For the first Data Challenge, we wanted to give users a basic introduction to COSI data analysis through 3 straightforward examples of COSI's science goals:
- Extracting energy spectra from the Crab, Cen A, Cygnus X-1, and Vela
- Imaging bright point sources, such as the Crab and Cygnus X-1
- Imaging diffuse emission from the positron-electron annihilation 511 keV and the Al-26 1.8 MeV gamma-ray lines
For each of these examples, we have provided a detailed description of the simulated sources and data products here in the data_products directory. Each of the sources was simulated at 10x the true astrophysical flux. Having a strong signal simplifies the analysis and allows us to focus on the workflow of the procedures. The COSI SMEX mission is expected to be 50x more sensitive than the balloon-borne mission.
The simulations were all performed in MEGAlib with an accurate mass model of the COSI Balloon instrument. The COSIBalloon.9Detector.geo.setup model was used, which accounts for the failure of three GeD detectors at different times during flight. Each of the continuum simulations was performed for 100 keV – 10 MeV, and an energy range selection of <5 MeV was used in MEGAlib’s mimrec event selection tool.
The source simulations include the real flight aspect information so that the balloon path and source exposure time is accurate. This can be seen in the below plot showing the Galactic longitude and latitude of the COSI balloon's zenith direction as a function of time.
Furthermore, the transmission probability of the source photons in the atmosphere is calculated for each instance of the simulation. The probability of transmission is taken at a constant altitude of 33 km, and is shown as a function of zenith angle and energy in the below figure.
The tools that were used for the simulations (including the source library, orientation file, transmission probability file, etc.) are available in the main branch of cositools/cosi-data-challenges (link).
We have included many combinations of the source simulations and background to allow for flexibility and further testing with these files. All files are either in the .npz zipped numpy array format for the CDS-binned response matrix or background model, or in MEGAlib's photon list .tra.gz format. MEGAlib was used to perform all of these simulations, and the source models are described in detail below.
Within this directory, 1 background model and 3 response matrices are provided:
- Scaled_Ling_BG_1x.npz: background model generated from C. Karwin's scaled 1x Ling background simulation
- Continuum_imaging_response.npz: 6º response used for spectral analysis and imaging continuum sources
- 511keV_imaging_response.npz: 6º imaging response required for RL imaging of Galactic positron annihilation
- 1809keV_imaging_response.npz: 6º imaging response required for RL imaging of Galactic Al-26
There is a combined simulation of all sources and background:
- DC1_combined_10x.tra.gz: 4 point sources with 10x flux (Crab, Cygnus X-1, Cen A, Vela), 511 keV & Al-26 lines, 1x Ling background
There is each of the sources individually with the background:
- Point_sources_10x_BG.tra.gz: 4 point sources with 10x flux (Crab, Cygnus X-1, Cen A, Vela) and 1x Ling background
- Crab_BG_10x.tra.gz: Crab with 10x flux and 1x Ling background
- CygX1_BG_10x.tra.gz: Cygnus X-1 with 10x flux and 1x Ling background
- CenA_BG_10x.tra.gz: Cen A with 10x flux and 1x Ling background
- Vela_BG_10x.tra.gz: Vela with 10x flux and 1x Ling background
- GC511_10xFlux_and_Ling.inc1.id1.extracted.tra.gz: 511 keV emission with 10x flux with Ling background
- Al26_10xFlux_and_Ling.inc1.id1.extracted.tra.gz: Al-26 emission with 10x flux with Ling background
Each of the sources is also included without background:
- Point_sources_10x.tra.gz: 4 point sources with 10x flux (Crab, Cygnus X-1, Cen A, Vela)
- Crab_only_10x.tra.gz: Crab with 10x flux
- CygX1_only_10x.tra.gz: Cygnus X-1 with 10x flux
- CenA_only_10x.tra.gz: Cen A with 10x flux
- Vela_only_10x.tra.gz: Vela with 10x flux
- GC_511_10xFlux_only.inc1.id1.extracted.tra.gz: 511 keV emission with 10x flux
- Al26_10xFlux_Only.inc1.id1.extracted.tra.gz: Al-26 emission with 10x flux
And finally, there is 1 background simulation corresponding to the .npz background model above (this is not required for any analysis, but is included here for posterity):
- Scaled_Ling_BG_1x.tra.gz: C. Karwin's scaled 1x Ling background simulation
All files have the same start and stop time, in unix time:
start time: 1463443400.0 s
stop time: 1467475400.0 s
total time: 4032000.0 s = 46.67 days
This corresponds to May 17 2016 00:03:20 GMT to July 02 2016 16:03:20 GMT, covering the full COSI Balloon flight in 2016.
