Browsing by Author "Singh, Mridweeka"

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    A tale of three cataclysmic variables with distinct superhumps
    (2025) Joshi, Arti; Tappert, Claus; Catelan, Márcio; Schmidtobreick, Linda; Singh, Mridweeka
    Superhumps are among the most commonly observed variable features in the light curves of cataclysmic variables (CVs). To study the superhump behaviour of CVs, we present Transiting Exoplanet Survey Satellite (TESS) observations of three CVs: CRTS J110014.7+131552, SDSS J093537.46+161950.8, and [PK2008] HalphaJ130559. Among them, a super-outburst has been observed in CRTS J110014.7+131552, which is associated with the precursor outburst, where prominent superhumps have been observed during maximum of the outburst with a mean period of 0.06786(1) d. We observed variations in the superhump period, along with changes in the shape of the light curve profile and the amplitude of the superhumps during different phases of the outburst, indicating disc-radius variation as well as periodically variable dissipation at the accretion stream’s bright spot. The data on SDSS J093537.46+161950.8 reveal previously unknown variations modulated with periods of 0.06584(2) d and 2.36(2) d, related to the positive superhump and the disc-precession periods, respectively, which can reasonably be interpreted as a result of the prograde precession of an eccentric accretion disc. Despite its short orbital period, the lack of outburst activity, its stable long-term brightness, discovery spectrum, and absolute magnitude suggest that the object might not be an SU UMa type dwarf nova. Instead, it could belong to the group of high-mass-transfer CVs below the period gap: either a rare class of nova-like variables or a high-luminosity intermediate polar, a subclass of magnetic CVs. For [PK2008] HalphaJ130559, a new average orbital period of 0.15092(1) d has been identified. Additionally, this system displays previously undetected average periods of 0.14517(3) d and 3.83(1) d, which could be provisionally identified as negative superhump and disc-precession periods, respectively. If the identified simultaneous signals do indeed reflect negative superhump and disc-precession period variations, then their origin might be associated with the retrograde precession of a tilted disc and its interaction with the secondary stream.
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    SN 2020udy: A New Piece of the Homogeneous Bright Group in the Diverse Iax Subclass
    (2024) Singh, Mridweeka; Sahu, Devendra K.; Barna, Barnabas; Gangopadhyay, Anjasha; Dastidar, Raya; Teja, Rishabh Singh; Misra, Kuntal; Howell, D. Andrew; Wang, Xiaofeng; Mo, Jun; Yan, Shengyu; Hiramatsu, Daichi; Pellegrino, Craig; Anupama, G. C.; Joshi, Arti; Bostroem, K. Azalee; Burke, Jamison; McCully, Curtis; Subramanian, Rama, V; Li, Gaici; Xi, Gaobo; Li, Xin; Li, Zhitong; Srivastav, Shubham; Im, Hyobin; Dutta, Anirban
    We present optical observations and analysis of the bright type Iax supernova SN 2020udy hosted by NGC 0812. The evolution of the light curve of SN 2020udy is similar to that of other bright type Iax SNe. Analytical modeling of the quasi-bolometric light curves of SN 2020udy suggests that 0.08 +/- 0.01 M circle dot of 56Ni would have been synthesized during the explosion. The spectral features of SN 2020udy are similar to those of the bright members of type Iax class, showing a weak Si ii line. The late-time spectral sequence is mostly dominated by iron group elements with broad emission lines. Abundance tomography modeling of the spectral time series of SN 2020udy using TARDIS indicates stratification in the outer ejecta; however, to confirm this, spectral modeling at a very early phase is required. After maximum light, uniform mixing of chemical elements is sufficient to explain the spectral evolution. Unlike in the case of normal type Ia SNe, the photospheric approximation remains robust until +100 days, requiring an additional continuum source. Overall, the observational features of SN 2020udy are consistent with the deflagration of a carbon-oxygen white dwarf.
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    Unveiling the nature of two dwarf novae: CRTS J080846.2+313106 and V416 Dra
    (2024) Joshi, Arti; Catelan, Marcio; Scaringi, Simone; Schwope, Axel; Anupama, G. C.; Rawat, Nikita; Sahu, Devendra K.; Singh, Mridweeka; Dastidar, Raya; Subramanian, Rama Venkata; Rao, Srinivas M.
    We present the analysis of optical photometric and spectroscopic observations of two non-magnetic cataclysmic variables, namely CRTS J080846.2+313106 and V416 Dra. We find CRTS J080846.2+313106 to vary with a period of 4.9116 +/- 0.0003 h, which was not found in earlier studies and which we provisionally suggest is the orbital period of the system. In both long-period systems, the observed dominant signal at the second harmonic of the orbital frequency and the orbital modulation during quiescence are suggestive of ellipsoidal variation from changing aspects of the secondary, with an additional contribution from the accretion stream or hotspot. However, during the outburst, the hotspot itself is overwhelmed by the increased brightness, which is possibly associated with the accretion disc. The mid-eclipse phase for V416 Dra occurs earlier and the width of the eclipse is greater during outbursts compared to quiescence, suggesting an increased accretion disc radius during outbursts. Furthermore, from our investigation of the accretion disc eclipse in V416 Dra, we find that a total disc eclipse is possible during quiescence, whereas the disc seems to be partially obscured during outbursts, which further signifies that the disc may grow in size as the outburst progresses. The optical spectra of CRTS J080846.2+313106 and V416 Dra are typical of dwarf novae during quiescence, and they both show a significant contribution from the M2-4V secondary. The light curve patterns, orbital periods, and spectra observed in the two systems look remarkably similar, and seem to resemble the characteristics of U Gem-type dwarf novae.