General information
We provide here data products from the POLAR collaboration to be used freely by the scientific community. In case the data downloaded here is used for any scientific publication please cite to M. Kole et al. "The POLAR Gamma-Ray Burst Polarization Catalog" A&A (2020) to refer to the data products, and acknowledge the Department of Astronomy of the University of Geneva for providing the data products.
The data products for the majority of the GRBs detected by POLAR can be downloaded from this website. A simple light curve of all 55 GRBs is presented here in order to illustrate the general characteristics of each GRB. For the 38 brightest of these GRBs the data products which allow for spectral analysis can be downloaded and for many of out of these the polarization response, which allows to perform polarization analysis, is also available. The list of these 38 GRBs, along with the spectral fit results, can be founeded in M. Kole et al. "The POLAR Gamma-Ray Burst Polarization Catalog" A&A, (2020), as well as the polarization results for 14 of these GRBs. We do however encourage additional spectral and polarization studies with the existing data. Additionally, requests for spectral and/or polarization responses for other GRBs can be send to Merlin Kole while requests for potentially other interesting events can be send there as well.
While the details of the different available data products can be found below, additional details can also be found in M. Kole et al. "The POLAR Gamma-Ray Burst Polarization Catalog" A&A (2020) as well as in J.M. Burgess et al. "Time-Resolved GRB Polarization with POLAR and GBM" A&A 627, A105 (2019) regarding how these were used in the POLAR analysis. We also encourage the user to study the information on the processing of the data products which can be found in Z.H. Li et al. "In-Orbit Instrument Performance Study and Calibration for POLAR Polarization Measurements" NIM, A 900C (2018) pp. 8-24.
The data products are compatible with the polarpy software (to be found here) which is part of the 3ML network. In the future HDF5 versions of the data will also be placed here while "fits" format data can be acquired on request as well.
Photon list data
The photon list (data_GRB_for_xxx.root): This root file contains a single TTree with a list of all photons (triggers) around the GRB. Typically the time period ranges T0-200 second to T0+200 seconds. The photon list is cleaned using the methods described in more detail in Z.H. Li et al. "In-Orbit Instrument Performance Study and Calibration for POLAR Polarization Measurements" NIM, A 900C (2018) pp. 8-24. This cleaning procedure is optimized to maximize the signal to background ratio for typical GRBs. For each photon the following 4 parameters are provided:
1) The photon detection time as measured in seconds in unix time ("tunix"). The jitter of the timing in POLAR is known to be better than 100 ns, while the absolute timing precision is known to be better than 1 ms.
2) The energy of the photon ("Energy"). This energy (in keV) is calculated based on the total energy deposited in the instrument, so it is the sum of the energy deposited in all 1600 bars. The energy of each bar is calculated using the methods described in Z.H. Li et al. This includes pedestal correction, common noise correction, front-end electronics non-linearity correction, temperature dependent gain correction and cross talk correction.
3) The fraction of the time the instrument was in dead time ("dead_fraction"). The value provided is the time the instrument was in dead time over the total time. The calculation of the dead time is complex in POLAR due to the indpendently triggering modules, it is therefore not simply linearly proportional to the trigger rate but calculated based on measurements of the deadtime performed continuously during the flight. Details can be found in N. Produit et al. "Design and construction of the POLAR detector" NIM A 877 (2018), pp. 259-268.
4) The scattering angle of the photon inside of the instrument ("scatter_angle"). The scattering angle (in degrees) is taken to be the angle between the two scintillator bars with the highest energy depositions in the full instrument. The first bar is that with the highest energy deposition, while the second bar is the bar with the highest energy which is non-adjacent to the first bar. If only bars adjacent to the first bar have energy depositions the event is not selected for polarization analysis (as this would reduce the M100). For such events the scattering angle is set to be -1. This is also the case for all events where there is only a single bar with an energy deposition or events where at least 1 bar has an energy deposition above the dynamic range (in overflow) as for such events no correct energy conversion can be performed.
