Publish and perish
Polarization leakage in epoch of reionization windows – III. Wide-field effects of narrow-field arrays
Leakage of polarized Galactic diffuse emission into total intensity can potentially mimic the 21-cm signal coming from the epoch of reionization (EoR), as both of them might have fluctuating spectral structure. Although we are sensitive to the EoR signal only in small fields of view, chromatic sidelobes from further away can contaminate the inner region. Here, we explore the effects of leakage into the ‘EoR window’ of the cylindrically averaged power spectra (PS) within wide fields of view using both observation and simulation of the 3C196 and NCP fields, two observing fields of the LOFAR-EoR project. We present the polarization PS of two one-night observations of the two fields and find that the NCP field has higher fluctuations along frequency, and consequently exhibits more power at high line-of-sight k that could potentially leak to Stokes I. Subsequently, we simulate LOFAR observations of Galactic diffuse polarized emission based on a model to assess what fraction of polarized power leaks into Stokes I because of the primary beam. We find that the rms fractional leakage over the instrumental k-space is 0.35% in the 3C196 field and 0.27% in the NCP field, and it does not change significantly within the diameters of 15°, 9° and 4°. Based on the observed PS and simulated fractional leakage, we show that a similar level of leakage into Stokes I is expected in the 3C196 and NCP fields, and the leakage can be considered to be a bias in the PS.
After a four-year long heady journey, I finished my PhD by the end of 2016, and defended the thesis successfully on 23 January, 2017. In the Netherlands, you are supposed to print more than a hundred copies of your PhD thesis in book format. I eventually came to like this strict rule because no matter what the quality of your research, it feels good when you have its tangible output in your hand as a professional-looking book. So here is the soft copy of the book as a pdf file. I do not know how people finish distributing their hundred (in some cases two hundred) copies because I have almost thirty copies left even after distributing away generously.
A summary of the thesis written for non-astronomers is also available in this website in English and Bengali. And the pdf of the complete thesis can be downloaded from here.
ISBN: 978-90-367-9528-9 (print) or 978-90-367-9529-6 (electronic).
Polarization leakage in epoch of reionization windows – II. Primary beam model and direction-dependent calibration
MNRAS 462, 4482–4494.
Leakage of diffuse polarized emission into Stokes I caused by the polarized primary beam of the instrument might mimic the spectral structure of the 21-cm signal coming from the epoch of reionization (EoR) making their separation difficult. Therefore, understanding polarimetric performance of the antenna is crucial for a successful detection of the EoR signal. Here, we have calculated the accuracy of the nominal model beam of Low Frequency ARray (LOFAR) in predicting the leakage from Stokes I to Q, U by comparing them with the corresponding leakage of compact sources actually observed in the 3C 295 field. We have found that the model beam has errors of ~10 per cent on the predicted levels of leakage of ~1 per cent within the field of view, i.e. if the leakage is taken out perfectly using this model the leakage will reduce to 0.001 of the Stokes I flux. If similar levels of accuracy can be obtained in removing leakage from Stokes Q, U to I, we can say, based on the results of our previous paper, that the removal of this leakage using this beam model would ensure that the leakage is well below the expected EoR signal in almost the whole instrumental k-space of the cylindrical power spectrum. We have also shown here that direction-dependent calibration can remove instrumentally polarized compact sources, given an unpolarized sky model, very close to the local noise level.
Paralia Katerini's, Greece.
I presented the results of my first paper. The presentation is embedded below.
Polarization leakage in epoch of reionization windows – I. Low Frequency Array observations of the 3C196 field
MNRAS 451, 3709–3727.
Detection of the 21-cm signal coming from the epoch of reionization (EoR) is challenging especially because, even after removing the foregrounds, the residual Stokes I maps contain leakage from polarized emission that can mimic the signal. Here, we discuss the instrumental polarization of Low Frequency Array (LOFAR) and present realistic simulations of the leakages between Stokes parameters. From the LOFAR observations of polarized emission in the 3C196 field, we have quantified the level of polarization leakage caused by the nominal model beam of LOFAR, and compared it with the EoR signal using power spectrum analysis. We found that at 134–166 MHz, within the central 4° of the field the (Q, U) → I leakage power is lower than the EoR signal at k < 0.3 /Mpc. The leakage was found to be localized around a Faraday depth of 0, and the rms of the leakage as a fraction of the rms of the polarized emission was shown to vary between 0.2 and 0.3 per cent, both of which could be utilized in the removal of leakage. Moreover, we could define an ‘EoR window’ in terms of the polarization leakage in the cylindrical power spectrum above the PSF-induced wedge and below line-of-sight k ~ 0.5 /Mpc, and the window extended up to line-of-sight k ~ 1 /Mpc at all transverse k when 70 per cent of the leakage had been removed. These LOFAR results show that even a modest polarimetric calibration over a field of view of ≲ 4° in the future arrays like Square Kilometre Array will ensure that the polarization leakage remains well below the expected EoR signal at the scales of 0.02–1 /Mpc.
I presented the the results of the first two years of my PhD. The presentation is embedded below.
Study of the formation of Cold Fronts and Radio Mini-halos induced by the Intergalactic Gas Sloshing in the Cores of Galaxy Clusters
Some cool-core non-major merging galaxy clusters host diffuse amorphous radio sources in their central regions (r = 100 − 300 kpc) named Radio Mini-halos (MH). MHs are characterized by steep synchrotron spectra. Their diffuse radio emission surrounds a bright radio source associated to the brightest cluster galaxy (BCG). The corresponding radio emitting particles cannot be connected to the central radio galaxy in terms of particle diffusion. It has been proposed that they could result from a relic population of relativistic electrons re-accelerated by MHD turbulence, necessary energetics being supplied by the cool-core region. Later the MHs in two clusters, namely MS1455.0+2232 and RX J1720.1+2638, were found to be confined within the region delimited by the cold fronts (CF), i.e. edges of dense and cold structures in the thermal intracluster gas observed in X-rays. These MHs have also been found to be spatially correlated with the X-ray spiral structure created by gas sloshing at the cluster centre. Gas sloshing is one of the possible mechanism behind the formation of the CFs. Being a turbulent mechanism, it has been suggested that gas sloshing should also be responsible for the re-acceleration of radio emitting electrons. To examine this possibility we analysed Chandra X-ray data of six clusters (including the aforementioned ones) that host MHs and correlated them with the corresponding radio data. We found that, beside R1720 and MS1455, the MHs and the CFs in R1504, R1347 and A1835 are also spatially correlated. This is consistent with the hypothesis that the electrons responsible for MH emission could be re- accelerated by sloshing induced MHD turbulence.