3ND INTERNATIONAL COSMIC RAY CONFERENCE, BEIJING 011 Observation of Crab Nebula by Nuclear Compton Telescope during 009 flight HUANG, MING-HUEY A. 1, FOR NCT COLLABORATION 1 National United University, Miaoli, Taiwan, 36003 mahuang@nuu.edu.tw Abstract: The Nuclear Compton Telescope (NCT) is a balloon-borne soft gamma ray (0.-10 MeV) telescope designed to study astrophysical sources of nuclear line emission and polarization. The heart of NCT is an array of crossstrip germanium detectors, designed to provide 3D positions for each photon interaction with full 3D position resolution to < 1.6 mm 3. Tracking individual interactions enables Compton imaging, effectively reduces background, and enables the measurement of polarization. A 10-GeD version was flown on May 17-1 009 from the Columbia Scientific Balloon Facility in Fort Sumner, NM, with total flight duration of 3.5 hours. The Crab Nebula was the primary target for the flight. Analysis of the data reveals an image of the Crab, which was detected at a significance of 4 in 9.3 ks of observation time. This is the first reported detection of an astrophysical source by a next generation Compton telescope. Keywords: Galactic source, Gamma ray, Compton scattering, Balloon experiment, Germanium detector, Crab nebula 1 Introduction Soft gamma rays (00 kev 0 MeV) are important probes of the most violent and extreme processes in the cosmos, such as neutron stars, X-ray binaries, and Active Galactic Nuclei (AGN). They also come from the decays of many radioactive nuclei that are released by supernova explosions, such as 6 Al, 60 Fe, and 44 Ti. Observation of these gamma rays could probe deep within these often obscured systems and make unique and complementary observations of their nature. At this energy range, Compton effect provides largest interaction cross-section. Gamma ray photon with energy Eγ 0 scatter with an electron and is deflected by an angle with a photon energy Eγ 1, 1 1 cos 1 m e c...(1) E 1 E 0 where m e c =511keV is the rest mass energy of the electron [1]. If both the direction of the scattered photon and cos can be measured, then the photon s direction can be constrained to a circle on the sky (the Compton circle ). The Compton effect is also sensitive to the polarization of the incident photon through the Klein-Nishina crosssection [], enabling a Compton telescope to potentially measure the polarization of a gamma-ray source. The first satellite-borne Compton telescope was COMP- TEL on board the Compton Gamma Ray Observatory (CGRO) [3]. CGRO retired in June 000 and some instruments had been upgrade to new satellite mission, such as Fermi gamma ray satellite [4] replaces the function of EGRET on CGRO. However, there is no replacement for COMPTEL now. Also, COMPTEL consisted of two layers of scintillator detectors spaced by1.5 meters; its angular resolution was poor and took out a large space. Many institutions are developing the compact Compton Telescopes (CCTs), which aim for a compact size detector with fine position and fine energy resolution to track individual gamma-ray interactions within the detector volume. The Nuclear Compton Telescope (NCT) is one of the active projects of CCT. In this paper we will briefly describe the NCT instruments and flight in 009 (Section), then present some physics results from 009 flight, which include image (Section3) and energy spectrum (Section4) of Crab nebula. The NCT instrument NCT use an array of 3D-positioning germanium detectors (GeDs) to detect gamma rays in the energy range of 0.-10 MeV [5, 6, 7]. Each GeD measures cm cm 1.5 cm and has 37 strips on top and bottom sides forming a orthogonal grid, with a strip pitch of mm []. Those Vol. 7, 145