The scattering angle provided is in the POLAR coordinate system where an angle of 0 implies scattering along the x-axis of the instrument which is perpendicular to the direction of flight of the instrument. The same convention is followed in the polarization response. At the end of the polarization analysis this implies that the polarization angle found will be in the POLAR coordinate system, transforming this to the IAU convention will require the coordinates of POLAR at the time of the GRB which can be requested to Merlin Kole or Nicolas Produit.
Spectral Response
The spectral response of 38 of the GRBs is provided using a name such as RMF_ARF_xxx.root. The file contains the energy response of POLAR consisting of the RMF and ARF for this specific GRB. The RMF for typically a 135 "true energy" (or monte carlo energy) bins are presented (called rsp_xxx). The lowest energy bin ranges from 7.5 keV to 12.5 keV while the highest one is typically from 677.5 keV to 682.5 keV. For the GRBs for which polarization analysis was performed (see the list in M. Kole et al. 2020) the energy range goes up to 752.5 keV with a total of 150 true energy bins. The lower bounds of all the bins can be found in the ER_Low histogram while the upper limits can be found in ER_high and finally EM_bounds gives both the lower and upper limits. The binning in the reconstructed energy is equal to that used for the true energy.
The RMF_xxx histograms are all normalized to have a total bin content of 1. The ARF an RMF are combined in the 2d "rsp" histogram in which the "true energy" is on the y-axis and the reconstructed one on the x-axis. The unit on the z-axis is cm^2/bin. The ARF for each true energy can therefore be calculated by summing over the bin contents along the x-axis for each true energy. The rsp histogram therefore contains by itself all the information to perform the spectral analysis.
The spectral response is produced for each GRB by simulating the incoming angle to be that corresponding to best known location of the GRB with respect to POLAR. It should be noted that the zenith of POLAR moved by 4 degrees per minute during flight. The attitude of POLAR chosen to calculate the relative incoming angle of the GRB was typically that corresponding to T50 of the GRB. For long GRBs several responses could be required to take the changing angle into consideration in which case this can be requested. Additionally, the temperature of all the 25 detector modules (which has an effect of the energy gain of each channel) as measured during the GRB is taken into account when producing the response. It should be noted that while for GRBs used for polarization analysis in the past (see the list in M.Kole et al. 2020) the incoming angle with respect to POLAR is taken to be within 0.1 degree of the best known location, for other GRBs the closest healpix bin center (with a grid of 400 points over half the sky) was used. Additionally for the latter GRBs the temperature was assumed to be uniformly 25 degrees over the full instrument. Updated more accurate responses can be requested in case it is found that more accuracy is needed for such GRBs.
Polarization Response
The polarization response of POLAR for each GRB is provided along with the photon list (for convenience) in a single root file ("polarization_plus_data_xxx.root" ): The polarization response contains scattering angle distributions for diferent (true) energy bins, polarization angles and polarization degrees (0% and 100%). The angles follow the same convention as those provided in the photon list.
The naming convention of the histograms is as follows: sim_a_b_c where "a" is the true energy of the incoming beam in keV, "b" is the polarization degree and "c" is the polarization angle (in the POLAR coordinate system). Each histrogram consists of 360 scattering angle bins where the bin content is in units of events/s/bin.
In the data analyis performed by the collaboration error bars were added to scattering angle bins in the response in order to propogate systematic errors such as those from the calibration (relative of 2%) as well as that from the location (ranging from <1% to 10% depending on the location accuracy). The user is adviced to artifically add such error bars in their analysis, or to set these to 0 (for the majority of the responses the error should be set to 0% in order for the user to have the freedom to set this to the desired value).
The response was produced for the relative incoming angle of the GRB calculated based on the best known location of the GRB at (typically) the time of T50 of the GRB. The temperatures of the 25 different detector modules were also taken into account when producing the response as these have an influence on the gain of each channel and therefore the detection efficiency in the different bars.