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Spectral analysis on faint extended sources: problems and strategies.

Gamma-ray Large Area Space Telescope. Spectral analysis on faint extended sources: problems and strategies. Omar Tibolla Padova University. DC2 Closeout Workshop, Goddard Space Flight Center, 31 May – 2 June 2006. Vela FoV. Vela. PSR0904-5008. RXJ0852.0-4622 (Vela Jr)?.

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Spectral analysis on faint extended sources: problems and strategies.

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  1. Gamma-ray Large Area Space Telescope Spectral analysis on faint extended sources: problems and strategies. Omar Tibolla Padova University DC2 Closeout Workshop, Goddard Space Flight Center, 31 May – 2 June 2006

  2. Vela FoV Vela PSR0904-5008 RXJ0852.0-4622 (Vela Jr)?

  3. Vela Jr? Is there really a source or is it only caused by Diffuse Galactic emission? Is it extended? Is it Vela Jr? Yes, it’s a source It seems so...

  4. Vela Jr? (2) Yes, it is extended. ...and it seems also to have a structure...

  5. Spectral analysis: directly Let’s consider a ROI centered in Vela Jr center ROI radius = 10o. So at the same time we study all the 3 sources and the backgrounds directly.

  6. Spectral analysis: directly (2) Galactic backgrounds Vela PSR0904-5008 Using the LAT source catalog? No (Pulsars should have a Broken Power Law Spectrum) Extragalactic background Residual components

  7. Spectral analysis: directly (3) Galactic backgrounds Freeing scale parameters? No Vela PSR0904-5008 The solution is trying to isolate sources and to study them separately... But the 3 sources are very close among them, so we must use small ROI, much smaller than LAT PSF. Extragalactic background Cut in energy! (and more attention to higher energies gammas) Residual components

  8. Another exemplum Galactic backgrounds This exemplum is more impressive; letting free too much the parameters (in this case scale parameters), we can get also non physical solutions! So, after isolating sources and studying them separately...we should freeze their parameters as soon as possible... Extragalactic background Residual components

  9. Backgrounds To know the backgrounds we consider a ROI near the sources we are studying but far enough to not be influenced by them = 2o ROI of radius = 2o Centered in: RA = 138o dec= -43.5o

  10. Backgrounds (2) Extragalactic background (fixed): constant diffuse emission Pref = 1.6 ( x 10-7) Sp. Index= -2.1 Galactic backgrounds: modeled with MapCube file GP_gamma.fits The scale factor is almost 1 never change very much (up to 1.037..) Residual component: modeled with MapCube file residual.fits The scale factor is more than 3 times grater than we was expecting... 3.309

  11. PSR0904-5008 Now we try to isolate PSR0904-5008. ROI centered in the source: RA = 136.058o dec= -50.1258o ROI radius = 2o (remember that now the backgrounds are totally fixed)

  12. PSR0904-5008 (2) PSR0904-5008 So we ask python likelihood to fit PSR0904-5008 spectral behaviour with a Broken Power Law: Pref. = 0.07026 ( x 10-9) Index 1 = -0.6664 Index 2= -1.6589 EB = 4991.4 Galactic backgrounds But we don’t like this fit... in particular if we look the behaviour at higher energies... Residual components Extragalactic background

  13. PSR0904-5008 (3) PSR0904-5008 with EB = 20 GeV So we fix manually the Energy Break at 20 GeV and after we let that EB run up to 25 GeV... With EB = 20 GeV we obtain: Pref. = 0.01374 + 0.00031 ( x 10-9) Index 1 = -0.924 + 0.012 Index 2= -2.656 + 0.049 Galactic backgrounds Residual components Extragalactic background

  14. PSR0904-5008 (4) PSR0904-5008 with EB = 25 GeV EB : 20 GeV  25 GeV With EB = 25 GeV we obtain: Pref. = 0.00988 + 0.00023 ( x 10-9) Index 1 = -0.9776 + 0.011 Index 2= -2.989 + 0.064 Galactic backgrounds For EB : 20 GeV  25 GeV, results are very similar among them. Residual components (log L increase for EB 5 GeV... but for EB < 20 GeV the gap between model and experimental data becomes relevant... So we’ll use EB = 20 GeV ) Extragalactic background

  15. PSR0904-5008 (5) PSR0904-5008 We tried (with Luis Reyes) to use a single Power Law with an exponential cut-off: and the this curve fits much better, with the following parameters: Galactic backgrounds Pref. = 1.46 + 0.31 ( x 10-9) Index = -0.832 + 0.073 EB = 7055 + 3884 P1= 26630 + 1744 Residual components Extragalactic background It looks much better…

  16. Vela Now it’s Vela turn. ROI centered in the source: RA = 128.842o dec= -45.1687o ROI radius = 2o

  17. Vela Vela doesn’t create any problem. Its spectrum is fitted very well with a Broken Power Law: Pref. = 0.0813 + 0.0049 ( x 10-9) Index 1 = -1.750 + 0.008 Index 2= -3.441 + 0.078 EB = (4624 + 133) MeV Vela Galactic backgrounds Residual components Extragalactic background

  18. RXJ0852.0-4622 Now we should know everything to face the study of RXJ0852.0-4622. So we go back to the ROI shown in slide 5 and we put in the model all the fixed parameters we have obtained until now... And what about Vela Jr model? We create a homogeneous disk fits file (MapSource)...and after we let that the radius of this circle runs from 1o down to 0.8o.

