Alert Notice 736: Nova in Sagittarius: V6594 Sgr = N Sgr 2021 = TCP J18490521-1902054 = ASASSN-21eh

March 30, 2021

AAVSO Forum threads (scroll to the bottom of a thread for latest posts):
- Time Sensitive Alerts: https://www.aavso.org/tcp-j18490521-1902054-asassn-21eh-nova-sgr
- Novae: https://www.aavso.org/nova-sagittarii-2021-mag-10

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Event: Nova in Sagittarius: V6594 Sgr = N Sgr 2021 = TCP J18490521-1902054 = ASASSN-21eh

Discovered independently by:
 - All-Sky Automated Survey for Supernovae (ASAS-SN) (via P. Schmeer)
 - Sizuo Kaneko (Shizuoka, Japan) (via S. Nakano) (IAUC 4950)
 - Hideo Nishimura (Kakegawa, Shizuoka-ken, Japan) (via S. Nakano) (IAUC 4950)
 - Yuji Nakamura (Kameyama, Mie, Japan) (via S. Naito and I. Endoh) (IAUC 4950)

Discovery magnitude:
 - ASAS-SN: 11.9 g
 - Kaneko: 11.2 unfiltered CCD (using a Canon EOS 6D camera + 200-mm-f.l. f/3.0 lens)
 - Nishimura: 10.8 unfiltered CCD (using a Canon EOS 6D digital camera (+ 200-mm-f.l. f/3.2 lens)
 - Nakamura: 11.2 unfiltered CCD (using a 135-mm-f.l. f/4.0 lens)

Discovery date:
 - ASAS-SN: 2021 Mar. 25.29 UT
 - Kaneko: 2021 Mar. 25.736 UT
 - Nishimura: 2021 Mar. 25.761 UT
 - Nakamura: 2021 Mar. 25.803 UT

Coordinates (2000.0): R.A. 18 49 05.07  Decl. -19 02 04.2  (from VSX page for V6594 Sgr)

Spectra: Spectroscopy indicating the object to be a classical nova was obtained as follows:
 - H. Maehara (National Astronomical Observatory of Japan) reports that O. Ohshima (Okayama, Japan) obtained a low-resolution spectrum on Mar. 25.838 UT using a 30-cm telescope (+ Alpy-600 spectrograph); the spectrum has been posted at http://otobs.org/hiki/?TCP_J18490521 (IAUC 4950).
 - Munari et al. (ATel #14488) obtained a high-resolution spectrum on Mar. 27.179 UT using a 50-cm telescope operated for the ANS Collaboration in Stroncone (Italy). Their data, in combination with photometry they obtained (see below), indicate the nova is only slightly reddened and was discovered pre-maximum.

Observing recommendations: Please observe N Sgr 2021 as it continues to evolve, with observations of all types (visual, CCD, DSLR, PEP, spectroscopy) and multiple bands as instrumentation permits. Dr. Fred Walter (Stony Brook University) recommends observations every few hours in the early stages as the nova continues to brighten. Frequency of observation once past peak depends on the rate of decline, but should be a minimum of one observation per night per band. For spectroscopy, he recommends spectra in blue to observe He II 4686, H-beta, and the Bowen blend (4640A), in addition to H-alpha, with a cadence of one spectrum per clear night.

