POLAR measurements of the Crab pulsar
PPOLAR measurements of the Crab pulsar
Hancheng Li ab , Mingyu Ge ∗ b , Bobing Wu b on behalf of the POLAR collaboration † a University of Chinese Academy of Sciences, Beijing 100049, China b Key Laboratory for Particle Astrophysics, Institute of High Energy Physics, Beijing 100049,ChinaE-mail: [email protected]
POLAR is a Compton polarimeter sensitive in the 50 to 500 keV energy range. The Crab pulsaris a scientific target for POLAR on board the Chinese space laboratory Tiangong-2 (TG-2). Withits large Field of View (FoV), POLAR detected significant pulsed signals from the Crab pulsarwhich is visible by POLAR in about half of observation time. In this work, we present thepreliminary results including the pulse profile, timing and polarization measuring method. First,we show the highly significant pulse profile observed by POLAR which is compared to the resultsof other detectors including Fermi/LAT and INTEGRAL. And the pulse profile as a function oftheta incident angle and as a function of channel number, which indicate that POLAR has a gooddetection performance, have been showed. Second, we find that the timing of the Crab pulses areaccurately measured, which provides a unique verification and calibration to the POLAR timingsystem. Finally, the potential polarization measurement of the Crab pulsar is also discussed. ∗ Speaker. † Abous us
PoS (ICRC2017) , 820 (2017), doi: 10.22323/1.301.0820 c (cid:13) Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). http://pos.sissa.it/ a r X i v : . [ a s t r o - ph . I M ] O c t OLAR Crab
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1. Introduction
The Crab pulsar is one of the most widely studied celestial objects which was born in 1054, hasa spin period of about 33 ms, with a bright feature over almost the full electromagnetic spectrumfrom radio to high energy γ -rays. At all wavelengths this pulsar shows a double-pulse structure,with the main pulse (P1) and the inter pulse (P2) separated by a phase of 144 ◦ [1] [2] [3] [4] [5]. Inaddition, the polarization of the Crab pulsar and Nebulae are also concerned since they have highlymagnetized filed. A sounding rocket mission [6] reported the first detection of X-ray polarization(5-20 keV) for the Crab nebula with a sounding rocket payload. Then, the polarization fractionwas measured with 19 . ± . ) at 2.6 keV by an instrument on-board the OSO-8 satellite mission[7] during a 256 ks observation. More recently, the INTEGRAL satellite [8] [9], AstroSAT andPoGO+ [10] instruments have also measured the polarization of Crab emissions.POLAR started its mission on-board the Chinese space laboratory TG-2 after the successfullaunch which was on 15th September, 2016 [11]. The installation diagram of POLAR on TG-2 isshown in Figure 1. POLAR has a high opportunity to capture photons from Crab pulsar thanks to itshigh sensitivity and large FoV when flying in-orbit, even if POLAR has no pointing control systemby itself. The structure of POLAR detector [12] is shown in Figure 2 (a). When photons incidenton POLAR, they tend to scatter in the detector. The deposited energy of these scattering photonswill be converted to digital signal, and if the signal value exceed certain thresholds, through ancomplex process by electronic system [13], these trigger information will be recorded.The polarization of Crab pulsed emission can be measured by POLAR since it is a polarimeterespecially for GRB with an energy range from 50 to 500 keV. The basic concept is to measure themean degree of polarization and the azimuthal angle of the polarization vector by analyzing theangular distribution of the Compton scattering azimuthal angle of a sample of photons from GRBor other X-ray/ Gamma-ray emissions. The emission of the Crab pulsar can also be observed be-cause the pulsed photon can be accumulated with the steady pulse phase considering the frequencyevolution, though it is very faint compared with the GRB flux in a short time interval. However,it is very difficult to acquire the polarization directly because the incident angle between the Craband POLAR varies with time and the signals could not be accumulated directly. Considering thissituation, some new methods should be developed together with the Monte Carlo simulation.
