Project Leader: Tom Richards

U Sco is a top priority target while it is still bright. CCD time series observations are especially important. Send to AAVSO and CBA. VSS participants please send all observations to Tom Richards for inclusion in our analyses, data tables and plots. See Brad Schaefer’s email below, and VSS data submission guidelines here. Keep abreast of developments in the frequently updated news item on U Sco.

Links:

• AAVSO VSX data sheet on U Sco
• Excel file of all U Sco Data to date, with plots (always promptly updated)
• U Sco Finder Charts
• Times of Eclipses (try to get time series on these!)
• U Sco Eclipse Data – 2010-02-08 (Bill Allen, Tom Richards [@tom-richards])
• A look at the VSS Data
• ATel 2452 report, 2010-02-20

RA 16 22 30.78, Dec -17 52 42.8 (J2000) GCVS type NR+E V mag range 8.7-19.3 Epoch JD 2447717.6145, Period 1.2305522 d.

The U Sco Eruption

More information regularly posted on CBA and AAVSO egroups.

Read AAVSO Alert Notice 415.

The following email from Brad Schaefer (Fri 2010/02/05) sets out the scientific importance of amateur observations of this eruption, and how to observe and submit to CBA.

Hi;

Just about now, the recurrent nova U Sco (now at V=11.5 in its eruption just one week old) will start showing eclipses, and there is high science from getting detailed time series of these eclipses. And the CBA is perfect for getting exactly that. So I am appealing that all hands take long time series of U Sco around these eclipses when U Sco is up in the pre-dawn sky. (Even northern hemisphere observers can get a few hours, and we need this to patch together the whole eclipse light curve.)

In quiescence, U Sco has a deep total eclipse. This provides a unique and wonderful opportunity to do important front-line science between now and early March, even with smallish-telescopes. Here are three big-time science items that CBA is perfect to get: (1) Eclipse mapping of the light distribution across the photosphere. As the companion star blocks out the optical light from the photosphere around the white dwarf, the variations will define the size and structure of the photosphere. Such has never been done before. (2) The center of the eclipse timings can be used with the center of eclipse timings in the tails of the last two eruptions (1987 and 1999) so as to determine the change in the orbital period across the 1999 eruption. This change will by Kepler’s Law translate directly into a mass ejected. This method is the only reliable way to get the ejected mass (the other old ways are all horribly assumption ridden and they are accurate only to perhaps 1-2 orders of magnitude). And the knowing the ejected mass will tell us whether the white dwarf is net gaining in mass over each eruption cycle, and this will directly answer the old uber-important Type Ia supernova progenitor problem. (3) The eclipse depth varies from early in the eruption when the expanding shell is optically thick until late in the tail when it is optically thin. A measure of the eclipse depth will directly give the optical depth from the center of the binary all the way out. This will then (for knowing the opacity of the shell from physics) translate into the total mass ejected. This is a totally new way of measuring the mass ejected, and I want to try this on the U Sco eruption.

This method is totally independent of the big-troubles of the old methods (like uncertain distances, filling factors, ionization states…), and hopefully will be good. Then, as in item 2, we get the mass ejected and this plays into the uber-important Type Ia SN progenitor problem.

So we’ve got big stakes to play for, and CBA is perfectly set up.

First, you have a good distribution in longitude, with this allowing for coverage of mulitple eclipses. Second, you have multiple observers with distribution in longitude, as this is required to patch together light curves to get a whole eclipse. The problem is that U Sco is close enough to the Sun, that any one observer can only cover 1-3 hours between U Sco rise and dawn, so multiple observers are required to cover the ~6 hour duration elcipses. Third, you don’t have large scopes, and you are set up perfectly for time series photometry. U Sco is now at V=11.5, and by the end of March it will be V=14. This is just perfect for CBA and useless for the big scopes.

