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LAT Data Analysis: the case of pulsars

LAT Data Analysis: the case of pulsars. Massimiliano Razzano (INFN - Pisa) Astrofisica gamma dallo spazio in Italia AGILE e GLAST (ESRIN, Frascati, 3 luglio 2007). The analysis tools we will use. LAT generic tool LAT pulsar-specific tool other analysis tool. Spatial Analysis

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LAT Data Analysis: the case of pulsars

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  1. LAT Data Analysis:the case of pulsars Massimiliano Razzano (INFN - Pisa) Astrofisica gamma dallo spazio in Italia AGILE e GLAST (ESRIN, Frascati, 3 luglio 2007)

  2. The analysis tools we will use LAT generic toolLAT pulsar-specific toolother analysis tool • Spatial Analysis • Select ROI gtselect; • Make countmaps gtbin; • Make exposure maps for likelihoodgtexpcube,gtexpmap,gtdiffresp; • Estimate flux using max. likelihoodgtlikelihood; • Temporal Analysis • Select photons to optimize periodicity testgtselect; • Barycentric correctionsgtbary; • Retrieve pulsar ephemerides from databasegtephcomp; • Periodicity testgtpsearch; • Phase assignmentgtpphase; • Spectral Analysis • Spectral analysis using max. likelihoodgtlikelihood; • Phase selectionfselect; • Generate PHA files for XSpecgtbin • Generate RMF files for XSpecgtrspgen • Spectral AnalysisXSpec

  3. Spatial Analysis with likelihood Unbinned likelihood can be used to determine source flux and point-source detection significance. Maximum Likelihood allow also spectral analysis (see Gino’s talk) Spatial Analysis: example of PSR B1706-44 ( 1 month sim.) Maximum likelihood gives: F(E>100 MeV)=(1.22±0.07) x 10-6 ph cm-2s-1 With TS1/2=31.2

  4. Effect of selection cuts Cutoff at 1GeV Cut B Cut A Cut B Cut A Cutoff at 30GeV • Cuts give different results depending on pulsar spectrum and on gamma-ray background (most of simulated pulsars are on the Galactic plane); • Example of 2 pulsars on the Galactic plane with ~the same flux (10-7 ph cm-2s-1) Sample cuts Cut A: E>100 MeV, r<3° CutB: E>1GeV, r <1°

  5. The barycentric corrections The photon arrival times are affected by the motion of GLAST through Solar System and by relativistic effects. These effects are compensated by the barycentric corrections Corrections are: • Conversion TTTDB; • Geometric corrections due to lighttravel time from GLAST location to Solar System Barycenter; • Relativistic delay due to gravitaional field of Sun (e.g. Shapiro delay); The barycentric corrections convert the photons arrival times, (expressed in Terrestrial Time TT at the spacecraft), to the arrival times at the Solar System Barycenter (expressed in Barycentric Dynamical Time TDB) Pay attention:gtbary usually overwrites the input file!

  6. Sample DC2 pulsar database (D4) from GSSC Data Center. The LAT Pulsar Database (D4) D4 will contain timing solution of candidate g-ray pulsars with counterparts (mainly from radio)

  7. Periodicity tests Once we know that D4 contains valid ephemerides, we can test if our gamma source has the same periodicity of the radio counterpart. In this case we have identified the source as a gamma-ray pulsar Tests against the null hypotesis: H0 = no periodicity Tests implemented: • Chi-squared test (Leahy et al. 1983,ApJ 266; • Z2n test (Buccheri et al. 1983 A&A128),Rayleigh test; • H test (De Jager et al., 1989 A&A 221) When we will find the periodicity, we can assign phases to each photon and build the lightcurve

  8. For each photon a phase is assigned for a trial frequency, then an histogram of phase bin mj is created; • In absence of pulsation every phase bin will have the same mean count mexpected; • The quantity: • is computed; • For large counts is every bin, S is distributed as a c2n-1 • Then it is possible to test the non-periodicity hypothesis and give a chance probability p(c2>S) Example ofc2periodicity test Frequency space is scanned in fractions of Fourier resolution fF=1/T, where T is the duration of the observation. It represents the spacing between 2 independent frequencies in FT. For example 1-week observation: fF 1/(86400*7)≈1.6E-06 Hz Periodicity test on the profile Here we use as example the c2 test

  9. The H test is more efficient for unknown-a-priori lightcurves (see for details: De Jager et al., 1989 A&A 221) The other 2 tests give similar results. The number of bins is Z2n is equivalent to the number of harmonics we want to consider. Z2n has p.d.f of χ22n (See for details: Buccheri et al. 1983, A&A128) Z2n test Htest Z2n test and H test Other 2 tests are implemented in gtpsearch: Z2n test and H-test. For more details, see the references.

  10. dt=t-t0, t0 is the epoch 1 1 = + - + - + - + 2 3 φ( t ) φ( t ) f ( t t ) f ( t t ) f ( t t ) ... 0 0 0 1 0 2 0 2 6 Folding at the radio ephemerides Once the periodicity is known, to each photon can be assigned a rotational phase as: A PULSE_PHASE column is added. If pulsar is in a binary orbit other timing corrections must be applied. The binary modulation can be corrected andan orbital phase can be assigned Here we choose to use the D4 ephemerides, but they can be also entered manually

  11. Counts predictions and integrated flux using maximum likelihood within 20° Spectral analysis: using Likelihood

  12. Spectrum of PSR B1706-44 obtained using Xspec for an 1-month observation in scanning mode. Spectral analysis using XSpec LAT Analysis tools can create also Spectrum Files (PHA) and Response Matrix Files (RMF) that can be used with XSpec Creation PHA file Creation RMF file

  13. Concluding remarks • These slides shows basic analysis steps using the Science Tools. More refined analysis can be done; • Data used for these analysis come from simulations of pulsars and other sources (e.g. DC2,SC2); • Some LAT generic tools are useful also for pulsar analysis (e.g. spectral); • Pulsar tools provide a complete set for specific analysis of pulsars (e.g. periodicity tests); • Process of developing tools is under continue development, including possible new methods. Simple PowSpectrum-based search tool has been included; • Integration with standard analysis tools is possible; • Possibility of automatic analysis using scripting languages; • Using fselect it is possible to cut in phase and perform phase-resolved analysis Where to look at: Link to Pulsar Tools dev page: http://glast.gsfc.nasa.gov/ssc/dev/psr_tools/ Tutorial on User Workbook: http://glast-ground.slac.stanford.edu/workbook/sciencetools/sciTools_Home.htm Pulsar Science Working Group: http://confluence.slac.stanford.edu/display/SCIGRPS/Pulsars%2C+SNRs%2C+and+Plerions Xspec web page: http://heasarc.gsfc.nasa.gov/docs/xanadu/xspec/

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