Pulsars, transients and relativistic astrophysics

Abstract:

Core-collapse supernovae (CCSNe) are the bright explosions of massive stars. During the explosion heavy elements are produced by nuclear burning. One of these products is 56Ni that radioactively decays into 56Co. The explosion energy and the produced 56Ni and its product 56Co are what powers the light curves of classical supernovae (SNe). Stripped envelope (SE) SNe have lost their hydrogen (H) and in some cases, helium (He) envelopes through mass loss at some point in their lives. These SE SNe are known to produce SNe of types IIb (weak H), Ib (He-rich), Ic (He-poor), and Ic-BL (He-poor broad lines). Type IIn SNe, on the other hand, are SNe that interact with circumstellar medium (CSM) around the progenitor. The CSM is thought to be caused by the progenitor’s mass loss. The mechanism of the mass loss for these can happen in a variety of ways. All stars have mass loss through stellar winds, but in some cases, it is not enough to produce interaction that would produce a type IIn. The mass loss can also happen because of pair instability pulsations, eruptions, or binary effects. The mass loss can be studied by analysing the observational properties of a SN and understanding the mass loss might shed light on what kind of progenitor produced the SN. In this thesis, photometric and spectroscopic data of SN 2017dio are analysed. The photometric data are used to study the explosion epoch, light curves, and color curves of SN 2017dio and it is compared with four other SNe. The spectroscopic data are used to verify the classification of SN 2017dio, to study the spectral evolution, and to discuss the possible CSM properties and progenitor scenario. The findings indicate that SN 2017dio is a SN of type Ic-BL interacting with H-rich CSM. Both spectroscopic and photometric analysis support the theory of the CSM not being close to the explosion site and the calculated mass loss rate 0.04M⊙/year indicates that the progenitor must have experienced massive mass loss periods in the decade before its explosion

Abstract:

Modern radio telescopes are routinely reaching depths where normal star-forming galaxies are the dominant observed population. Realizing the potential of radio as a tracer of star formation and black hole activity over cosmic time involves achieving such depths over representative volumes, with radio forming part of a larger multiwavelength campaign. In pursuit of this, we used the Karl G. Jansky Very Large Array (VLA) to image ∼5 deg2 of the VIDEO/XMM-LSS extragalactic deep field at 1–2 GHz. We achieve a median depth of 16 µJy beam−1 with an angular resolution of 4.5 arcsec. Comparisons with existing radio observations of XMM-LSS showcase the improved survey speed of the upgraded VLA: we cover 2.5 times the area and increase the depth by ∼20 per cent in 40 per cent of the time. Direction-dependent calibration and wide-field imaging were required to suppress the error patterns from off-axis sources of even modest brightness. We derive a catalogue containing 5762 sources from the final mosaic. Sub-band imaging provides in-band spectral indices for 3458 (60 per cent) sources, with the average spectrum becoming flatter than the canonical synchrotron slope below 1 mJy. Positional and flux density accuracy of the observations, and the differential source counts are in excellent agreement with those of existing measurements. A public release of the images and catalogue accompanies this article.

Abstract:

We present high-cadence multifrequency radio observations of the long gamma-ray burst (GRB) 190829A, which was detected at photon energies above 100 GeV by the High Energy Stereoscopic System (H.E.S.S.). Observations with the Meer Karoo Array Telescope (MeerKAT, 1.3 GHz) and Arcminute Microkelvin Imager – Large Array (AMI-LA, 15.5 GHz) began one day post-burst and lasted nearly 200 d. We used complementary data from Swift X-Ray Telescope (XRT), which ran to 100 d post-burst. We detected a likely forward shock component with both MeerKAT and XRT up to over 100 d post-burst. Conversely, the AMI-LA light curve appears to be dominated by reverse shock emission until around 70 d post-burst when the afterglow flux drops below the level of the host galaxy. We also present previously unpublished observations of the other H.E.S.S.-detected GRB, GRB 180720B from AMI-LA, which shows likely forward shock emission that fades in less than 10 d. We present a comparison between the radio emission from the three GRBs with detected very high energy (VHE) gamma-ray emission and a sensitivity-limited radio afterglow sample. GRB 190829A has the lowest isotropic radio luminosity of any GRB in our sample, but the distribution of luminosities is otherwise consistent, as expected, with the VHE GRBs being drawn from the same parent distribution as the other radio-detected long GRBs.