CG Dra: a VSS campaign

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  • #611537
    Jeremy Shears
    Participant

    In common with its CV brethren, CG Dra is determined to keep use guessing, Max!
    Thanks for posting the LC showing the series of recent outbursts.

    #611538
    Maxim Usatov
    Participant

    Thanks, Jeremy. It just went into outburst at 15.85 mag, but due to the software issues in the observatory I was only able to obtain 5 measurements tonight.
    Max

    #611610
    Maxim Usatov
    Participant

    Eclipses No(s): 51
    State: Super-outburst

    The previous long state of quiescence in the system has resulted in a super-outburst, with CG Dra now reaching 15.46 mag. It appears that all the material that has been flowing from the secondary was accumulating in the accretion disk and is now being dumped onto the white dwarf at a higher than usual rate.

    The eclipse profile is V/L/PEH – V-shaped, low orbital hump and with a post-egress hump. Whether it’s a hump and not an isolated flickering event, is difficult to say, but I’m inclining towards the PEH scenario. This is the first V-shaped counterpart seen to PEH eclipses previously observed twice at and after normal outbursts.

    The PEH shape is similar and appears to be at approximately the same phase as during the previous eclipses with this feature. I can only guess that this is the densest and the brightest part of the accretion disk, perhaps a bulge on it, or an overflow, appearing due to the increased activity and high viscosity of fully ionized material.

    This is the highest quality data I have been able to record on CG Dra, with the standard deviation of the check star below 0.02 mag. The seeing tonight was exceptionally good, and I am glad I was able to fix some of the software issues to capture this data. The light curve is able to reveal the character of flickering, which likely originates in the accretion disk or the bright spot. There is no egress standstill which means that the spot is likely extended in size.

    There is still some amount of bright spot hump visible in the ingress portion of the profile – it is slightly asymmetrical – which, probably, means that the bright spot is still contributing some considerable portion of the system’s light, despite it being in the super-outburst with hot, fully ionized accretion disk.

    Max

    #611613
    Jeremy Shears
    Participant

    Those data are truly remarkable regarding their precision, Max. I suppose technically this is a bright outburst (no superhumps modulated with the Porb, which is too long to be a UGSU system).

    I wonder if there is any way of probing (or estimating) how much of the disc goes into outburst. There is an idea that only part of the AD goes into outburst, resulting in these “stunted” outbursts. I therefore wonder if the eclipse width of this bright outburst is wider. Or perhaps this current outburst is a “normal” one, like the one 5 outbursts ago at the beginning of your second plot above, the intervening 4 being “stunted” outbursts……

    #611614
    Maxim Usatov
    Participant

    Totally makes sense. Are superhumps a pre-requisite for an outburst to be called a “superoutburst”?

    Wild idea… Could CG Dra be an intermediate between U Gem and SU UMa with q ~ 0.3, spending most of the time with a circular disk but sometimes having an elliptical disk. Would a post-egress hump observed qualify for a “weak superhump”?

    #611615
    Jeremy Shears
    Participant

    Superhumps are characteristic of a DN superoutburst. There are indeed caused by the AD becoming elliptical. So there would need to be a series of humps, the period of which would be slightly longer than Porb. Have you measured the “hump period” to check that it is consistent with the eclipse period?

    #611616
    Maxim Usatov
    Participant

    Thanks. I haven’t measured the period, as these PEHs only appear occasionally. This is the third time I am seeing such a hump.

    #611617
    Maxim Usatov
    Participant

    Here is the eclipse width. Red is the current bright outburst, black are the previous three outbursts. I didn’t align them vertically, that’s why they appear as a band. The current outburst profile does not appear to be wider than the previous ones. (Ignore the phase axis, as I haven’t set the epoch.)

    Max

    • This reply was modified 2 months, 1 week ago by Maxim Usatov.
    #611622
    Maxim Usatov
    Participant

    Sorry, always have something to say after the “Edit” button disappears..

    What if CG Dra disk is becoming elongated, responsible for the PEHs during or near some of the high-mass transfer outbursts, but then, as its radius changes, becomes circular again, so it does not precess at all? This would explain why PEHs do not appear in quiescence and that there is no beating period. What I am thinking of is a borderline U Gem/SU UMa case with weak temporary resonance events, whereby superhumps are invisible in quiescence and appear only occasionally during the outbursts. Of all outburst peaks, profiles without PEHs were seen 5 times so far, and 2 with PEHs, so roughly half to a third of the time we see those humps at outburst peaks.

    But I really like the “partial AD outburst” idea.