Observations reported to the AAVSO (selected from over 240 submitted):
2021 Mar. 19.80417 UT, <11.5 (A. Pearce, Nedlands, W. Australia);
20.82986, <11.5 (Pearce);
21.82778, <11.5 (Pearce);
22.81181, <11.5 (Pearce);
23.31, 20.0 g (ASAS-SN, via P. Schmeer);
23.81111, <11.5 (Pearce);
24.82014, <11.5 (Pearce);
25.29, 11.9 g (ASAS-SN, independent discovery; via Schmeer);
25.736, 11.2 (S. Kaneko, Shizuoka, Japan, independent discovery, via Nakano) (IAUC 4950);
25.761, 10.8 unfiltered CCD (H. Nishimura, Kakegawa, Shizuoka-ken, Japan, independent discovery, via S. Nakano) (IAUC 4950);
25.744, 11.2 (Kaneko);
25.755, 10.9 (Kaneko);
25.803, 11.2 unfiltered CCD (Y. Nakamura, Kameyama, Mie, Japan, independent discovery, via S. Naito and I. Endoh) (IAUC 4950);
25.80694, 11.3 (Pearce);
26.188, 11.3 (F. Kugel, Banon, France) (IAUC 4950);
26.206, 11.0 V (Kaneko, remotely with an iTelescope 0.32-m f/8 telescope at Nerpio, Spain) (IAUC 4950);
26.27546, 11.075 CV +/-0.006 (F.-J. Hambsch, Mol, Belgium);
26.40804, 10.956 CV +/-0.004 (Hambsch);
26.50284, 10.237 I +/-0.016 (R. Fidrich, Budapest, Hungary);
26.50378, 10.568 R +/-0.019 (Fidrich);
26.50379, 10.869 V +/-0.004 (Fidrich);
26.50586, 11.177 B +/-0.001 (Fidrich);
26.740, 11.0 (T. Noguchi, Katori, Japan) (IAUC 4950);
26.75216, 10.933 B +/-0.062 (Pearce, remotely using a 0.43-m f/6.8 reflector at Siding Spring);
26.75331, 10.634 V +/-0.002 (Pearce, remotely using a 0.43-m f/6.8 reflector at Siding Spring);
26.75440, 10.171 I +/-0.013 (Pearce, remotely using a 0.43-m f/6.8 reflector at Siding Spring);
26.80417, 10.4 (Pearce);
27.11806, 10.3 (V. Cseh, Debrecen, Hungary);
27.21100, 10.505 CV (G. Poyner, Birmingham, UK);
27.23674, 10.217 TG +/-0.043 (R. Tyson, Glen Burnie, MD);
27.33045, 10.134 CV +/-0.005 (Hambsch);
27.33220, 10.175 V (C. Colesanti, Sao Paulo, Brazil);
27.40946, 10.016 CV +/-0.004  (Hambsch);
27.80278, 10.2 (Pearce);
28.26803, 10.182 CV +/-0.010 (Hambsch);
28.29167, 10.3 (J. De Souza Aguiar, Campinas, Brazil);
28.40926, 10.159 CV +/-0.006 (Hambsch);
28.51327, 10.056 V +/-0.028  (F. Sims, Gilbert, AZ);
29.12153, 10.0 (Cseh);
29.15139, 9.8 CV (K. Wenzel, Grossostheim, Germany);
29.26526, 9.613 CV +/-0.011 (Hambsch);
29.30833, 9.7 (L. Araujo, Pelotas, Brazil);
29.40922, 9.646 CV +/-0.006 (Hambsch);
30.13542, 9.4 (Wenzel);
30.21500, 9.577 CV (Poyner);
30.26438, 9.371 V +/-0.027 (Hambsch);
30.39780, 9.489 V +/-0.009 (Hambsch);
30.40247, 9.503 V +/-0.009 (Hambsch);

Charts: Charts with comparison stars for V6594 Sgr may be created using the AAVSO Variable Star Plotter (VSP).

Submit observations: Please submit observations using the name V6594 SGR.
 - Submit optical observations to the AAVSO International Database using WebObs (see below).
 - Submit spectra to the AAVSO Spectroscopy Database (AVSpec).

Notes:
a. Designated TCP J18490521-1902054 when posted to the IAU Central Bureau for Astronomical Telegrams Transient Object Confirmation Page (TOCP).

b. Information in this Alert Notice is taken in part from IAU Circular 4950 (D. W. E. Green, Ed.).

c. H. Nishimura reports that nothing is visible down to magnitude 15 at this location on three frames taken 2021 Mar. 23.778 UT (IAUC 4950).

d. Y. Nakamura reports that a faint star is visible at this location on his 15-sec frames taken on 2021 Mar. 23.799 UT (limiting mag 13.0) (IAUC 4950).

e. N. Samus and E. V. Kazarovets report that the GCVS team has assigned the permanent name V6594 Sgr to this nova (IAUC 4950).

f. P. Schmeer (Saarbruecken-Bischmisheim, Germany) reports that a catalogued star of red magnitude 19.5 (PSO J184905.064-190204.346 = VHS J184905.06-190204.3) is very close to the position measured by F. Kugel (see below) (IAUC 4950).  

g. Position end figures:
 - H. Nishimura (2021 Mar. 25.761 UT, via IAUC 4950): 05.21s, 05.4"
 - Y. Nakamura (2021 Mar. 25.803 UT, via IAUC 4950): 05.2s, 06"
 - F. Kugel (2021 Mar. 26.188 UT, via IAUC 4950): 05.06s, 04.0"
 - R. Fidrich (2021 Mar. 26.504 UT): 05.07s, 04.04"
 - T. Noguchi (2021 Mar. 26.740 UT, via IAUC 4950): 05.06s, 03.9"
 - A. Pearce (2021 Mar. 26.752 UT): 05.08s, 04.04"

h. Images
 - R. Fidrich (2021 Mar. 26.504 UT): https://tinyurl.com/2hvzk8ja
 - T. Noguchi (2021 Mar. 26.740 UT, via IAUC 4950): http://park8.wakwak.com/~ngc/images/TCPinSgr_20210326.jpg

Congratulations to the ASAS-SN team, S. Kaneko, H. Nishimura, and Y. Nakamura on their independent discoveries!

This AAVSO Alert Notice was compiled by Elizabeth O. Waagen.
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