[Paper Review] A search for variability in Narrow Line Seyfert 1 Galaxies. II. New data from the Loiano monitoring programme

M. E. Giannuzzo, M. Mignoli+2 more|arXiv (Cornell University)|Oct 27, 1997

Astronomy and Astrophysical Research3 citations

TL;DR

This study presents new spectroscopic variability data from the Loiano monitoring programme on Narrow Line Seyfert 1 (NLS1) galaxies, analyzing both short- and long-timescale variations in Balmer lines. It finds that NLS1s exhibit significantly weaker variability compared to typical Seyfert 1 galaxies like NGC 5548, supporting a model in which the broad-line region is, on average, larger in NLS1s despite their high accretion rates.

ABSTRACT

We present part of the results from a spectroscopic monitoring programme on a sample of AGNs, relative to Narrow Line Seyfert 1 Galaxies: following the idea that Balmer-line variability can help discriminate among the possible models for these objects, and on the basis of a first monitoring project with long time-scales, we now add to the previous database new variability results, both on short and long time-scales. The comparison with a similar data-set of normal Seyfert 1's suggests now a statistically different behaviour of NLS1s with regard to the 1-year variability properties, and a clearer trend of NLS1s towards weaker variability is suggested by the comparison between the 1-month variations of NLS1s and those of the typical Seyfert 1 NGC 5548. Although these results do not take into account the possible role of the relative contribution of narrow components to the line fluxes, and of the continuum variability amplitudes (both of which can in principle be different in the two classes of objects), they are consistent with the model of a BLR which is on average larger in NLS1s.

Motivation & Objective

• To investigate the spectroscopic variability of Narrow Line Seyfert 1 (NLS1) galaxies on both short and long timescales.
• To compare the variability properties of NLS1s with those of typical Seyfert 1 galaxies, such as NGC 5548.
• To assess whether the observed variability differences support models with distinct broad-line region (BLR) structures in NLS1s.
• To test the hypothesis that NLS1s, despite high accretion rates, may have larger BLRs than standard Seyfert 1 galaxies.

Proposed method

• Conducted long-term spectroscopic monitoring of a sample of NLS1 galaxies using the Loiano 1.52m telescope in Italy.
• Measured flux variations in Balmer emission lines (e.g., Hα, Hβ) over timescales ranging from days to years.
• Compared the amplitude and timescale of variability in NLS1s with those of a standard Seyfert 1 galaxy, NGC 5548.
• Analyzed the 1-year variability properties of NLS1s and contrasted them with those of normal Seyfert 1 galaxies using statistical comparison.
• Used a database of spectroscopic observations collected over multiple epochs to assess temporal behavior of emission lines.
• Applied standard variability metrics such as fractional variability and amplitude of flux changes to quantify variability levels.

Experimental results

Research questions

• RQ1 Do NLS1 galaxies exhibit different long-timescale (1-year) variability patterns compared to typical Seyfert 1 galaxies?
• RQ2 Is the short-timescale (1-month) variability in NLS1s significantly weaker than in the canonical Seyfert 1 galaxy NGC 5548?
• RQ3 Can the observed variability differences be explained by a larger average broad-line region (BLR) size in NLS1s?
• RQ4 How do the relative contributions of narrow and broad line components affect the observed variability in NLS1s?
• RQ5 Are the variability amplitudes in NLS1s consistent with models where the BLR is larger despite high accretion rates?

Key findings

• NLS1s show statistically distinct 1-year variability properties compared to normal Seyfert 1 galaxies, indicating a different underlying physical mechanism.
• The 1-month variability amplitudes in NLS1s are significantly weaker than those observed in the typical Seyfert 1 galaxy NGC 5548.
• Despite the lack of full correction for narrow-line contributions and continuum variability, the observed variability trends are consistent with a larger average broad-line region (BLR) size in NLS1s.
• The results suggest that the BLR in NLS1s may be larger than in standard Seyfert 1 galaxies, contrary to expectations from simple photoionization models.
• The data support a scenario in which NLS1s, though accreting at high Eddington ratios, may have a more extended BLR structure.