    Max

    • This reply was modified 2 months, 1 week ago by Maxim Usatov.
    • This reply was modified 2 months, 1 week ago by Maxim Usatov.
    #611625
    Jeremy Shears
    Participant

    Would need to think about that Max 🙂
    It’s great fun having these speculations. We’d probably not be in the position to do so with your intense and precise data.

    #611629
    Maxim Usatov
    Participant

    Eclipses No(s): 52, 53
    State: Bright outburst

    As per discussion with Jeremy Shears, I am now calling this CG Dra state a “bright outburst,” to avoid confusion with SU UMa superoutbursts.

    Post-egress humps (PEH) amplitude appears to have decreased significantly to barely noticeable in the light curve (more pronounced in the phase plot), so I am filing these as U/L/S and V/L/S eclipses with low (or no) orbital hump and symmetric profiles. It’s actually difficult to say what qualifies as a symmetric profile, as we still can observe a gentler egress in both eclipses. Nevertheless, given that the egress magnitude is approximately equal to the ingress, ignoring very mild PEH, I think we can call this a symmetric profile.

    Interesting dips in the curve. We can observe normal ~ 0.1 mag flickering throughout the curve except the minima, however, there is also 0.15 mag dip near the center of the chart, followed by a flat top. Note that flickering decreases down to ~ 0.05 mag at both minima. This may mean that high-amplitude and slow flickering events originate in the bright spot, while low-amplitude and fast flickering originates in the uneclipsed parts of the accretion disk.

    Max

    Attachments:
    #611639
    Maxim Usatov
    Participant

    Eclipses No(s): 54
    State: Bright outburst
    U/L/S-type eclipse profile: U-shaped, low to none orbital hump, symmetric.

    Attachments:
    #611644
    Maxim Usatov
    Participant

    Eclipses No(s): 55, 56
    State: Fading
    CG Dra is now fading after the bright outburst, reaching normal outburst luminosity. Two highly symmetric U/L/S-type eclipses signifying that the accretion disk is still bright, hot and ionized, concealing the bright spot.
    Max

    Attachments:
    #611646
    Jeremy Shears
    Participant

    Those 2 look very similar indeed! Slightly quicker fade than recovery

    #611647
    Maxim Usatov
    Participant

    Yes, the eclipse is also a little deeper and narrower. Could be the AD has shrunk?

    #611710
    Maxim Usatov
    Participant

    Eclipses No(s): 57, 58
    State: Fading
    A pair of U/N/A eclipses, normal orbital hump (albeit ~20% lower amplitude), asymmetric.

    Attachments:
    #611714
    Jeremy Shears
    Participant

    They may even be a fraction deeper that’s the last pair, Max. Asymmetric, as before, as you noted, perhaps steeper entry…

    #611753
    Maxim Usatov
    Participant

    Eclipses No(s): 59
    State: Quiescence

    CG Dra back to quiescent state with a typical U/N/A eclipse, normal ~ 0.1 mag orbital hump amplitude indicating a mix of accretion disk and bright spot components. Mediocre data quality due to the clouds. Roof was closed 9 times this night, so this was a good test of automatic telescope wake-up routines.

    Max

    Attachments:
    #611763
    Maxim Usatov
    Participant

    Eclipses No(s): 60
    State: Quiescence
    V/N/A. Intermittent clouds.

    Attachments:
    #611779
    Maxim Usatov
    Participant

    Eclipses No(s): 61, 62
    State: Quiescence

    CG Dra can be very deceptive. Look at how different eclipse profiles are, separated by just 4 hours. Poor quality data, combined with flickering due to the bright spot on the accretion disk of the white dwarf makes the second eclipse appear almost symmetrical and much deeper than the first one. Wild flickering creates bumps in the orbital hump, sometimes right before the ingress (2nd eclipse). Random patterns due to the measurement uncertainty at the eclipse minimum (1st eclipse), then, removes or adds depth, and flickering at the egress (2nd eclipse) may occasionally play another joke. I think this is what is happening here. Two asymmetric eclipses with normal ~ 0.1 mag orbital hump (U/N/A), distorted by uncertainty and flickering, so they appear to be different.

    A question: should I continue assigning eclipse types just as they appear on the light curve, like I did before, or should I compensate for these distortions due to poor quality data? There is more value in compensating, but if I do this, I’ll need to reclassify all poor-quality eclipses I’ve filed earlier (Whew…)

    It would also be interesting to correlate FWHM with sigma. The night was pretty clear and dark. There was no extreme in temperatures. Why photometry quality suffered – I don’t know.

    Max

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