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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)?. - PowerPoint PPT Presentation
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Spectral analysis on faint extended
sources: problems and strategies.
Gamma-ray Large AreaGamma-ray Large AreaSpace TelescopeSpace Telescope
Omar TibollaPadova University
DC2 Closeout Workshop,Goddard Space Flight Center,
31 May – 2 June 2006
Vela FoV
Vela
PSR0904-5008 RXJ0852.0-4622 (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 = 10o.
So at the same time we study all the 3 sources and the backgrounds directly.
Spectral analysis: directly (2)
Vela
PSR0904-5008
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
PSR0904-5008
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 backgroundResidual 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 = 2o
Centered in: RA = 138o
dec = -43.5o
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 10-7)Sp. Index= -2.1Galactic backgrounds: modeled with MapCube file GP_gamma.fitsThe scale factor is almost 1 never change very much(up to 1.037..)
Residual component:modeled with MapCube file residual.fitsThe scale factor is more than 3 times grater than we was expecting... 3.309
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)
PSR0904-5008 (2)
So we ask python likelihood to fit PSR0904-5008 spectral behaviour with a Broken Power Law:
Pref. = 0.07026 ( x 10-9)Index 1 = -0.6664Index 2= -1.6589
EB = 4991.4
Extragalactic background Residual components
Galactic backgrounds
But we don’t like this fit... in particular if we look the behaviour at higher energies...
PSR0904-5008
PSR0904-5008 (3)
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 + 0.012Index 2= -2.656 + 0.049 Residual components
Extragalactic background
Galactic backgrounds
PSR0904-5008 with EB = 20 GeV
EB : 20 GeV 25 GeV PSR0904-5008 with EB = 25 GeV
Galactic backgrounds
Residual components
Extragalactic background
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
For EB : 20 GeV 25 GeV, results
are very similar among them.
(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 )
PSR0904-5008 (4)
PSR0904-5008
Galactic backgrounds
Residual components
Extragalactic background
We tried (with Luis Reyes) to use a single Power Law with an exponential cut-off:
PSR0904-5008 (5)
1P
EE B
eand the this curve fits much better, with the following parameters:
Pref. = 1.46 + 0.31 ( x 10-9) Index = -0.832 + 0.073
EB = 7055 + 3884
P1= 26630 + 1744
It looks much better…
Vela
Now it’s Vela turn.
ROI centered in the source: RA = 128.842o
dec = -45.1687o
ROI radius = 2o
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. = 0.0813 + 0.0049 ( x 10-9)Index 1 = -1.750 + 0.008Index 2= -3.441 + 0.078
EB = (4624 + 133) MeV
RXJ0852.0-4622
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.
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...
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
RXJ0852.0-4622 (2)
Vela
PSR0904-5008
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 PSR0904-5008 (almost one order of magnitude brighter than Vela Jr) make impossible the study of Vela Jr, amplifying too much its uncertainity..
PSR0904-5008
Galactic backgrounds
Vela
Extragalactic background
Residual components
Using the model Vela Jr with
radius = 0.8o, we obtain:
Prefactor > 10000 ( + 2 ) ( x 10-9) Spectral Index > -1 ( + 0.00002 )(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...
RXJ0852.0-4622 (3)
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
Galactic backgrounds
Vela
Residual components
Extragalactic background
PSR0904-5008
Vela Jr
RXJ0852.0-4622 avoiding PSR0904-5008 (2)
Using the model Vela Jr with
radius = 1o, we obtain:
Prefactor = 9813 + 986 ( x 10-9)Spectral Index = -2.165 + 0.042
And I like this result...
(Note that, even if PSR0904-5008 is not in the ROI, we should insert it in the model)
Galactic backgrounds
Vela
Vela Jr
Extragalactic background
Residual components
PSR0904-5008
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
RXJ0852.0-4622 avoiding PSR0904-5008 (3)
This seems to be the correct way to follow, but we should know much better the geometrical shape (or structure) of RXJ0852.0-4622...
OK
Galactic backgrounds
Vela
Vela Jr
Extragalactic background
Residual components
PSR0904-5008
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
RXJ0852.0-4622 avoiding PSR0904-5008 (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
PSR0904-5008
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
RXJ0852.0-4622 avoiding PSR0904-5008 (5)
This seems to be the correct way to follow, but we should know much better the geometrical shape (or structure) of RXJ0852.0-4622...
OK
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)
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.