Simulating HESS SNRs

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

Summary -RXJ1713.7-3946 (astro-ph/0511678v2, 2005) -RXJ0852.0-4622 (A&A, 437, L7-L10, 2005) -HESS galactic survey (ApJ, 636, 777-797, 2006): -HESSJ1634-472 -HESSJ1640-465 -HESSJ1713-381 -HESSJ1813-178 -HESSJ1834-087

Intro: RXJ1713.7-3946 H.E.S.S. and Cangaroo H.E.S.S. collaboration resolved spatially and morphologically 2 Shell-type Supernova Remnants: -RXJ1713.7-3946 -RXJ0852.0-4622 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. RXJ1713.7-3946 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 RXJ1713.7-3946 was recently studied and resolved by H.E.S.S. Collaboration. VHE Gamma-ray emission of RXJ1713.7-3946 is discussed in 2 scenarios (very common in all SNRs models): -via Inverse Compton scattering -due to neutral pion decay from proton-proton interactions

RXJ1713.7-3946 spatial resolution H.E.S.S. array didn’t find any variation in spectral index in all the disk of RXJ1713.7-3946, 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 RXJ1713.7-3946:

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 EC: 2-a power law with an energy dependent exponent: 3-a broken power law (transition from G1 to G2 at break energy EB, 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 4.9×10-11 erg cm-2 s-1 (in the range 1-10 GeV): (Note: the “unidentified” EGRET source 3EG 1714-3857 is extremely close to RXJ1713.7-3946! 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:

Simulation Starting from the simplest spectral model (4; the “CANGAROO one”) and from the simple geometrical model shown before, I simulated (MapSource) the behavior of RXJ1713.7-3946 using H.E.S.S. (and CANGAROO) results: 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=265,625o, dec=-28,92o), 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 RXJ1713.7-3946 is 3EG 1714-3857 and so we can use directly EGRET fluxes (in worst case EGRET didn’t see RXJ1713.7-3946 and so his flux at GeV energies must be much smaller). The total flux of 3EG 1714-3857 above 20 MeV is almost 0.03533 m-2 s-1 and it means a luminosity that is almost 1/5 of Crab luminosity. GLAST should will be able to see 2500-3000 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 0.038 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) In fact, using gtobsim (GlastRelease v7r0p3, ScienceTools v6r0p2) to simulate one month of observation of RXJ1713.7-3946 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) (full sky view) (particular)

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: RXJ0582.0-4622 RXJ0852.0-4622, known also as G266.2-1.2 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 (RXJ1713.7-3946 is close to galactic plane; RXJ0852.0-4622 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. RXJ0852.0-4622 was seen in X-ray by ROSAT, in g-ray by CANGAROO and by HESS, but, according to me, it was not seen by EGRET.

RXJ0582.0-4622 spectral model HESS group presents only a spectral model for RXJ0852.0-4622 emission, a Power Law Spectrum: So the total flux: according to HESS paper.

RXJ0582.0-4622 spatial resolution This is the Count Map of gammas from RXJ0852.0-4622:

Simulation So I simulated RXJ0852.0-4622, 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 RXJ0852.0-4622 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: 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. All in all, using gtobsim with that flux, we simulated one month of observation of RXJ0852.0-4622 and we should see 2285 gammas in the range of energy from 20 MeV to 200 GeV.

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! 277424 gammas above 20 MeV:

curiosity (2) 126554 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: -HESSJ1634-472 -HESSJ1640-465 -HESSJ1713-381 -HESSJ1713-397: this is RXJ1713.7-3946, we have just described! -HESSJ1745-290: this is Sgr A East/ SgrA* (≈ center of our Galaxy) -HESSJ1813-178 -HESSJ1834-087 (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) HESSJ1834-087 HESSJ1713-381 HESSJ1813-178 HESSJ1634-472 HESSJ1640-465

HESSJ1634-472 There are 2 possible counterparts of HESSJ1634-472: -one is a source seen by Integral: IGRJ16358-4726 -the other is G337.2+0.1 (seen by ASCA and also called AXJ1635.9-4719) HESSJ1634-472 looks like a round with radius of 0,2 degrees. His spectrum is well described by a Power Law: with G= 2.38 + 0.27; and the Total Flux above 200 GeV: F0= 13.2 × 10-12 cm-2 s-1

HESSJ1634-472 (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:

HESSJ1634-472 (3) 669 gammas above 200 MeV and 39 above 2 GeV

HESSJ1640-465 HESSJ1640-465 is identified with G338.3-0.0 and probably it is also the unidentified EGRET source 3EGJ1639-4702. HESSJ1640-465 looks like a round with radius of 0,1 degrees. His spectrum is also well described by a Power Law: with G= 2.42 + 0.15; and the Total Flux above 200 GeV: F0= 20.9 × 10-12 cm-2 s-1

HESSJ1640-465 (2) Above 20 MeV we see 5756 gammas : (Note: extrapolating fluxes from HESS Power Laws, we obtain that HESSJ1634-472 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.)

HESSJ1640-465 (3) 1426 gammas above 200 MeV and 39 above 2 GeV

HESSJ1713-381 HESSJ1713-381 is identified with the SNR G348.7+0.3, i.e. the SNR CTB 37B (=Part of the SNR Complex CTB 37 studied by ASCA). HESSJ1713-381 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 G= 2.27 + 0.48; and the Total Flux above 200 GeV: F0= 4.2 × 10-12 cm-2 s-1

HESSJ1713-381 (2) (Note: extrapolating fluxes we obtain that HESSJ1713-381 is very weak in luminosity! But maybe, small angular size and small flux can make of HESSJ1713-381 a good test for LAT performances..) In fact above 20 MeV we see only 363 gammas :

HESSJ1713-381 (3) only 77 gammas above 200 MeV and 6 above 2 GeV

HESSJ1813-178 HESSJ1813-178 has a very precise coincidence of position with ASCA source AXJ1813-178, recently seen by INTEGRAL; this source is also identified with a VLA radio faint source with shell structure: G12.82-0.02. HESSJ1813-178 looks like a round with radius of 0,05 degrees (so we can say the same comment we have just done for HESSJ1713-381). His spectrum is also well described by a Power Law: with G= 2.09 + 0.08; and the Total Flux above 200 GeV: F0=14.2 × 10-12 cm-2 s-1

HESSJ1813-178 (2) The source model looks like the exact copy of HESSJ1813-178’S one. The Total flux above 20 MeV extrapolated from HESS one is very weak in luminosity: even smaller than HESSJ1713-381’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 HESSJ1713-381):

HESSJ1813-178 (3) 85 gammas above 200 MeV (a little bit more than HESSJ1713-381) and 11 above 2 GeV

HESSJ1834-087 HESSJ1834-087 is identified with G23.3-0.3 seen by VLA; it should be also possible that HESSJ1834-087 is connected to old Pulsar PSRJ1833-0827 (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 HESSJ1834-087 looks like a round with radius of 0,05 degrees (so we can say the same comment we have just done for HESSJ1713-381 and HESSJ1813-178). His spectrum is also well described by a Power Law: with G= 2.45 + 0.16; and the Total Flux above 200 GeV: F0=18.7 × 10-12 cm-2 s-1

HESSJ1834-087 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 HESSJ1713-381 and to HESSJ1813-178. We see 6487 gammas above 20 MeV:

HESSJ1834-087 1491 gammas above 200 MeV and 50 above 2 GeV

Conclusions

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.

Simulating HESS SNRs