So here are the details of what to do. Use the list below to select which mornings will have U Sco observable for your location for times within 5 hours of the quoted eclipse center times. (We must have observations around 5 hours of so on either side of eclipse so as to know the uneclipsed level.) When U Sco rises high enough (push this a little if you can), start taking your normal time series and just keep at this until dawn makes you stop. Choose the shortest reasonable exposure time such that U Sco and one of the comparison stars (preferrably COMP) are not saturated. For the comparison stars, see the attached files (in PDF and WORD formats) for finder charts on all scales as well and the magnitudes for all the indicated stars. If you have a V-filter then use it (R-band is the only other option to use). If you need more signal or do not have a V or R filter, then simply run without any filter. After dawn, process your images (bias subtract, flat field…) in the usual way. Extract the magnitude of U Sco with aperture photometry in comparison with one or more of the stated comparison stars (preferably COMP). Send these magnitudes to the usual CBA data site. It is all easy, and much of what you already do so well.

U Sco Telegram

The Astronomer’s Telegram http://www.astronomerstelegram.org

ATEL #2452

Title:	Recurrent Nova U Sco Shows Deep Optical Eclipses During Plateau
Phase
Author:	Bradley E. Schaefer, Ashley Pagnotta (Louisiana State University), Bill Allen, Tut Campbell , Tom Krajci, Thomas Richards, George Roberts, William Stein, Chris Stockdale (Center for Backyard Astrophysics), Shawn Dvorak, Tomas Gomez, Barbara G. Harris, George Sjoberg, Thiam Guan Tan (AAVSO), Arto Oksanen (Caisey Harlingten Observatory), and Gerald Handler (University of Vienna)
Queries:	schaefer@lsu.edu
Posted:	20 Feb 2010 8:44 UT
Subjects:	Optical, Novae

U Sco is a recurrent nova whose long-predicted eruption started on 2010 Jan 28.4385 UT (IAUC #9111), peaked brighter than V=7.8, faded with the fastest rate of all known novae until Feb 9 whereupon the light curve leveled off in a plateau at V=14.0. The optical light curve in 2010 is closely following those of all nine previous eruptions (Schaefer 2010, ApJSupp, in press, arXiv:0912.4426). Flickering in the light curve was first reported on Feb 5.174 (IAUC #9114). U Sco started as a weak relatively hard X-ray source (ATEL #2419), then became a supersoft X-ray source (ATEL #2430) at the same time the plateau started. U Sco produces deep eclipses in the UV (with depth 0.5-0.7 mag and FWHM duration around 5.3 hours) and probably shows broad X-ray eclipses with ~40% amplitude on Feb 12.46, 13.69, and 14.92 (ATEL #2442).

We have measured 5109 magnitudes, from the discovery images up to Feb 18.30. Observations were made with many telescopes in the United States (Florida, Arkansas, and New Mexico), New Zealand, Australia, South Africa, Spain, and Chile. The magnitudes are all from CCD images, either through a V-band filter or without a filter (resulting in effective wavelengths of near 6100A). Each unfiltered data set was normalized to the Johnson V-band by means of simultaneous magnitudes taken with classical V-band photometry. We constructed a smooth light curve template and subtracted this from all magnitudes.

Before Feb 5.174, our folded light curve (see the first figure in the link below) has a RMS scatter of 0.10, which is consistent with the uncertainties in the detrending of the light curve. On Feb 3.86, we see a flicker with duration of ~1 hour and amplitude of at least 0.15 mag. No eclipses are evident.

From Feb 5.174 to Feb 11.0, U Sco had frequent large amplitude flickering (second figure). No flickers were seen after Feb 9.8. The flickers have a typical time scale of 30 minutes and amplitude of 0.4 mag. These flickers have the same amplitude and time scale as in quiescence, but with U Sco being ~60 times brighter than in quiescence, so the energy in the flickers is greatly larger than what is available in the normal disk. The RMS scatter during this time interval is 0.28 mag. No eclipses are visible in the folded light curve.

Between Feb 8.8 and Feb 11.3, U Sco goes from no eclipse to deep eclipse. The folded light curve for Feb 11.0-18.3 (third figure) shows a highly significant eclipse roughly 0.6 mag deep. The light curve shape appears to be constant over this entire interval. The FWHM of the eclipse is 0.18+-0.02 in phase (5.3+-0.6 hours), while the total duration of the eclipse is 0.29+-0.06 in phase (8.6+-1.8 hours). This can be compared to the eclipses in quiescence (see Figures 45-47 in Schaefer 2010), where the V-band depth is 1.3 mag, the FWHM is 0.10 in phase (3.0 hours), and the total duration is 0.18 in phase (5.3 hours). We attribute the sudden onset of eclipses to the recession of the photosphere to a radius smaller than the binary separation (i.e., ~6.4 R_sun), whereas the relative small depth and long FWHM of the eclipse show that the photosphere is somewhat larger than the size of the companion star (~2.3 R_sun). Another implication is that the early flickering arises from the shell, not the accretion disk.

