Catching NGC 4051 in the Low State with Chandra

We report the results from a Chandra target-of-opportunity observation of the low-luminosity narrow-line Seyfert 1 galaxy NGC 4051, obtained during a 6 week duration low-flux state in 2001 February. During the Chandra observation, the 2-10 keV source flux was 7 × 10^-12 ergs cm^-2 s^-1, corresponding to a 2-10 keV luminosity of ~8 × 10⁴⁰ ergs s^-1. We confirm the absence of strong extended soft X-ray emission in NGC 4051 and that the low-state spectrum is dominated by the central point source. The X-ray spectrum has an unusual, hybrid shape, very soft below ~3 keV (and which we model with a blackbody of temperature of 0.14 keV) and very hard at higher energies (power-law slope of Γ ~ 1). The light curves in both soft and hard bands are significantly variable and correlated, implying a connection between hard and soft components and proving that the hard component is dominated by primary continuum emission and is not due to pure reflection from distant cold material. However, a comparison with RXTE data obtained during the same 2001 low state suggests the presence of unusually prominent disk reflection features, which may help to explain the apparent upward curvature and extreme hardness of the Chandra spectrum above a few keV. The shape of the 2001 low-state spectrum is consistent with that observed in a brief (<3 days) low-flux excursion during the source's normal state in 2000 April, suggesting that emission processes during the low state are not significantly different to those in the normal state. The unusual spectral shape observed in the low state may be a continuation to low fluxes of the normal, flux-dependent spectral variability of the source.