Improved upper limits of 44Ti emission from G1.9+0.3 with NuSTAR

G1.9+0.3 is the youngest known supernova remnant (SNR) in the Milky Way. NuSTAR observed the SNR for 350 ks in 2013 and 600 ks in 2021. Combining all data, no significant detection of the 68 keV or the 78 keV line of the ⁴⁴Ti decay chain could be detected, neither in the whole remnant nor in any subsection (east or west lobe, center). The improved NuSTAR upper limits of the 44-Ti emission depend on the assumed line width, which is not well constrained. The upper limit (95% confidence interval) is 4.4 × 10^-6 photons cm^-2 s^-1 assuming narrow lines, 7.5 × 10^-6 photons cm^-2 s^-1 assuming a 2 keV broadening (1σ) of the 68 keV line, and 1.1 × 10^-5 photons cm^-2 s^-1 assuming 4 keV broadening. The limits are in agreement with previous observations from Chandra, INTEGRAL, and COMPTEL. Observing G1.9+0.3 for 1 Ms with NuSTAR shows that only the next generation of telescopes will be able to settle the question of ⁴⁴Ti production in G1.9+0.3, such as HEX-P in the hard X-ray range, or COSI in the gamma-ray range observing the 1.157-MeV line.