Simulating HESS SNRs Gamma-ray Large Area Space Telescope Omar Tibolla Padova University DC2 Closeout Workshop, Goddard Space Flight Center, 31 May – 2 June 2006

Summary -RXJ (astro-ph/ v2, 2005) -HESS galactic survey (ApJ, 636, , 2006): -HESSJ HESSJ HESSJ HESSJ HESSJ RXJ (A&A, 437, L7-L10, 2005)

Intro: RXJ H.E.S.S. and Cangaroo H.E.S.S. collaboration resolved spatially and morphologically 2 Shell-type Supernova Remnants: -RXJ RXJ These 2 objects are the only two spatially resolved VHE gamma-ray SNRs with a shell-like structure which agrees with that seen in X-rays and they may well be the brightest SNRs in the VHE gamma-ray domain in the whole sky. These 2 SNRs are in a very peculiar class of Shell-type SNRs with dominantely non-thermal X-ray and only very faint radio emission. RXJ was discovered by ASCA X-ray observations and after was studied in X- rays by XMM and Chandra. In VHE gamma rays it was detected by CANGAROO Collaboration in 1998 and re- observed by CANGAROO-II in 2000 and 2001; finally RXJ was recently studied and resolved by H.E.S.S. Collaboration. VHE Gamma-ray emission of RXJ is discussed in 2 scenarios (very common in all SNRs models): -via Inverse Compton scattering -due to neutral pion decay from proton-proton interactions

RXJ spatial resolution H.E.S.S. array didn’t find any variation in spectral index in all the disk of RXJ , but they found different fluxes (see the picture). So they did a simple geometrical model for the emission from a thick sphere matched to the dimensions and relative fluxes of RXJ :

Spectral models H.E.S.S. Collaboration fitted their experimental results for different spectral models. They used three alternative shapes: 1-a power law with an exponential cutoff E C : 2-a power law with an energy dependent exponent: 3-a broken power law (transition from  1 to  2 at break energy E B, S quantifies the sharpness of the transition):

Spectral models (2) The three spectral shapes look quite different at GeV energies. They extrapolated the three curves to 1 GeV, in order to compare them with the EGRET upper limit on the energy flux of 4.9 × erg cm -2 s -1 (in the range 1-10 GeV): (Note: the “unidentified” EGRET source 3EG is extremely close to RXJ ! So the comparisons they did are immediate)

Spectral models (3) We must add a fourth spectral model: the model used by CANGAROO Collaboration and revised (in 2004 and 2005) by CANGAROO-II. This model is more simple and it fits H.E.S.S datas not as well as the 3 models previously shown did before, but not so badly. 4-a simple power law:

Starting from the simplest spectral model (4; the “CANGAROO one”) and from the simple geometrical model shown before, I simulated (MapSource) the behavior of RXJ using H.E.S.S. (and CANGAROO) results: Simulation With spatial resolution of 0,05 degrees (the same order of magnitude of the best LAT sensitivity, at highest energies), from 20 MeV to 200 GeV. (The different colors have the same meaning of H.E.S.S. simple geometrical model) (in order to simplify procedures of first simulations and first Data Analysis, I put the SNRs in the center of galaxy (RA=265,625 o, dec=-28,92 o ), but it’s trivial to change its position)

gtobsim The first thing to do for simulating an observation of that source is calculating his total flux; but we know that in best case RXJ is 3EG and so we can use directly EGRET fluxes (in worst case EGRET didn’t see RXJ and so his flux at GeV energies must be much smaller). The total flux of 3EG above 20 MeV is almost m -2 s -1 and it means a luminosity that is almost 1/5 of Crab luminosity. GLAST should will be able to see gammas from Crab Nebula in a week: so we are waiting to see a number of gammas a little smaller than this in one month. In DC2 Sky we used the H.E.S.S. Broken Power Law Spectrum: The Break Energy is at 6.7 TeV, so for our model a single Power Law is perfectly fine and we used the lower energy part of the Broken Power Law: spectral index is 2.06 and the flux above 10 MeV integrates to m -2 s -1 (a little smaller but very close to EGRET values). (the exempla, we will see, will follow the Cangaroo Single PL)

gtobsim (2) (full sky view) (particular) In fact, using gtobsim (GlastRelease v7r0p3, ScienceTools v6r0p2) to simulate one month of observation of RXJ I’ve just modeled, we obtain 2439 gammas in the range of energy from 20 MeV to 200 GeV. and their spatial distribution is the following: (Note: GlastRelease v8r0 and ScienceTools v7r0p3 give much different results)

