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

Vela FoV Vela PSR RXJ (Vela Jr)?

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

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

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

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

Spectral analysis: directly (3) Vela PSR Galactic backgrounds Extragalactic background Residual components Freeing scale parameters? No 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. Cut in energy! (and more attention to higher energies gammas)

Another exemplum Extragalactic background Residual components 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...

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

Backgrounds (2) Extragalactic background (fixed): constant diffuse emission Pref = 1.6 ( x ) 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 ) Residual component: modeled with MapCube file residual.fits The scale factor is more than 3 times grater than we was expecting

PSR Now we try to isolate PSR ROI centered in the source: RA = o dec = o ROI radius = 2 o (remember that now the backgrounds are totally fixed)

PSR (2) So we ask python likelihood to fit PSR spectral behaviour with a Broken Power Law: Pref. = ( x ) Index 1 = Index 2= E B = Extragalactic background Residual components Galactic backgrounds But we don’t like this fit... in particular if we look the behaviour at higher energies... PSR

PSR (3) So we fix manually the Energy Break at 20 GeV and after we let that E B run up to 25 GeV... With E B = 20 GeV we obtain: Pref. = ( x ) Index 1 = Index 2= Residual components Extragalactic background Galactic backgrounds PSR with E B = 20 GeV

E B : 20 GeV  25 GeV PSR with E B = 25 GeV Galactic backgrounds Residual components Extragalactic background With E B = 25 GeV we obtain: Pref. = ( x ) Index 1 = Index 2= For E B : 20 GeV  25 GeV, results are very similar among them. (log L increase for E B  5 GeV... but for E B < 20 GeV the gap between model and experimental data becomes relevant... So we’ll use E B = 20 GeV ) PSR (4)

PSR Galactic backgrounds Residual components Extragalactic background We tried (with Luis Reyes) to use a single Power Law with an exponential cut-off: PSR (5) and the this curve fits much better, with the following parameters: Pref. = ( x ) Index = E B = P1= It looks much better…

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

Vela Galactic backgrounds Extragalactic background Residual components Vela Vela doesn’t create any problem. Its spectrum is fitted very well with a Broken Power Law: Pref. = ( x ) Index 1 = Index 2= E B = ( ) MeV

RXJ 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 1 o down to 0.8 o. Now we should know everything to face the study of RXJ 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...

Using the model Vela Jr (using a Single Power Low Spectrum hypothesis) with radius = 1 o, we obtain: Prefactor = ( x ) Spectral Index = RXJ (2) Vela PSR Residual components Galactic backgrounds Extragalactic background 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...)... Something seems to be wrong...

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

RXJ avoiding PSR In order to simplify the problem we could try to exclude PSR 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 = 130 o dec = o ROI radius = 4.3 o

Galactic backgrounds Vela Residual components Extragalactic background PSR Vela Jr RXJ avoiding PSR (2) Using the model Vela Jr with radius = 1 o, we obtain: Prefactor = ( x ) Spectral Index = And I like this result... (Note that, even if PSR is not in the ROI, we should insert it in the model)

Galactic backgrounds Vela Vela Jr Extragalactic background Residual components PSR And finally let’s do the check moving the radius of the model of Vela Jr down to 0.8 o...both Prefactor and Spectral Index decrease slowly. With radius = 0.8 o we obtain: Prefactor = ( x ) Spectral Index = RXJ avoiding PSR (3) This seems to be the correct way to follow, but we should know much better the geometrical shape (or structure) of RXJ  OK

Galactic backgrounds Vela Vela Jr Extragalactic background Residual components PSR Let’s look if using the Power Law with the exponential cut-off for PSR , we’ll get some improvements; using for Vela Jr, radius = 1 o, we have: Prefactor = ( x ) Spectral Index = RXJ avoiding PSR (4) (The same results we obtained with the Broken Power Low; but we could expect it, excluding that Pulsar from the ROI)

Galactic backgrounds Vela Vela Jr Extragalactic background Residual components PSR For Vela Jr with radius = 0.8 o we obtain: Prefactor = ( x ) Spectral Index = again the same results we obtained before RXJ avoiding PSR (5) This seems to be the correct way to follow, but we should know much better the geometrical shape (or structure) of RXJ  OK

Next steps In the last slide we end saying that the next step should be, having a better spatial resolution of RXJ : 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)

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.