2. Observations and Data Reduction
The Crab observation period for POLAR can obtained only through the judgement of IN orOUT FoV. So we can calculate the incident angle from Crab to POLAR detector by use of platformparameters data (PPD) which is provided in every second. And we can get an exposure map ofCrab observation by accumulating the same incident angle of observation together, as shown inFigure 2 (b). Above all, in scientific observation data (SCI), for each event, it has a lot of usefulinformation, such as trigger time, deposited energy signal, trigger position, trigger number, etc.These information support us to perform the following analysis. OLAR Crab
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Figure 1:
The installation diagram of POLAR on TG-2. The outboard part is called obox, it’s connectedwith ibox which is supporting electronics system inboard. (a) (b)
Figure 2: (a) is the structure of POLAR detector. It consists of 25 modules, and each module has 8*8detecting channels made of plastic scintillator bars. More working schematics of POLAR can be found in[12] and [13]. The incident angles are defined as theta and phi. Theta angle is the angle of Z-axis positivedirection and the source vector from origin of coordinates to source point. Projecting the source vector ontoX-Y plane, and phi is the angle between projection and X axis positive direction. (b) is the Crab exposuremap on POLAR. It does not cover all incident directions. OLAR Crab
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Crab photons are drown in a sea of background. Data reduction is necessary to reduce back-ground. First, the acquisited data lack of PPD should be rejected, since we are not clear the incidentangle without PPD, and we don’t know whether there is crab photon in SCI. Second, in order toensure that crab is within our visible region, the event whose theta incident angle larger than 100degrees should be rejected. At last, in view of the scattering times of Crab photons in POLAR de-tector rarely more than 5, we throw away the event whose trigger number more than 5. The abovereduction methods can improve ratio of signal and noise for Crab analysis.
We study the timing properties of the Crab pulsars with the POLAR observations. First, thearrival time for each photon was converted to the Solar System Barycentre using ephemeris DE405.For each observation we obtained the period of a pulsar by folding the observed counts to reachthe maximum Pearson χ . Second, the pulse profile was folded with the spin frequency and thetime of arrival (TOA) was calculated from the peak phase of the pulse and the reference time. Thentiming parameters were solved by the phase coherent timing method utilizing TEMPO2 [14] [15].At the end, the Fermi-LAT observations were analysed with the same process to check whetherthe observation by POLAR was performed appropriately especially for the timing system.
3. Results from POLAR
From the almost consecutive observation from POLAR, we have obtained the frequency evo-lution and timing results as shown in Figure 3 and Table 1. First of all, the spin frequency of theCrab pulsar is checked. Due to the large background for the Crab pulsar, we combined the observa-tion data in every day to search the spin frequency of the pulsar. As shown in Figure 3 (a), the spinfrequency of the pulsar decreases with time significantly with time, which is consistent with theephemeris supplied by Jodrell Bank [16] . Then, TOAs were calculated with the spin parametersand fitted utilizing TEMPO2. With the best parameters, the timing residuals distribute near zeroswith the root mean squared value 85 µ s. However, the timing residuals show slow variations withtime as the Crab pulsar has the large timing noise as shown in Figure 3 (b). In order to verify theseresults, we also checked timing residuals observed from Fermi-LAT the at the same time intervalwith the same spin parameters. As illustrated in Figure 3 (b), the timing residuals observed byPOLAR are remarkably consistent with Fermi-LAT results, which suggests that timing system ofPOLAR is reliable. With the accurate timing parameters, the total pulse profile of the Crab pulsar was folded fromthe all observed events. As shown in Figure 4 (a), the pulse profile shows the typical double-peak https://fermi.gsfc.nasa.gov/ssc/data/analysis/LAT_essentials.html OLAR Crab
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Table 1:
The timing parameters of the Crab pulsar
Parameters ValuePEPOCH(MJD) 57697.040344079745F0(Hz) 29.6484272934(4)F1(10 − Hz s − ) -3.689865(1)F2(10 − Hz s − ) 1.16(1)F3(10 − Hz s − ) 3.4(3) Figure 3:
Panel (a): the evolution of the spin frequency of the Crab pulsar observed by POLAR. Each datapoint is subtracted by ( . − t ∗ . × − ) to show the details of its frequency evolution. Thegreen line represents the fitted result. Panel (b): The time residuals of the Crab pulsar observed by Fermiand POLAR, as represented by the blue and red squares respectively. structure with high significance, which is also consistent with the results of RXTE [17]. As thelarge of view of POLAR, Crab could be observed in every orbit and pulse profile could be obtainedthough with lower significance. Therefore, all pulse profiles observed in every day are co-alignedwith the same phase as illustrated in Figure 4 (b). These results also confirm that POLAR hasdetected the pulsed photons from the compact objects.The pulse profile as a function of theta incident angle as shown in Figure 5 (a), which indicatethat POLAR’s FoV in its energy range is larger than 2* π . And the pulse profile as a function of1600 channels as shown in Figure 5 (b), which show that the pulsed photons of Crab are captured5 OLAR Crab
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Figure 4:
The pulse profile detected by POLAR as function of time. Panel (a) shows the total pulse profileaccumulated from all observations. Panel (b) exhibits the normalized profiles with background subtractionobserved in every day. by every channel.