The folded light curve also appears to show a shallow secondary ‘eclipse’. While the companion star (a G5 IV star) is normally at V=18.9 alone, perhaps the apparent secondary minimum is caused by the photosphere around the white dwarf covering the brilliantly irradiated companion star. Another intriguing feature is the asymmetry where U Sco is brighter at phase 0.25 by 0.11+-0.04 mag than at phase 0.75. This poorly-understood situation is similar to that of the recurrent novae CI Aql and V394 CrA in quiescence (Figs 44 and 48, Schaefer 2010).

U Sco detrended light curve folded on the orbital period for three time intervals: http://web.me.com/bradschaefer/Site/U_Sco.html

Password Certification: Bradley E. Schaefer (schaefer@lsu.edu) http://www.astronomerstelegram.org/?read=2452

Eclispe Times

Observations 2010-02-08

W.H. Allen, Vintage Lane Observatory, Blenheim, New Zealand T.J. Richards, Woodridge Observatory, Eltham, Victoria, Australia

Updated 2010-02-15.

A comparison of photometric data on the current eruption of the eclipsing recurrent nova U Sco by the authors on 2008-02-08 UTC indicated that a broad eclipse of the primary had been observed. Central eclipse is estimated at HJD 2455236.18(1).

Allen used a 0.4 m Cassegrain and SBIG STL 1001E CCD camera, unfiltered; and Richards an RCOS 0.4 m R-C and Apogee U9 CCD camera, also unfiltered. Allen’s data covered HJD 2455000+236.0799 to 236.1985 (171 m), and Richards’ 236.2164 to 236.2767 (87 m), leaving a gap of 26 m between data sets. Allen used as comparison star 146 in the comparison sequence for chart 1969dbj (#UC145-168958, 2MASS 1266639630, AAVSO 000-BBX-420), V=12.588; and Richards, AAVSO comp 145 (3UC145-168856, 2MASS 1266605515, AAVSO 000-BBX-407), V=14.489. Allen’s check star was AAVSO 145 with a standard deviation of 0.023 mag. Richards’s was AAVSO 143 with a standard deviation of 0.013 mag.

The data shows a smoothly decelerating decline of 0.5 mag to a broad floor, where modulations commence of up to 0.1 mag at a timescale of ~0.02 d, followed by a shallower 0.15 mag rise after the data gap, with similar modulations. Any estimation of the epoch of central eclipse is highly uncertain under the circumstances. Uncertainty in Richards’s data is 0.013 mag, derived from the standard deviation of the check star AAVSO .

Data for the plot below is available for download as a tab-separated text file: U Sco Eclipse Data – 2010-02-08.

VSS Data

Please refer to the U Sco Data Excel Spreadsheet. This is updated as data is received.

Eclipse on 2010-02-08

See also the Notice about this. This eclipse was observed jointly by Bill Allen in New Zealand, then when his window closed, by Tom Richards in Victoria. This gave sufficient time coverage to ascertain it was indeed an eclipse.

There’s a lot of interest in the data plot. Bill’s decline is quite smooth with a gentle deceleration, indicating a gradation in the eclipsed object (whatever it is – expanding shell, re-forming accretion disc? An eclipse of a white dwarf would be very sudden.) Then some modulations start, of up to 0.1 mag, and lasting up to 0.03 days (45 minutes). These continue in the gentler climb out of eclipse, but a bit more erratically. Modulations had already been observed, out of eclipse, a day or so before this. This may indicate that the accretion disk, undoubtedly blown away in the explosion, is re-forming. Or not: modulations like this – flickering – are a feature of the inner accretion disk which forms last, or the bright spot where the accretion stream hits the accretion disk. The eclipse of the latter is always phased a little later than the eclipse of the primary, so flickering should be absent on egress not ingress.

