In this section the calorimetric γ-ray emissivity of NGC 253 will be computed. Given
the CR energy input in the form of SNe of FCR as calculated from Eq. 3.8, and a energy
conversion into π0 production and subsequent decay into γ rays, the measurable F
γ on
Earth can be calculated as: Fγ = 1
3· 1
4πD2 · FCR ≈ 8.2 × 10
−11 TeV cm−2 s−1 , (3.13)
considering a distance of 2.6 Mpc to NGC 253 (Engelbracht et al. 1998). Neglecting diffusion and convection and under the assumption of a proton distribution which follows a power law in energy with index Γ = 2.0, the integral flux above 1 TeV can be calculated as:
Fγ(> 1 TeV) ≈ 2.3 × 10−12 cm−2 s−1 . (3.14)
Aharonian et al. (2005c) modelled the expected VHE γ-ray emission considering convection (but neglecting diffusion) and estimated the γ-ray flux from NGC 253 to Fγ(> 1 TeV) ≈
9.8 × 10−13 cm−2 s−1. Domingo-Santamar´ıa & Torres (2005) have performed a full mod-
elling, taking into account all energy-loss processes by CRs and obtained integral fluxes of Fγ(> 1 TeV) ≈ 2.0×10−13cm−2s−1, which is an order of magnitude lower compared to the
calorimetric limit. Fig. 3.6(a) and 3.6(b) show the results of the Domingo-Santamar´ıa & Torres (2005) model. Also Rephaeli et al. (2010) recently performed a detailed modelling, taking into account convective and diffusive losses. The estimated fluxes are compared to the measurement performed with H.E.S.S. and the spectral results as obtained with the ζ analysis, which are presented in Chapter 6.
As outlined in this Chapter, VHE γ-ray emission is presumably expected from star forming environments since these are able to serve as particle acceleration sites and provide at the same time target material and/or radiation fields for the production of HE and VHE γ rays. In the following chapters, data from VHE γ-ray observations performed with H.E.S.S. of the CWB binary η Carinae, the young massive stellar cluster Westerlund 1 and the SB galaxy NGC 253 are analysed with the BDT method. Spectral and morphological results are presented and used to draw conclusions on the origin of the potential VHE γ-ray emission from these objects.
1E-3 0,01 0,1 1 10 100 1000 10000 1E-23 1E-21 1E-19 1E-17 1E-15 1E-13 1E-11 1E-9 1E-7 1E-5 1E-3 E (GeV) F ( G e V - 1 cm - 2 s - 1 )
total -ray flux
Bremsstrahlung IS
Invers Compton IS
0
decay IS
total -ray flux SD
EGRET 2 upper limits
(a) 100 1000 10000 1E-14 1E-13 1E-12 1E-11
integrated -ray flux
CANGAROO data
HESS upper limits (28h)
HESS sensitivity (5 in 50h) E 0 (GeV) F ( E > E 0 ) ( p h o t o n s cm - 2 s - 1 ) (b)
Figure 3.6: Results of the simulated model as obtained by Domingo-Santamar´ıa & Torres (2005). (a) Predicted differential γ-ray flux split by different contributions and between disc (SD) and SB nucleus (IS). Also indicated is the 2σ upper limit (UL) as obtained by EGRET. (b) Integral γ-ray flux in the region between 100 GeV and 10 TeV. Also shown are the CANGAROO results as well as the H.E.S.S. UL and the H.E.S.S. sensitivity (Figures taken from Domingo-Santamar´ıa & Torres (2005)).
4 VHE γ-ray observations of η Carinae and
the Carina region
4.1 The Carina region and the CWB η Carinae
The Carina Nebula is one of the largest and most active HII regions in our Galaxy and is a place of ongoing star formation. It is located in the constellation Carina, at a distance of ∼2.3 kpc harbouring eight massive stellar clusters with more than 64 O-type stars (Feinstein 1995; Smith 2006). A Hubble Space Telescope image of the Carina nebula showing the most prominent regions is displayed in Fig. 4.1. Based on the most evolved stars inside and the size of the nebula, its age is estimated to 3 Myrs. Diffuse X-ray emission was reported by Hamaguchi et al. (2007) based on observations from Suzaku, XMM-Newton and Chandra. The authors concluded that apart from two thermal, lower- temperature components originating from diffuse plasma, the 5 keV component is most likely generated by one or multiple SNRs. The Carina nebula also harbours four WR stars as well as the extreme-type luminous blue variable (LBV) η Carinae.
η Carinae is one of the most peculiar objects in our Galaxy. In the 1840’s and 1890’s a giant outburst (also known as the great eruption) and a smaller outburst produced the Homunculus- and Little Homunculus nebula (Ishibashi et al. 2003, see e.g.). η Carinae and the Homunculus nebula in optical and X-ray wavelengths is depicted in Fig. 4.2. The material expelled from the central star in the great eruption adds up to a mass of ∼ 12 M⊙
which moves outwards at an average speed of ∼ 650 km s−1 implying a kinetic energy of
the giant outburst of roughly (4 − 10) × 1049erg (Smith et al. 2003). Interestingly, Smith
(2008) found material which is moving ahead of the expanding Homunculus nebula at speeds of 3500−6000 km s−1. The existence of this material basically doubles the estimate
of the kinetic energy of the giant outburst and can be interpreted as a low-energy SNR shell (Smith 2008) with a fast blast wave moving into the ISM with velocities comparable to e.g. SN 1006 (Vink 2005).
For a long time it was believed that the central object, η Carinae, is a hypergiant LBV star, though recent observations (Hillier et al. 2001; Pittard & Corcoran 2002) suggest
a binary system composed of a massive WR star (M ≥ 90M⊙) and an O- or B-type
star with a mass of M ≤ 30M⊙. The optical (Damineli 1996; Damineli et al. 2000) and
IR (Whitelock et al. 1994, 2004) light curves infer a long period of ∼2023 days (5.54 years) with a long time in a “high spectroscopic state” characterised by an emission line spectrum and a short time (typically a few months) in a “low spectroscopic state” – also called “spectroscopic events”1. An analysis of the X-ray light curves point to an highly
eccentric orbit of e ∼ 0.9 (Corcoran et al. 2001; Falceta-Gon¸calves et al. 2005). In contrast to the WR star, which has a very high mass loss rate of ˙M1 ≈ 2.5 × 10−4 M⊙ yr−1 and a
1
This low spectroscopic state is believed to be caused by the collapse of the colliding wind region.
Figure 4.1: Optical image of the Carina nebula as seen by Hubble. The image also depicts prominent regions such as the stellar clusters Trumpler 14 and Trumpler 16, Herbig-Haro objects as well as two of the most massive stars known in the Milky Way: η Carinae and HD 93250. Credit: NASA/ESA.
(a) (b)
Figure 4.2: (a) Composite Chandra/HST image. X-ray emission as measured with the Chandra satellite from η Carinae is shown as yellow (0.5 − 1.2 keV) and white (1.2−11 keV). The yellowish ring is interpreted as material blown away from the star in an eruption ∼ 1000 yrs ago (Credit: NASA/CXC/GSFC/M.Corcoran et al). Blue depicts the emission as seen by Hubble. (b) Zoom into the central region of image (a); Optical emission as detected by the Hubble Space Telescope. The Homunculus nebula is clearly visible as double-lobe-like structure (Credit: NASA/ESA/STScI).
terminal wind velocity of v1 ≈ (500−700) km s−1, the companion star has a thin fast wind