gtobsim (3): higher energies If we cut gammas at higher energy? We will consider now gammas between 200 MeV and 200 GeV (we increase lower limit of one order of magnitude). The only thing we have to pay attention to is changing fluxes: we obtain new flux multiplying it for the ratio between the two integrals of flux. So in this energy range (200 MeV -200 GeV), we see 749 gammas (more concentrated, as we expected, around the source position):

gtobsim (4): higher energies “Zooming” the source region, we obtain a clearer image: as we expect, we see that our spread is much smaller than in the previous case, i.e. gammas are more concentrated around the real position of our source:

gtobsim (5): highest energy Let’s do the last test increasing lower energies: we consider gammas only above 2 GeV; the flux is much smaller and infact we see only 33 gammas above 2 GeV.

DC2 particular and source location DC2 sky

Intro: RXJ RXJ , known also as G or Vela Junior, is the second shell-type SNR spatially resolved by H.E.S.S. Collaboration. As well as our previous source, it’s in a very “noisy” place of the sky (RXJ is close to galactic plane; RXJ is also in the galactic plane and it’s very close to Vela...), so it will be interesting to see how GLAST will be able to work on it. These 2 SNRs are in a very peculiar class of Shell-type SNRs with dominantely non-thermal X-ray and only very faint radio emission. RXJ was seen in X-ray by ROSAT, in  -ray by CANGAROO and by HESS, but, according to me, it was not seen by EGRET.

RXJ spectral model HESS group presents only a spectral model for RXJ emission, a Power Law Spectrum: So the total flux: according to HESS paper.

This is the Count Map of gammas from RXJ : RXJ spatial resolution

Simulation So I simulated RXJ , doing a simple geometrical model and using the Power Law Spectra we have seen in previous slides (extrapolating it from TeV energies of HESS to the energies of LAT): The spatial resolution, as you see in the picture, is 0,1 degrees (same order of magnitude of the best LAT angular resolution at higher energies). (Also in this case, I put RXJ in the center of Galaxy, in order to simplify the first test about simulation and about analysis)

gtobsim An so I extrapolated the total flux we should have at LAT energies, using HESS Power Law: All in all, using gtobsim with that flux, we simulated one month of observation of RXJ and we should see 2285 gammas in the range of energy from 20 MeV to 200 GeV. almost 1/5 - 1/6 of the luminosity of the Crab Nebula... And so, why didn’t EGRET see it? Probably it was “obscured” by Vela (EGRET angular resolution was 5.8 degrees at 100 MeV). Or it could be that the Power Law Spectrum is not the correct way of working.

gtobsim (2) Their spatial distribution:

gtobsim (3) At higher energy, >200MeV, we see 1080 gammas:

gtobsim (4) And above 2 GeV we see 61 gammas:

curiosity 61 gammas are too few for speaking about a structure, but I’m curious to see if and how we will be able to see a structure of the source we have just simulated. So I increase very much the time of observation: 10 (5+5) years! gammas above 20 MeV:

curiosity (2) gammas above 200 MeV:

curiosity (3) We saw a structure also at lower energies, but it will be more clear above 2 GeV: And looking obsim results at higher resolution, we obtain: The same structure of our model!

DC2 Source location DC2 sky

Other HESS sources Making a survey of inner Galaxy in VHE Gamma Rays, HESS array found a lot of sources, and some of them are considered Shell-type SNRs (not Plerions SNRs!). The SNRs should be: -HESSJ HESSJ HESSJ HESSJ : this is RXJ , we have just described! -HESSJ : this is Sgr A East/ SgrA* (≈ center of our Galaxy) -HESSJ HESSJ (Modeling these sources will be much simpler, because, according to HESS data, they have not a complex structure as the 2 sources I had previously implemented)

Other HESS sources (2) HESSJ HESSJ HESSJ HESSJ HESSJ

HESSJ There are 2 possible counterparts of HESSJ : -one is a source seen by Integral: IGRJ the other is G (seen by ASCA and also called AXJ ) HESSJ looks like a round with radius of 0,2 degrees. His spectrum is well described by a Power Law: with  = ; and the Total Flux above 200 GeV: F 0 = 13.2 × cm -2 s -1