A better explanation may be that the diffuse expanding photospheric nova shell has some local pulsations which were obscured on ingress but reappeared during totality. Certainly, these modulations can be seen when the system is out of eclipse. If this is right, they should die away as the shell expands and becomes more transparent.

The vertical scale in the plot “CV mag”, by the way, means Clear (unfiltered) data, using a V magnitude for the comparison star. Both of us were using 0.4 m telescopes. Bill’s precision error is 0.023 mag, and Tom’s 0.013 mag. In addition to the target photometry, we both carried out extensive photometry on ~10 comparison stars in the Henden sequence for this star, from which Brad Schaefer very kindly derived a magnitude correction of -0.71, the same for each system, to be made for our magnitude data for the target. We both have KAF chips (though different models), and the analysis indicated they both have an effective wavelength of 6100 angstroms.

It is impossible to say from this data when the moment of mid-eclipse occurred, since we can’t tell what is being eclipsed when. Maybe HJD 236.18 ± 0.01 is about as good a guess as any. Brad Schaefer reports (personal communication) that it looks a bit early, but that the phase has wandered a bit in earlier eruptions too, so it’s not surprising.

Eclipse Ingress, 2010-02-09

Again, Bill Allen and Tom Richards ended-on in their observing windows, producing a plot that is much more interesting than either half would be alone. Bill has captured a plateau stage of the system, exhibiting the small modulations discussed above. Then I observed a large hump of ~0.2 mag, followed by a decline that seems to dive below the plateau. Bill’s comparison star was AAVSO 126 (V=12.588), and mine AAVSO 145 (V=14.489). Bill’s precision error (standard deviation of AAVSO 145) is 0.013 mag, and mine 0.110 mag (standard deviation of AAVSO 143). Both used 60 s exposures, unfiltered.

Taking our mid-eclipse yesterday as HJD 236.18 and Porb=1.23 d, then the next eclipse is at HJD 237.41. If Bill’s data shows a plateau at V=14.25 then the next eclipse is beginning at HJD 237.25, preceded by wht looks like an orbital hump. That would make the leading half-eclipse width 0.16 d or 0.13 x Porb. This is consistent with our eclipse data of the day before. However, the leading “plateau” if that’s what it is, is at the same mag as the floor of the previous day’s eclipse. So either that wasn’t an eclipse, or the nova has faded over 0.4 mag in one day. Both seem unlikely to me unless the nova declined sharply due to entry into a transition phase, which seems even more unlikely. There is no CCD data in the AAVSO database for Feb 9th except mine, so we have no help there. Nothing on the 10th. On Feb 11th it’s around 14.2 V. It was brighter on the 7th.

The main conclusion to draw from this is that we are dealing with a very erratic star, and the very restricted amount of optical data coming in to the AAVSO, let alone VSS, makes it very difficult to draw inferences about the astrophysics behind what we are observing.

Project News

• U Sco fading with enigmatic eclipses February 28, 2010: Brad Schaefer reports (priv. comm.) that eclipses are getting broader and deeper, as we have (partially) observed. He writes: Perfect. I’ve just constructed the light curve for the last two days, and the eclipses are getting broader and deeper. I don’t understand this… We’re getting day-to-day changes, so it is valuable to keep looking. And the out of eclipse light ...
• U Sco Eclipse Observed February 13, 2010: A comparison of photometric data on U Sco by Bill Allen and Tom Richards on 2008-02-08 UTC indicated that a broad eclipse of the primary had been observed. Central eclipse is estimated at HJD 2455236.18(1). For more information click here.
• Eruption 2010-01-08 – Observations are top priority February 5, 2010: * The recurrent nova U Sco erupted on 2010 Jan 28th at mag 8.8. A week later it faded to V=11.5. Only seven recurrent novae (NR) are listed in the GCVS, and this one exhibits total eclipses. Because of the crucial importance of this rare class of star to understanding the mechanisms underlying novae and Type ...
• Important international campaign to detect the (very fast) rise of the recurrent Nova U Sco September 29, 2009: * The AAVSO has been promoting since 2008 a multi-wavelength campaign by Brad Schaefer (Louisiana Starte University) to monitor the next outburst of the recurrent nova U Scorpii. Since this NR erupts only about every 10 years (and is due now) and has a 4-hour rise, it is vital to have a campaign at all longitudes ...