRPs, respectively, and from the top to the bottom, the data taken by the SXS, HXI, SGD-1, and combined data were plotted, respectively.

Fig. 4.

The top plots in each panel show the same pulse profiles of NORMAL and on/near GRP cycles represented by blue and red lines, respectively. The counts in off-phase (φ = 0.6 – 0.8) of the NORMAL cycles were subtracted from these pulse profiles. The bottom plots in each panel represent the ratio of the enhancement of near GRP data relative to the NORMAL cycles, which was shown in green. The data during the OFF phase were not plotted here. The statistical uncertainties of each phase bin at 1 and 2 sigma were shown in thick and thin black lines, respectively. The SXS, HXI, SGD-1, and combined data were shown from top to bottom panels, respectively.

Fig. 5.

The enhancement of inter or main pulses on the MP-GRP or IP-GRP were shown in the left and right plots, respectively. From the top to bottom panels, the SXS, HXI, SGD-1, and combined data were shown, respectively. The enhancements of GRP relative to normal cycles were measured at the corresponding pulse peaks (i.e., inter pulse of main pulse) with 0.0078, 0.0322, 0.0909, and 0.200 phase widths, which were shown from top to bottom in each plot. The enhancement, as a percentage, was shown in red, and statistical uncertainties of 1 and 2 sigma were shown in blue and green colors, respectively.

Fig. 6.

Left and right panels show the X-ray spectra during the main and inter pulses, respectively. The spectra with the SXS and the HXI are shown in black and red crosses, respectively. The best-fit power-law models are shown in red and black lines for the SXS and HXI, respectively. The bottom panels represent the ratio between the data and the model.

Fig. 7.

The enhancement of MP-GRPs of Crab pulsar in various energy bands obtained by the Hilliam Herschel Telescope (Shearer et al. 2003), Hale telescope (Strader et al. 2013), Chandra (Bilous et al. 2012), Suzaku (Mikami et al. 2014), Hitomi (this work), CGRO (Lundgren et al. 1995; Ramanamurthy & Thompson 1998), Fermi (Bilous et al. 2011), and VERITAS (Aliu et al. 2012) All the upper limits shown in arrows represent 3 σ values. The red and blue cases indicate the enhancement measured in a short-phase width (Δφ = 0.02 – 0.03 phase) and wider-phase width (Δφ > 0.1), respectively. Note that the thresholds of GRP detections in the radio band were different among them.