HESSJ (2) So I created the source model with the spectrum, the shape and the size, shown in previous slide. I calculated the Total Fluxes at different Energies and simulated with obsim one month of observation. We see 2571 gammas above 20 MeV:

669 gammas above 200 MeVand 39 above 2 GeV HESSJ (3)

DC2 Source location DC2 sky

HESSJ HESSJ is identified with G and probably it is also the unidentified EGRET source 3EGJ HESSJ looks like a round with radius of 0,1 degrees. His spectrum is also well described by a Power Law: with  = ; and the Total Flux above 200 GeV: F 0 = 20.9 × cm -2 s -1

HESSJ (2) (Note: extrapolating fluxes from HESS Power Laws, we obtain that HESSJ is very luminous at lower energies! For gammas above 20 MeV the Total Flux is 2/3 of Crab’s one! But note also that the spectral index is higher, so we don’t expect very much gammas at higher energies.) Above 20 MeV we see 5756 gammas :

HESSJ (3) 1426 gammas above 200 MeVand 39 above 2 GeV

DC2 Source location DC2 sky

HESSJ HESSJ is identified with the SNR G , i.e. the SNR CTB 37B (=Part of the SNR Complex CTB 37 studied by ASCA). HESSJ looks like a round with radius of 0,05 degrees (so, in principle, LAT shouldn’t be able to distinguish if it is an Extended source or a Point source! Because its radius is of the same order of magnitude of LAT best PSF..) His spectrum is also well described by a Power Law: with  = ; and the Total Flux above 200 GeV: F 0 = 4.2 × cm -2 s -1

HESSJ (2) (Note: extrapolating fluxes we obtain that HESSJ is very weak in luminosity! But maybe, small angular size and small flux can make of HESSJ a good test for LAT performances..) In fact above 20 MeV we see only 363 gammas :

HESSJ (3) only 77 gammas above 200 MeVand 6 above 2 GeV

DC2 Source location DC2 sky

HESSJ HESSJ has a very precise coincidence of position with ASCA source AXJ , recently seen by INTEGRAL; this source is also identified with a VLA radio faint source with shell structure: G HESSJ looks like a round with radius of 0,05 degrees (so we can say the same comment we have just done for HESSJ ). His spectrum is also well described by a Power Law: with  = ; and the Total Flux above 200 GeV: F 0 =14.2 × cm -2 s -1

HESSJ (2) The source model looks like the exact copy of HESSJ ’S one. The Total flux above 20 MeV extrapolated from HESS one is very weak in luminosity: even smaller than HESSJ ’s one! But the spectral index is smaller, so we can expect bigger fluxes at higher energies (also according to Hess Total Flux above 200 GeV...) In fact above 20 MeV we see only 287 gammas (less than HESSJ ):

HESSJ (3) 85 gammas above 200 MeV (a little bit more than HESSJ ) and 11 above 2 GeV

DC2 Source location DC2 sky

HESSJ HESSJ is identified with G seen by VLA; it should be also possible that HESSJ is connected to old Pulsar PSRJ (and if this connection is true, the things are more complicated and a Plerion Model should be better than my Shell-type SNR simulation...) Also HESSJ looks like a round with radius of 0,05 degrees (so we can say the same comment we have just done for HESSJ and HESSJ ). His spectrum is also well described by a Power Law: with  = ; and the Total Flux above 200 GeV: F 0 =18.7 × cm -2 s -1

HESSJ The source model is looks like the previous ones. Instead the Total flux above 20 MeV extrapolated from HESS one is very strong in luminosity, compared to HESSJ and to HESSJ We see 6487 gammas above 20 MeV:

HESSJ gammas above 200 MeVand 50 above 2 GeV

DC2 Source location DC2 sky

Conclusions Source Name“Structure”Radius (degrees) Spectral Index Flux at 20 MeV (#/m 2 s) RXJ yes RXJ yes HESS no HESS no HESS no HESS no HESS no

Acknowledgements In alphabetic order: - Giovanni Busetto; Padova University, Italy. - Bernard Degrange; Ecole Polytechnique, Palaiseau, France. - Seth Digel; SLAC, Stanford, USA. - Francesco Longo; Trieste University, Italy. - Elisa Mosconi; Padova University, Italy. - Riccardo Rando; Padova University, Italy. - Francesca Maria Toma; Padova University, Italy.