  19. RXJ0852.0-4622 (2) PSR0904-5008 Using the model Vela Jr (using a Single Power Low Spectrum hypothesis) with radius = 1o, we obtain: Prefactor = 8.94 + 26.13 ( x 10-9) Spectral Index = -4.99 + 0.27 Galactic backgrounds Vela Note the 300% of uncertainity in the prefactor... In fact, we can’t see Vela Jr in the plot...and, all in all, even if the result would be correct, I don’t like it (in particular for this SNR...)... Residual components Extragalactic background Something seems to be wrong...

  20. RXJ0852.0-4622 (3) PSR0904-5008 Using the model Vela Jr with radius = 0.8o, we obtain: Prefactor > 10000 ( + 2 ) ( x 10-9) Spectral Index > -1 ( + 0.00002 ) Galactic backgrounds Vela (Also in this case we can’t see Vela Jr in the plot...) There is really something wrong somewhere. Maybe having 2 source bright like Vela and PSR0904-5008 (almost one order of magnitude brighter than Vela Jr) make impossible the study of Vela Jr, amplifying too much its uncertainity.. Residual components Extragalactic background We could try to simplify the problem...

  21. RXJ0852.0-4622 avoiding PSR0904-5008 In order to simplify the problem we could try to exclude PSR0904-5008 from the ROI. 2 reasons: -it’s impossible to exclude Vela -at higher energies it’s the only relevant source (so there we can hope to see traces of Vela Jr) ROI centered in : RA = 130o dec= -45.5o ROI radius = 4.3o

  22. RXJ0852.0-4622 avoiding PSR0904-5008 (2) Galactic backgrounds Using the model Vela Jr with radius = 1o, we obtain: Prefactor = 9813 + 986 ( x 10-9) Spectral Index = -2.165 + 0.042 Vela Vela Jr And I like this result... Residual components (Note that, even if PSR0904-5008 is not in the ROI, we should insert it in the model) Extragalactic background PSR0904-5008

  23. RXJ0852.0-4622 avoiding PSR0904-5008 (3) Galactic backgrounds And finally let’s do the check moving the radius of the model of Vela Jr down to 0.8o...both Prefactor and Spectral Index decrease slowly. With radius = 0.8o we obtain: Prefactor = 5224 + 801 ( x 10-9) Spectral Index = -2.018 + 0.053 Vela Vela Jr  OK Residual components This seems to be the correct way to follow, but we should know much better the geometrical shape (or structure) of RXJ0852.0-4622... Extragalactic background PSR0904-5008

  24. RXJ0852.0-4622 avoiding PSR0904-5008 (4) Galactic backgrounds Let’s look if using the Power Law with the exponential cut-off for PSR0904-5008, we’ll get some improvements; using for Vela Jr, radius = 1o, we have: Prefactor = 9769 + 982 ( x 10-9) Spectral Index = -2.163 + 0.042 Vela Vela Jr Residual components (The same results we obtained with the Broken Power Low; but we could expect it, excluding that Pulsar from the ROI) Extragalactic background PSR0904-5008

  25. RXJ0852.0-4622 avoiding PSR0904-5008 (5) Galactic backgrounds For Vela Jr with radius = 0.8o we obtain: Prefactor = 5180 + 797 ( x 10-9) Spectral Index = -2.014 + 0.053 again the same results we obtained before Vela Vela Jr  OK Residual components This seems to be the correct way to follow, but we should know much better the geometrical shape (or structure) of RXJ0852.0-4622... Extragalactic background PSR0904-5008

  26. Next steps In the last slide we end saying that the next step should be, having a better spatial resolution of RXJ0852.0-4622: 1- using better classes of gammas: using only gammas of class A could be very useful also for re-doing the spectral analysis we have just performed. 2- using higher cut in energy, in order to reduce the PSF 3- more detailed TS Maps 4- increase the observation time 5- try the new release of Science Tools (v7r2...here I used Science Tools v7r0p3) 6-in order to separate much better Vela and Vela Jr, it should be useful to have more cuts on CTB core (see Bill’s talk)

  27. Acknowledgements In alphabetic order: -Bill Atwood; USFC, USA. - Giovanni Busetto; Padova University, Italy. - Seth Digel; SLAC, Stanford, USA. - Francesco Longo; Trieste University , Italy. - Elisa Mosconi; Padova University, Italy. - Riccardo Rando; Padova University, Italy. - Luis Reyes; GSFC, USA. - Francesca Maria Toma; Padova University, Italy.

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