two layers of electrodes are bias in opposite potential and GeD is operated as fully depleted p-i-n junctions. When a Compton scattering happens, the scattered electron and remaining hole drift to two electrodes, which are than collected and their two coordinate are determined. The Collections Time Difference (CTD) between two layers can be used to determine depth of interaction. The individual scatter positions in 3 dimension and energy deposits can be measured. This design provides 3D position resolution <1.6 mm 3. Then 6 GeDs form an array to increase aperture and chance of multiple Compton scattering. The purpose of this design is that a gamma ray undergoes at least one Compton scatter and is fully absorbed inside the detectors. Compton Kinematic Discrimination algorithms [9] are used to find the most likely scattering order, and then the total gamma-ray energy can be reconstructed and the initial direction can be traced to a ring on the sky. The combined GeD array enable high spectral resolution, moderate angular resolution, and novel sensitivity to gamma-ray polarization. The entire set of detectors and their cryostat are enclosed inside a well of anticoincidence BGO shields to reduce the Earth albedo and atmospheric backgrounds. The resulting overall field of view is primarily limited by the BGO shields to 3. sr. The ten NCT GeDs are housed in a single cryostat that successfully flew on the prototype flight. The cryostat is attached to a single 50-liter liquid nitrogen dewar which cools the GeDs to ~5 K for ~7 days. The dewar is vented through a 5 psi valve keeping the liquid nitrogen under pressure at float. The entire instrument and readout electronics are mounted in a pointed, autonomous balloon gondola [7]. Figure 1 shows GeD housing, the cryostat, liquid Nitrogen tank, BGO shield inside the gondola. HUANG ET AL. NCT CRAB IMAGE To date, NCT has flown on two conventional balloon flights. The first was a prototype flight in 005 that succeeded in measuring the soft gamma-ray atmospheric background and galactic anticenter region [14]. The NCT electronics had a major upgrade from the version used in 005 flight [15]. The full 10-GeD version of NCT was flown on its second flight in May009 [16]. The first results was submitted to journal [17] and summarized in this paper. The third flight attempt was in April 010 at Alice Spring, Australia. However, a launch failure causes the gondola crash to ground [1]. We are rebuilding the gondola and part of NCT instruments now and aim at the next flight in 01 ~ 013. 3 Observation of Crab Nebula 3.1 Data selection and results NCT was launched from Fort Sumner, New Mexico at 1330UT 5/17/009 and was terminated at 0400UT 5/19/009 near Kingman, Arizona. Altitude was maintained between 35 and 40 km for the entire time the detectors were operational. The total flight time was 3.5 hours, with nine of the ten detectors on for a total of hours at float. Data from the flight were selected only when the Crab was above 40 elevation, approximately 33 ks of observation time. Taking account of livetime of 9% the effective Crab observation time was 9.3 ks [16]. We select energy range of 300 495 kev and 50 1500 kev, which excluded very strong atmospheric 511 kev background line. Below 300 kev, NCT observed fewer counts of background continuum than that predicted by simulation [19]. Possible reason is the simulations underestimate the shielding effect of the detector guard rings. This low-energy region (<300 kev) was excluded from the analysis. The strictest data cut is the distance between the first and second interactions in the detectors. Based on simulations of the Crab, setting a lower limit of 1. cm for the first interaction distance improves the angular width of the Crab(10 ARM FWHM to 7.4 ARM FWHM), while eliminating only 40% of the events within the central FWHM of the ARM distribution. The cut is also meant to improve the likelihood that an event is properly reconstructed to attain the best signal-to-noise ratio. Figure 1: Top view of NCT instruments. The software analysis tools for NCT are built using the Medium Energy Gamma-ray Astronomy library (MEGAlib) [10]. MEGAlib provides utilities for simulations [11], geometry modeling, event reconstruction, and imaging reconstruction [1], in addition to tools for building a custom analysis pipeline [13]. We plotted a total of 9,1 event circles in a 0 0 back projection, of which approximately 3,00 event circles (1.3%) are expected from the Crab based on simulations. Then use the List-Mode Maximum Likelihood Expectation Maximization (MLEM) [0, 1] to refine the raw back projection. The result is that point sources become clearer and other features are sharpened. We performed a total of five iterations of the MLEM algorithm on the image, which is the approximate number of iterations needed so that the FWHM of a point source image Vol. 7, 146