As described in the main cosi-data-challenge-1 README, these files are stored on Git’s Large File Server. One must install git-lfs to access them.
There are four bright point sources included in these simulations: the Crab nebula, Cygnus X-1, Centaurus A, and Vela.
The Crab nebula is considered both a pulsar wind nebula (PWN) and supernova remnant. The PWN surrounds the Crab Pulsar, a rapidly rotating and magnetized neutron star in the Milky Way constellation Taurus. The supernova remnant was produced by SN 1054. The Crab entered COSI-balloon's field of view for only ~12 of the 46 days of the 2016 flight (Sleator 2019); the balloon remained largely in Earth's Southern Hemisphere and the Crab is more easily viewed from the Northern Hemisphere. Nevertheless, as the brightest persistent
Cygnus X-1 is a bright hard X-ray source in the Cygnus constellation of the Milky Way. It is believed to be a black hole in an X-ray binary system. Cygnus X-1 emits in COSI's bandpass as well, and like the Crab is simulated here at 10x its true flux of 0.041 ph cm
Centaurus A is a galaxy in the constellation of Centaurus. Also called NGC 5128, Centaurus A has a supermassive black hole which emits radio waves, X-rays, and gamma-rays. It is simulated with 10x its true flux of 0.0036 ph cm
The Vela pulsar is simulated based on a power law extrapolation of the corresponding Fermi-LAT source (4FGL J0835.3-4510) to lower energy. We have verified that the extrapolation is consistent with the observations from OSSE+COMPTEL+EGRET, e.g. see Pavlov+01. It is fainter than the other three sources but is included as a source which could be observed by an instrument like COSI-balloon with more observation time.
A strong 511 keV signal emanating from the direction of the Galactic Center
- What is the underlying nature of the emission morphology?
The 511 keV image from the INTEGRAL SPI satellite (Bouchet et al. 2010, shown below as a significance map) reveals two seemingly distinct components: an extended "disk" which traces the Galactic Plane and a central, bright "bulge" around the Galactic Center. The notable difference between these structures is not understood. The extended disk emission may suggest that positrons are propagating away from and annihilating at a distance from their production sites, thereby smearing out the emission into a diffuse presentation. It is also possible, however, that there could be a collection of point-like sources emitting positrons which annihilate close to their progenitors; it is the collection of these sources together which may form a total diffuse structure. However, no individual point source of positrons has been detected. It is likely that positrons propagate away from their production sites and slow down to low enough energies to form positronium.
Imaging with fine angular resolution can help disentangle this dichotomy. The detection of an individual point source, for example, could revolutionize our understanding of the morphology. High-resolution spectroscopy is also critical to understanding the transport and eventual annihilation sites of these positrons: a significant ortho-Positronum (o-Ps;
- Where are all of these positrons coming from?
The bulge emission exhibits a positron annihilation rate of
Ultimately, we can boil the "positron puzzle" down to uncertainty around the source, transport, and sink (the annihilation itself) of these particles.
The COSI-balloon flight in 2016 detected the 511 keV signature of the positron puzzle with
In this data challenge, you will image the Galactic 511 keV emission, simulated at 10x its true flux (10x flux =
As MeV gamma-ray instruments, COSI-balloon and the COSI satellite are uniquely equipped to study the 1.809 MeV signature emission from this radioisotope, which traces stellar nucleosynthesis over millions of years.
In the 1980s, NASA's High Energy Astrophysical Observatory (HEAO-3) satellite mission detected 1.809 MeV emission emanating from the direction of the Galactic Center Mahoney et al. 1984. This marked the discovery of Galactic Al-26. The spectrum is shown below.
