Alert Notice 729: Photometry and Spectroscopy requested for V406 Vul (XTE J1859+226)

February 12, 2021

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Dr. Eric Bellm (University of Washington) has requested AAVSO assistance in monitoring the black hole Low-Mass X-ray Binary V406 Vul (XTE J1859+226), which has brightened significantly in recent days and may be undergoing a major outburst. CCD observations and spectroscopy are requested.

Dr. Bellm writes that the purpose of this AAVSO campaign is to "provide a well-sampled long-term outburst light curve for V406 Vul (XTE J1859+226), a black hole Low-Mass X-ray Binary that last had a major outburst in 1999.  Starting on 2021-02-04 the Zwicky Transient Facility detected a significant optical brightening (to an r-band magnitude of 18.9; the quiescent magnitude is ~22nd mag.) which has continued in subsequent days. The source has also brightened in X-rays by more than three orders of magnitude relative to its quiescent value.

"We expect that a rare major outburst of XTE J1859+226 may be beginning.  A well-sampled optical light curve during the rise from quiescence can help shed light on where in the accretion disk the instability which causes the outburst occurs.

"The duration of the outburst is unknown, at present. If the object continues to brighten the outburst may last for hundreds of days...[It is anticipated] that the campaign will continue until the optical flux has returned to quiescent levels (V > 20)."

Dr. Bellm also notes that "we will invite all observers [whose data are] used in the published work as coauthors."

CCD observations are requested on a nightly basis. Dr. Bellm writes: "While this object shows short-timescale variations on timescales as short as seconds, the most useful observations are long-term monitoring to track brightness variations. Nightly monitoring will be particularly valuable during the next few weeks as we trace the seldom-probed early rise of the outburst." One observation per filter per night per observer is requested at this time. The priority of filters is V, then B, then Rc, Ic; unfiltered observations are welcome although filtered are strongly preferred. The historical range for V406 Vul is 15.3 - <22 V. As of February 9, V406 Vul was V~18 (~18.5 in DSS r,i; 19 in g).

Low-resolution (R~1000) spectroscopic observations are requested. They "...are useful to track changes in the emission lines in the accretion disk throughout the outburst, particularly He II at 4686 Angstrom, the Bowen blend at 4630-45640, and the Balmer series emission lines."

It is understood that V406 Vul is a difficult target at present because of its faintness and its location, but its observability will improve over the coming weeks. Please do the best you can now and as the target brightens in order to maximize the information learned during this critical period.

Coordinates (2000.0): 18 58 41.51 +22 39 30.2 (from VSX page for V406 Vul)

Finder charts with comparison stars may be created using the AAVSO Variable Star Plotter (VSP).

Please submit observations using the name V406 Vul as follows:
 - Photometry: Use WebObs to submit to the AAVSO International Database
 - Spectroscopy: Submit to the AAVSO Spectroscopic Database (AVSpec)

This AAVSO Alert Notice was compiled by Elizabeth O. Waagen, using material provided by Dr. Bellm.


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