All information of an event with its scattered photon (simultaneous trigger) is recorded bydetector. Generally, the first reaction point has maximum deposited energy, and the second reac-tion point takes second place. Projecting the direction from the first reaction point to the secondreaction point onto X-Y plane, the angle between projection and X-axis is the azimuthal angle ofthis compton scattering event. For GRB, since it’s short duration, accumulating the events in dif-ferent azimuthal angle, we obtain modulation curve to reveal polarization information. However,for Crab pulsar the incident angle between the Crab and POLAR varies with time, so we have toobtain modulation curve as a function of incident angle. Then, we use POLAR Monte Carlo simu-lation software package [18] to reappear Crab observation on POLAR. This software package wasdeveloped from a calibration experiment on POLAR at European Synchrotron Radiation Facility.Simulating different polarization Crab source in parallel, and comparing these simulation resultswith observation results. If simulation results of one presuppose polarization are consistent withobservation results, then we consider this presuppose polarization as measured polarization of Crabpulsar.However, the precondition is that we should try to use Monte Carlo simulation to reappear6
OLAR Crab
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Figure 5: (a) is Pulse profile as function of θ ; (b) is pulse profile for 1600 channels. the observation results. In other words, we need a probably calibration process. The responseof Crab detection varies with many factors, including but not limited to detection efficiency, pro-jected area, visible efficiency, counts Rate and so on. Look for as many factors as possible fromobservation-simulation conjoint analysis, and make them decouple with polarization. Then polar-ization measurement of Crab pulsar is the potential target.
4. Summary
Firstly, from the observations, POLAR detected highly significant pulsed signals from theCrab pulsar which is similar with the known results. The pulse profile as a function of theta inci-dent angle indicate that POLAR’s FoV larger than 2 π , and as a function of 1600 channels showsthat photons of the Crab pulsar were captured by every channel. Second, the accurate timingbehavior observed by POLAR is highly consistent with Fermi-LAT observation, it verified thatPOLAR’s clock precision was more stable than 85 µ s. The above results show that POLAR has agood detection performance. And lastly, we have potential to measure the polarization of Crab onPOLAR. Acknowledgments
We thank the High Energy Astrophysics Science Archive Research Center(HEASARC) atNASA/Goddard Space Flight Center for maintaining its online archive service that provided thedata used in this research. This work is supported by the National Key Research and Develop-ment Program of China (2016YFA0400802), National Science Foundation of China (11233001,11503027, 11303069 and 11503028), and the Strategic Priority Research Program on Space Sci-ence, the Chinese Academy of Sciences,Grant No. XDA04010300 and XDB23000000.7
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References [1] Eikenberry, S. S., and Fazio, G. G. 1997,
ApJ , 476, 281[2] Kuiper, L., Hermsen, W., Cusumano, G., et al. 2001,
A&A , 378, 918[3] Rots, A. H., Jahoda, K., Macomb, D. J., Kawai, N., Saito, et al. 1998,
ApJ , 501, 749[4] Molkov, S., Jourdain, E., and Roques, J. P. 2010,
ApJ , 708, 403[5] Ge, M. Y., Lu, F. J., Qu, J. L., Zheng, S. J., Chen, Y. and Han, D. W. 2012,
ApJS , 199, 32[6] Novick R., et al., 1972,
ApJ , 174, L1[7] Weisskopf M. C., et al., 1976,
ApJ , 208, 125[8] M. Forot, et al.,
Astrophys. J.
688 (2008) L29[9] A.J. Dean, et al.,
Science
321 (2008) 1183.[10] M. Chauvin, et al. 2017, arXiv:1706.09203 [astro-ph.HE].[11] M. Kole et al.,
ICRC Conf. Proc.
Nucl. Instr. and Meth. A
550 (2005) 616.[14] Edwards, R. T., Hobbs, G. B., & Manchester, R. N. 2006,
MNRAS , 372, 1549[15] Hobbs, G. B., Edwards, R. T., & Manchester, R. N. 2006,
MNRAS , 369, 655[16] Lyne, A. G., Pritchard, R. S. and Graham-Smith, F. 1993.
MNRAS , 265, 1003[17] Ge, M. Y., Yan, L. L., Lu, F. J., et al. 2016,
ApJ , 818,48[18] M. Kole, Li,Z. H., et al., submitted to