Browsing by Author "Mukadam, Anjum S."

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    Constraining the Angular Momentum Evolution of V455 Andromedae
    (2016-04-10) Mukadam, Anjum S.; Pyrzas, Stylianos; Townsley, D. M.; Gänsicke, B. T.; Hermes, J. J.; Szkody, Paula; Kemp, Jonathan; Patterson, J.; Ding, Claire; Wolf, Katie; Gemma, Marina; Karamehmetoglu, Emir; Rock, John; University of Alabama Tuscaloosa
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    Enigmatic Recurrent Pulsational Variability of the Accreting White Dwarf EQ LYN (SDSS J074531.92+453829.6)
    (2013-09) Mukadam, Anjum S.; Townsley, D. M.; Szkody, Paula; Gänsicke, B. T.; Southworth, J.; Brockett, T.; Parsons, S.; Hermes, J. J.; Montgomery, M. H.; Winget, D. E.; Harrold, S.; Tovmassian, G.; Zharikov, S.; Drake, A. J.; Henden, A.; Rodriguez-Gil, P.; Sion, E. M.; Zola, S.; Szymanski, T.; Pavlenko, E.; Aungwerojwit, A.; Qian, S.-B.; University of Alabama Tuscaloosa
    Photometric observations of the cataclysmic variable EQ Lyn (SDSS J074531.92+453829.6), acquired from 2005 October to 2006 January, revealed high-amplitude variability in the range 1166–1290 s. This accreting white dwarf underwent an outburst in 2006 October, during which its brightness increased by at least five magnitudes, and it started exhibiting superhumps in its light curve. Upon cooling to quiescence, the superhumps disappeared and it displayed the same periods in 2010 February as prior to the outburst within the uncertainties of a couple of seconds. This behavior suggests that the observed variability is likely due to nonradial pulsations in the white dwarf star, whose core structure has not been significantly affected by the outburst. The enigmatic observations begin with an absence of pulsational variability during a multi-site campaign conducted in 2011 January–February without any evidence of a new outburst; the light curve is instead dominated by superhumps with periods in the range of 83–87 minutes. Ultraviolet Hubble Space Telescope time-series spectroscopy acquired in 2011 March reveals an effective temperature of 15,400 K, placing EQ Lyn within the broad instability strip of 10,500–16,000 K for accreting pulsators. The ultraviolet light curve with 90% flux from the white dwarf shows no evidence of any pulsations. Optical photometry acquired during 2011 and Spring 2012 continues to reflect the presence of superhumps and an absence of pulsations. Subsequent observations acquired in 2012 December and 2013 January finally indicate the disappearance of superhumps and the return of pulsational variability with similar periods as previous data. However, our most recent data from 2013 March to May reveal superhumps yet again with no sign of pulsations. We speculate that this enigmatic post-outburst behavior of the frequent disappearance of pulsational variability in EQ Lyn is caused either by heating the white dwarf beyond the instability strip due to an elevated accretion rate, disrupting pulsations associated with the He ii instability strip by lowering the He abundance of the convection zone, free geometric precession of the entire system, or appearing and disappearing disk pulsations.
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    First Unambiguous Detection of the Return of Pulsations in the Accreting White Dwarf SDSS J074531.92+453829.6 After an Outburst
    (2011-02-20) Mukadam, Anjum S.; Townsley, D. M.; Szkody, P.; Gänsicke, B. T.; Winget, D. E.; Hermes, J. J.; Howell, Steve B.; Teske, J.; Patterson, Joseph; Kemp, Jonathan; Armstrong, Eve; University of Alabama Tuscaloosa
    The primary white dwarf of the cataclysmic variable SDSS J074531.92+453829.6 was discovered to exhibit nonradial pulsations in 2006 January. This accreting white dwarf underwent its first recorded dwarf nova outburst in 2006 October, during which its brightness increased by more than 5 mag. A Hubble Space Telescope (HST) ultraviolet spectrum, obtained one year after the outburst, revealed a white dwarf temperature of 16,500 K, hotter than all other known accreting white dwarf pulsators. This implies that the accreting primary white dwarf of SDSS J074531.92+453829.6 was heated to temperatures beyond the instability strip during the outburst. Optical observations acquired a year after the outburst did not reveal any evidence of pulsations, suggesting that the white dwarf had not cooled to quiescence by then. We recently acquired optical high-speed time-series photometry on this cataclysmic variable SDSS J074531.92+453829.6 more than three years after its outburst to find that pulsations have now returned to the primary white dwarf. Moreover, the observed pulsation periods agree with pre-outburst periods within the uncertainties of a few seconds. This discovery is significant because it indicates that the outburst did not affect the interior stellar structure, which governs the observed pulsation frequencies. It also suggests that the surface of the white dwarf has now cooled to quiescence. Using this discovery in addition to the prior HST temperature measurement of 16,500 K, we have been able to constrain the matter accreted during the 2006 outburst. This is the first time an accreting white dwarf was unambiguously observed to resume pulsating after an outburst.
