| 1. |
- Ho, Anna Y. Q., et al.
(author)
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iPTF Archival Search for Fast Optical Transients
- 2018
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In: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 854:1
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Journal article (peer-reviewed)abstract
- There has been speculation about a class of relativistic explosions with an initial Lorentz factor Gamma(init) smaller than that of classical gamma-ray bursts (GRBs). These dirty fireballs would lack prompt GRB emission but could be pursued via their optical afterglow, appearing as transients that fade overnight. Here we report a search for such transients (that fade by 5-sigma in magnitude overnight) in four years of archival photometric data from the intermediate Palomar Transient Factory (iPTF). Our search criteria yielded 50 candidates. Of these, two were afterglows to GRBs that had been found in dedicated follow-up observations to triggers from the Fermi GRB Monitor. Another (iPTF14yb) was a GRB afterglow discovered serendipitously. Eight were spurious artifacts of reference image subtraction, and one was an asteroid. The remaining 38 candidates have red stellar counterparts in external catalogs. The photometric and spectroscopic properties of the counterparts identify these transients as strong flares from M dwarfs of spectral type M3-M7 at distances of d approximate to 0.15-2.1 kpc; three counterparts were already spectroscopically classified as late-type M stars. With iPTF14yb as the only confirmed relativistic outflow discovered independently of a high-energy trigger, we constrain the all-sky rate of transients that peak at m = 18 and fade by Delta m = 2 mag in Delta t = 3 hr to be 680 yr(-1), with a 68% confidence interval of 119-2236 yr(-1). This implies that the rate of visible dirty fireballs is at most comparable to that of the known population of long-duration GRBs.
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| 2. |
- Hussain, Muhammad Waqar, 1985-
(author)
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High-Temperature Radio Circuits in Silicon Carbide Bipolar Technology
- 2019
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Doctoral thesis (other academic/artistic)abstract
- High-temperature electronics find many niche applications in downhole drilling, aviation, automotive and future exploration of inner planets like Venus and Mercury. Past studies have shown the potential of silicon carbide (SiC) electronics for catering these extreme temperature applications. In particular, analog, digital and mixed-signal integrated circuits, based on in-house SiC bipolar technology, have been shown to operate successfully for temperatures as high as 500 oC. This thesis aims at exploring the potential of in-house SiC bipolar technology for realizing high-temperature radio frequency (RF) circuits.To that end, the in-house SiC bipolar junction transistors (BJTs) are first characterized up to 300 oC for RF figures of merit like unity current gain bandwidth and unity power gain bandwidth. The measurement results showed the feasibility of the current batch of SiC BJTs for developing RF circuits operating at low-end of very high frequency (VHF) band. Thereafter, three fundamental blocks of a high-temperature radio receiver, i.e. an intermediate-frequency amplifier, an oscillator and a down-conversion mixer were implemented. Firstly, an intermediate-frequency amplifier has been designed and measurement results demonstrated operation up to 251 oC. The proposed amplifier achieved a gain, input, and output matching of 16 dB, -7.5 dB and -11.2 dB, respectively, at 54.6 MHz and 251 oC. Next, 500 oC operation of an active down-conversion mixer has been exhibited. Measurements have shown that the conversion gain of the proposed mixer is 4.7 dB at 500 oC. Lastly, a negative resistance oscillator has been designed and tested successfully up to 400 oC. It has been shown that at 400 oC, the proposed oscillator delivers an output power of 8.4 dBm into a 50 Ω load.In addition to SiC BJTs, the aforementioned circuits also employed spiral inductors implemented on PCBs, commercially available ceramic capacitors and thick-film resistors. Therefore, this thesis presents the evaluation of passives to assess their feasibility for high temperature operation. This work also identifies and addresses several challenges associated with the development flow of high-temperature RF circuits.
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