3ND INTERNATIONAL COSMIC RAY CONFERENCE, BEIJING 011 matches the 7.4 ARM FWHM. The result is that the We select points in a 0 circle around Crab for offsource points. Then follow the same procedures as on- Crab appears in the image Fig. [17]. An independent study [1] tries to veto the first interaction in the four outer most GeDs, which are most noisy sections. The Compton events circle are counted in two source (Crab) events described in the last two sub- ones. It also produce image of Crab, although this method has slightly worse ARM than the previous method. C k : An event intersects k off-source points groups: and not the Crab point, C k : An event intersects k off-source points and the Crab point, where k=0 for Crab and k=1 for off-source points. Then the number of excess Crab counts can be written in the following way 1 Nexcess NC Noffsource NC Ni Figure : Crab image obtained by five iterations of the MLEM algorithm on the image [17]. 3. Simulation A detailed mass model for NCT has been developed, along with code that applies the detector response to the ideal simulated detector hits [13, ]. The MEGAlib simulation tool Cosima was used, a point source with a spectrum identical to the Crab Nebula spectrum measured by INTEGRAL/SPI [3] was simulated at infinity along the center of the field of view for an equivalent real time of 100,000 seconds. The total Crab flux at top of atmosphere (TOA) between 00 kev and 10 MeV was calculated to be.194 10 - ph cm - s -1. The total number of events between 00 kev and 10 MeV that interacted in the detectors were 19,97, with 1,6 events passing the event cuts (.5%). Atmospheric attenuation was simulated with a mean vertical depth of 3. g/cm, than effect of slanted depth at 45, 55, 65, and 75 was calculated. The expected flux at float altitude is 71% of that at TOA. Simulated events are reconstructed with the same software pipe-line. The input direction and the reconstructed direction are used to find Angular Resolution Measure (ARM), which is defined by ARM geo recon. Doppler broadening and detector positional resolution plays dominate contribution in NCT s ARM. The FWHM of ARM with this data cuts is 7.4, so we use 0.77.4 = 5. as cuts on ARM. 3.3 Significance of Crab detection 1 NCk knck kn C k k0 k1 k1 k 1 NC0 1 NCk kn...() C k k1 k1 These number counts for on/off sources are listed in Table 1. The statistical uncertainty for the excess counts becomes excess excess N C0 163.4 k1 k 1 N Ck i1 k1 k N C k...(3)...(4) The result of this calculation is that the Crab was detected at the 4.1σ level, which is within statistical error of the expected 3.9σ from simulations. k NC N k C k 0 N/A 57 1 9,09 1,01 66,44 14,373 3 11,75 51 4 03 5 5 0 6 7 0 7 0 0 Total 103,357 9,0 Flight Simulation Excess on-source 667 644 Background noise () 163.4 Significance 4.1 3.9 Table 1. Number of counts for on/off sources. 4 Crab energy spectrum To isolate flux from Crab, fluxes and error bars are calculated from on/off sources as described in the last section. Figure 3 show the simulated spectrum for events within 5. of the Crab. Then the similar processes apply to detected data. The difference between data and simulation for on source are plotted in histogram, shown in Fig. 4, and then Chi-square statistics are calculated. It is consistent with the simulated spectrum [0]. For off-source spectrum, similar histograms are calculated and find it is Vol. 7, 147
