While a broad profile of the Fe \mathrm { K \alpha } emission line is frequently found in the X-ray spectra of typical Seyfert galaxies , the situation is unclear in the case of Narrow Line Seyfert 1 galaxies ( NLS1s ) —an extreme subset which are generally thought to harbor less massive black holes with higher accretion rates .
In this paper , the ensemble property of the Fe \mathrm { K \alpha } line in NLS1s is investigated by stacking the X-ray spectra of a large sample of 51 NLS1s observed with XMM-Newton .
The composite X-ray spectrum reveals a prominent , broad emission feature over 4–7 keV , characteristic of the broad Fe \mathrm { K \alpha } line .
In addition , there is an indication for a possible superimposing narrow ( unresolved ) line , either emission or absorption , corresponding to Fe XXVI or Fe XXV , respectively .
The profile of the broad emission feature can well be fitted with relativistic broad-line models , with the line energy consistent either with 6.4 keV ( i.e. , neutral Fe ) or with 6.67 keV ( i.e. , highly ionized Fe ) , in the case of the narrow line being emission and absorption , respectively .
Interestingly , there are tentative indications for low or intermediate values of the average spins of the black holes ( a < 0.84 ) , as inferred from the profile of the composite broad line .
If the observed feature is indeed a broad line rather than resulting from partial covering absorption , our results suggest that a relativistic Fe line may in fact be common in NLS1s ; and there are tentative indications that black holes in NLS1s may not spin very fast in general .