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    GW Librae: Still Hot Eight Years Post-Outburst
    (2016-08) Szkody, Paula; Mukadam, Anjum S.; Gänsicke, Boris T.; Chote, Paul; Nelson, Peter; Myers, Gordon; Toloza, Odette; Waagen, Elizabeth O.; Sion, Edward M.; Sullivan, Denis J.; Townsley, Dean M.; University of Alabama Tuscaloosa
    We report continued Hubble Space Telescope (HST) ultraviolet spectra and ground-based optical photometry and spectroscopy of GW Librae eight years after its largest known dwarf nova outburst in 2007. This represents the longest cooling timescale measured for any dwarf nova. The spectra reveal that the white dwarf still remains about 3000 K hotter than its quiescent value. Both ultraviolet and optical light curves show a short period of 364–373 s, similar to one of the non-radial pulsation periods present for years prior to the outburst, and with a similar large UV/optical amplitude ratio. A large modulation at a period of 2 hr (also similar to that observed prior to outburst) is present in the optical data preceding and during the HST observations, but the satellite observation intervals did not cover the peaks of the optical modulation, and so it is not possible to determine its corresponding UV amplitude. The similarity of the short and long periods to quiescent values implies that the pulsating, fast spinning white dwarf in GW Lib may finally be nearing its quiescent configuration.
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    HST and Optical Data Reveal White Dwarf Cooling, Spin, and Periodicities in GW Librae 3-4 Years After Outburst
    (2012-07-10) Szkody, Paula; Mukadam, Anjum S.; Gänsicke, Boris T.; Henden, Arne; Sion, Edward M.; Townsley, Dean M.; Chote, Paul; Harmer, Diane; Harpe, Eric J.; Hermes, J. J.; Sullivan, Denis J.; Winget, D. E.; University of Alabama Tuscaloosa
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    Hubble Space Telescope and Ground-Based Observations of V455 Andromedae Post-Outburst
    (2013-09-20) Szkody, Paula; Mukadam, Anjum S.; Gänsicke, Boris T.; Henden, Arne; Sion, Edward M.; Townsley, Dean M.; Christian, Damian; Falcon, Ross E.; Pyrzas, Stylianos; Brown, Justin; Funkhouser, Kelsey; University of Alabama Tuscaloosa
    Hubble Space Telescope spectra obtained in 2010 and 2011, 3 and 4 yr after the large amplitude dwarf nova outburst of V455 And, were combined with optical photometry and spectra to study the cooling of the white dwarf, its spin, and possible pulsation periods after the outburst. The modeling of the ultraviolet (UV) spectra shows that the white dwarf temperature remains ∼600 K hotter than its quiescent value at 3 yr post-outburst, and still a few hundred degrees hotter at 4 yr post-outburst. The white dwarf spin at 67.6 s and its second harmonic at 33.8 s are visible in the optical within a month of outburst and are obvious in the later UV observations in the shortest wavelength continuum and the UV emission lines, indicating an origin in high-temperature regions near the accretion curtains. The UV light curves folded on the spin period show a double-humped modulation consistent with two-pole accretion. The optical photometry 2 yr after outburst shows a group of frequencies present at shorter periods (250–263 s) than the periods ascribed to pulsation at quiescence, and these gradually shift toward the quiescent frequencies (300–360 s) as time progresses past outburst. The most surprising result is that the frequencies near this period in the UV data are only prominent in the emission lines, not the UV continuum, implying an origin away from the white dwarf photosphere. Thus, the connection of this group of periods with non-radial pulsations of the white dwarf remains elusive.
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    Hubble Space Telescope and Optical Data on SDSSJ0804+5103 (EZ Lyn) One Year After Outburst
    (2013-05) Szkody, Paula; Mukadam, Anjum S.; Sion, Edward M.; Gänsicke, Boris T.; Henden, Arne; Townsley, Dean M.; University of Alabama Tuscaloosa
    We present an ultraviolet (UV) spectrum and light curve of the short orbital period cataclysmic variable EZ Lyn obtained with the Cosmic Origins Spectrograph on the Hubble Space Telescope 14 months after its dwarf nova outburst, along with ground-based optical photometry. The UV spectrum can be fit with a 13,100 K, log g = 8 white dwarf using 0.5 solar composition, while fits to the individual lines are consistent with solar abundance for Si and Al, but only 0.3 solar for C. The Discrete Fourier Transforms of the UV and optical light curves at 14 months following outburst show a prominent period at 256 s. This is the same period reported by Pavlenko in optical data obtained seven months and one year after outburst, indicating its long-term stability over several months, but this period is not evident in the pre-outburst data and is much shorter than the 12.6 minute period that was seen in observations obtained during an interval from 8 months to 2.5 yr after the 2006 outburst. In some respects, the long and short periods are similar to the behavior seen in GW Lib after its outburst but the detailed explanation for the appearance and disappearance of these periods and their relation to non-radial pulsation modes remain to be explored with theoretical models.
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    Multi-Site Observations of Pulsation in the Accreting White Dwarf SDSS J161033.64-010223.3 (V386 Ser)
    (2010-05-10) Townsley, Dean M.; Mukadam, Anjum S.; Gänsicke, B. T.; Szkody, P.; Marsh, T. R.; Robinson, E. L.; Bildsten, L.; Aungwerojwit, A.; Schreiber, M. R.; Southworth, J.; Schwope, A.; For, B. Q.; Tovmassian, G.; Zharikov, S. V.; Hidas, M. G.; Baliber, N.; Brown, T.; Woudt, P. A.; Warner, B.; O'Donoghue, D.; Buckley, D. A. H.; Sefako, R.; Sion, E. M.; University of Alabama Tuscaloosa