Subsequent observations by the Compton telescope (COMPTEL) on-board NASA's Compton Gamma Ray Observatory (CGRO) yielded the first image of Al-26 emission (Oberlack et al. 1996, Oberlack 1997, Pluschke et al. 2001). Emission is concentrated in the Inner Galaxy
The SPectrometer on INTEGRAL (SPI) largely corroborated the features seen in the COMPTEL image with over a decade of observation time from ESA's INTEGRAL satellite (Bouchet et al. 2015). Emission is concentrated in the Inner Galaxy with a reported flux of
| 
COMPTEL 1.8 MeV image (Pluschke et al. 2001) | SPI 1.8 MeV image (Bouchet et al. 2015)
The COSI-balloon flight in 2016 measured the 1.809 MeV signature of Al-26 with
Furthermore, the COSI satellite's full-sky observations with fine angular resolution have potential to more closely study individual regions of massive star activity; in particular, resolving individual sites of emission within Cygnus is a promising goal of the mission. Detailed imaging and spectroscopic studies of the region may inform better understanding of the dynamics of Al-26 after it is produced and ejected from massive stars.
In this data challenge, you will image the Galactic Al-26 emission (traced by the DIRBE 240 um image) as seen during the COSI-balloon flight in 2016. The flux is simulated at 10x the observed Al-26 flux (10x Inner Galaxy flux =
There are four bright point sources that are included in these simulations: the Crab nebula, Cygnus X-1, Centaurus A, and Vela. The spectra and flux for each of these sources was determined from the literature. As explained above, we have used 10x the flux values for these sources in order to simplify the analysis. Please keep this in mind when interpreting the results of your analyses in this Data Challenge.
Crab:
(l,b) = (184.56, -5.78)
Spectral shape: Band function from Jourdain et al. 2020
Flux = 0.48977 ph cm
Cen A:
(l,b) = (309.52, 19.42)
Spectral shape: SED from HESS+LAT collaboration 2018
Flux: 0.03609 ph cm
Cygnus X-1:
(l,b) = (71.33, 3.07)
Spectral shape: SED from Kantzas+21
Flux = 0.40644 ph cm
Vela:
(l,b) = (263.55, -2.79)
Spectral shape: Power law extrapolation of the Fermi-LAT Vela pulsar (4FGL J0835.3-4510), see Abdollahi+20
Flux = 0.00120 ph cm
The input spectra for these point sources is shown below. Note that the plot shows the true spectra, whereas the flux values reported above are for the 10x simulations.
The simulations are run in MEGAlib’s cosima tool. A list-mode image created in mimrec confirms the correct point source locations:
The morphology of the 511 keV emission from positron annihilation is not well constrained. For this first Data Challenge, we have used the model defined in Knödlseder et al. 2005, where the emission was fit with a 2-D asymmetric Gaussian spatial model with the following parameters (the reported flux is the true value and not the 10x value):
The emission was simulated as a 511 keV mono-energetic source, and the image from mimrec confirms the extended emission in the Galactic Center.
The Diffuse Infrared Background Experiment (DIRBE) 240 um map has been shown to be a good tracer for the Al-26 emission, as measured by COMPTEL and INTEGRAL/SPI. We thus assume the DIRBE 240 um distribution as the spatial sky model for the Al-26 emission and normalize the Inner Galaxy flux of this map to SPI's measured Al-26 flux in this region:
The emission was simulated with a 1.8 MeV mono-energetic source, and the image from mimrec confirms the extended emission along the Galactic disk.
Note that the list-mode imaging method employed in MEGAlib is not optimized for diffuse sources, and the extra structure out of the Galactic plane is an imaging artifact.
The background radiation model used for the Data Challenge is based on the semi-empirical model from Ling 1975, which has been used for all COSI balloon analyses. The model describes the angular and atmospheric depth dependence of gamma-rays within 0.3 – 10 MeV and includes 3 primary gamma-ray sources: continuum emission (Bremsstrahlung from cosmic ray interactions in the atmosphere), a 511 keV line component (from electron-positron annihilation in the atmosphere, and a cosmic diffuse component (down-scattered gamma-rays from the extragalactic background and Galactic plane). The Ling model requires a description of the atmosphere’s density, interaction depth, and mass absorption coefficients, which are obtained through an NRLMSISE-00 atmospheric model. An altitude of 33.5 km was assumed for these models - altitude drops and changes in longitude and latitude were not taken into account for this simulation.
The amplitude of the Ling background was scaled so the total integrated background spectrum from simulation matched closely to what was measured during flight, as can been seen in the figure below. The flight data (“All Data” label) shows a time-variable background count rate that was influenced by the geomagnetic cutoff and balloon altitude drops. The cyan line shows the scaled Ling background model. As a reference, the count rate from the 1x flux point sources is shown, and the signal is at most a few percent of the background count rate.
An image of the background simulation traces the exposure map, since the orientation of the COSI Balloon in Galactic coordinates was included in the simulation. Note that a
