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To: stefano@bicar.it
From: atel@astronomerstelegram.org
Subject: ATel  17140
Message-Id: <20250409070528.44BD428807C9@server.astronomerstelegram.org>
Date: Wed,  9 Apr 2025 03:05:28 -0400 (EDT)

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ATEL #17140							     ATEL #17140

Title:	Multiwavelength observations of KT Eri (=Nova Eri 2009) during
		the ongoing luminosity rise
Author:	N. Masetti (INAF-OAS, Bologna), U. Munari (INAF Padova)
Queries:	ulisse.munari@inaf.it
Posted:	8 Apr 2025; 14:17 UT
Subjects:Optical, Ultra-Violet, X-ray, Cataclysmic Variable, Nova

Following the announcement in ATel #17125 of KT Eri (=Nova Eri 2009) beginning
a steep rise toward record brightness and a new emission state, we observed
this source in X-rays and ultraviolet (UV) with the <it>Neil Gehrels Swift</it>
satellite, starting at 10:40 UT on 2025 Apr. 06. The source field was acquired
in UV using UVOT with filters UVW1, UVW2 and UVM2 for 580 s, 720 s and
720 s, respectively; preliminary analysis shows that the source was well
detected, with Vega system magnitudes UVW1=13.09+-0.04, UVM2=13.08+-0.04
and UVW2=13.10+-0.04. This corresponds to UVW1=14.60+-0.04, UVM2=14.77+-0.04
and UVW2=14.83+-0.04 in the AB system. No X-rays were instead observed
from the source: a simultaneous XRT pointing of about 2000 s did not reveal
any emission in the 0.3-10 keV band down to a 3-sigma limit of ~5e-3 counts/s.

Assuming a collisionally ionized plasma X-ray spectrum with kT~0.06 keV
as in  Pei et al. (2021, MNRAS, 507, 2073), adopting the Gaia DR3 catalog
distance  to the source of 5.1 kpc and considering a negligible hydrogen
column density given  the high Galactic latitude of KT Eri, the XRT observation
implies a flux upper limit of <3e-13 erg/cm2/s and a luminosity limit of
<9e32 erg/s.

The spectral energy distribution (SED) of KT Eri for the current April
2025 bright phase and the 2011-2024 quiescence following the 2009 outburst
are compared in <a href="https://www.astrogeo.va.it/ans_spectroscopy/sup/KT_Eri_April_2025.png">
<em>this picture</a></em>. The SED for the 2011-2024 quiescence combines
median UBVRI data from ATel #17125 with 2MASS JHKs values, and median NeoWISE
W1 and W2 magnitudes. The SED for April 2025 is built from the UVOT observations
of  April 6, 2025 above described, and the UBVRI magnitudes for March 24,
2025  from ATel #17125. Integrating the SED for April 2025 over the 1800-8500
Ang covered by the UVOT-UBVRI bands, and adopting the above distance under
negligible  reddening, we found a radiated luminosity of 2.5e35 erg/s equivalent
to 65 Solar  luminosities. The source luminosity from the SED in quiescence
over this range of  wavelengths is a factor of 2.5 less.

<it>We thank the Swift Team for the quick approval, scheduling and acquisition
of the observation presented in this communication.</it>

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