inconsistent with input spectrum. This study concludes that the spectrum measured by NCT is consistent with measurements from Fermi/SPI [0]. Fig. 3 Simulated spectrum for events within 5. of the Crab using data cuts given in the text. The averaged spectrum measured at points 0 away is also plotted. The difference of these two distributions is the signature we expect in the flight data after estimating the spectrum at nearby points. Fig. 4 Measured spectrum for the Crab plotted with the expected excess from simulations. The spectrum is measured by subtracting the average spectrum from off-source points. Error bars are calculated using Poisson statistics. The measured spectrum is inconsistent with zero (.7σ) but is consistent with the simulated Crab spectrum (P Crab =0.7). The bins are 00 kev wide. 5 Conclusion The Crab Nebula was detected by NCT at a significance of at least 4σ during 9.3 ks of observation time in the energy range 300 1500 kev. This result is the first significant detection of a celestial source by a CCT and is an important step in establishing the viability of the compact Compton telescope design for future space-based widesurvey instruments. Acknowledgement The NCT project is funded by NASA under Grant #NNG04WC3G for the NCT-US team and by the National Space Organization (NSPO) in Taiwan under Grants96-NSPO(B)-SP-FA04 01 and 9-NSPO-145 for the NCT-Taiwan team. The author M.A. Huang is also HUANG ET AL. NCT CRAB IMAGE supported by National Science Council of R.O.C. Taiwan under grant NSC99-11-M-00-006-MY3. References [1] Compton, A., Phys. Rev., 193, 1: 43 [] Klein, O., & Nishina, T., Zeitschrift für Physik, 199, 5: 53 [3] Schönfelder, V., et al., A&AS, 1993, 6: 657 [4] Fermi/LAT Collaboration: W. B. Atwood, et al. Astrophys.J., 009, 697:1071-110 [5] M.A. Huang, et al., Proc. 31 st ICRC, 009, paper ID 095 and reference therein. [6] Chang, H.-K., for NCT collaboration, Adv. Space Res., 007, 40: 11 and reference therein. [7] Bellm, E.C. for NCT collaboration, IEEE Trans. Nucl. Sci., 009, 56: 150-156 [] Amman, M., Luke, P. N., & Boggs, S. E., Nucl. Inst.Meth. - A, 007, 579: 6 [9] Boggs, S.E., Jean, P., Astron. Astrophys. Suppl. Ser. 145, 000, 311: 31 [10] Zoglauer A., et al.: 00, Space Telescopes and Instrumentation 00: Ultraviolet to Gamma Ray, ed. M. Turner & K. Flanagan, 7011: 101; Zoglauer A., et al., New Astron. Rev., 006, 50: 69 [11] Zoglauer A., et al., IEEE Nucl.Sci.Symp.Conf.Rec., 009, 053 [1] Zoglauer A., et al., Nucl. Inst. Meth.- A, 010, in pres, DOI: 10.1016/j.nima.010.0.043. [13] Bandstra, M. S., et al., IEEE Nucl.Sci.Symp.Conf. Rec., 009: 131 [14] Bowen, J., et al., IEEE Nucl.Sci.Symp.Conf.Rec., 007, 1: 436; Bowen, J.D., PhD thesis, U.C. Berkeley, 009 [15] Hung, W.-C., et al., IEEE Trans. Nucl. Sci., 009, 56: 303 [16] Bandstra, M. S., et al., IEEE Nucl.Sci.Symp.Conf. Rec., 009, 131 [17] Bandstra, M. S., et al., submitted to Astrophys. J. Nov. 010. [1] NASA incident report: http://www.nasa.gov/centers/goddard/pdf/491345main_ Master Report Vol I-Final.pdf [19] Chiu, J.-L. 010, PhD thesis, Nat l Tsing-Hua Univ. [0] Wilderman, S., Clinthorne, N., Fessler, J., Rogers, W.199, IEEE Nucl.Sci.Symp.Conf.Rec., 199, 3: 1716 [1] Liang J.S., 010, PhD thesis, Nat l Tsing-Hua Univ. [] Chiu, J.-L., et al., IEEE Nucl.Sci.Symp.Conf.Rec., 009, 47 [3] Jourdain, E., & Roques, J. P., ApJ, 009, 704: 17 NCT Collaboration: AMMAN, M. 1, BANDSTRA, M.S., BALLMOOS, P.Von 3, BELLM, E.C., BOGGS, S.E., CHANG, H.-K. 4, CHANG, Y.-H. 5, CHIANG, S.J. 6, CHIU, J.-L. 4, HUNG, W.-C. 5, JEAN, P. 3, LIANG, J.-S. 4, LIN, C.- H. 7, LIU, Z.-K. 5, LUKE, P.N. 1, PEREZ-BECKER, D., RUN, R.-S. 6, WUNDERER, C.B., ZOGLAUER, A. 1. Lawrence Berkeley National Laboratory, USA. Space Sciences Laboratory, University of California, Berkeley, USA 3. Centre d Etude Spatiale des Rayonnements, France 4. National Tsing Hua University, Taiwan 5. National Central University, Taiwan 6. National United University, Taiwan Vol. 7, 14
3ND INTERNATIONAL COSMIC RAY CONFERENCE, BEIJING 011 7. Institute of Physics, Academia Sinica, Taiwan. Deutsches Elektronen-Synchrotron (DESY), Germany Vol. 7, 149