arXiv:2610.35173v1 Announce Type: new Abstract: We consider an inverse source problem for the heat equation on the unit disk $\Omega\subset\mathbb R^2$ with flux measurements at finitely many boundary points. The source is piecewise constant in time on prescribed intervals and, on each interval, is the characteristic function of an unknown region with Lipschitz boundary. The measurements on each interval split into the contribution of the current source and a history term from earlier sources; subtracting the latter leaves a linear inverse problem for a forward operator $F_\tau$ that is densely defined and unbounded on $L^2(\Omega)$. We prove that $F_\tau$ is not closable, so the classical regularization theory for closed unbounded operators does not apply. We regularize instead with a Hilbert-scale penalty that compactifies it. The resulting Galerkin--Tikhonov reconstruction is sequential: each source is recovered from the flux observed up to the end of its interval and from the previously reconstructed sources. We prove its convergence and uniqueness, show that earlier reconstruction errors propagate into later data with exponential decay in time, and establish stability estimates with respect to the number of sensors. A numerical experiment on noisy measurements shows the feasibility of the sequential approximations, refined by Fourier approximation and boundary correction by a neural network.