The Narrow-line Seyfert 1 galaxy Mrk 335 is switching on again after years in a deep low-state

ATel #17547; S. Komossa (MPIfR), Dirk Grupe (Northern Kentucky University), L. Gallo (St. Mary's University)
on 17 Dec 2025; 00:25 UT
Credential Certification: St. Komossa (stefanie.komossa@gmx.de)

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We report on recent results from a dense multi-year monitoring campaign of the NLS1 galaxy Mrk 335 (z=0.026) that is getting brighter again after years in an exceptionally deep low-state. In particular, it has reached its brightest X-ray state within the last 4 yrs on 2025 October 29.

Mrk 335 is highly variable in all wavebands of the Neil Gehrels Swift observatory. While initially a bright X-ray source (e.g., Tananbaum et al. 1978), it went increasingly often into deep X-ray low-states when observed with Swift, and we have monitored it densely since 2007 (e.g., Grupe et al. 2007, 2008, 2012, Gallo et al. 2018, Komossa et al. 2020, Komossa & Grupe 2024). The X-ray variability is decoupled from the optical-UV bands most of the time. In the years 2018-2019, Mrk 335 went into its deepest, longest-lasting X-ray low-state. Several rapid flare events in 2020 represented the first sign of emergence from that low-state (Komossa et al. 2020) but did not yet lead to a rapid recovery to the brightest state. We have continued to monitor Mrk 335 with Swift since then, and we see a persistent trend that the average X-ray flux is rising again. Still, weekly fluctuations by a factor of a few are present.

Mrk 335 reached its brightest X-ray state for 4 years on 2025 October 29. At that epoch, we measured an XRT countrate of 0.56 cts/s, a hardness ratio HR = 0.10+/-0.02, an absorbed X-ray flux f = (1.54+/-0.30)e-11 erg/s (1.54e-14 W/m^2), and a powerlaw photon index of 2.27+/-0.40. That fit is based on the use of a single powerlaw model with absorption fixed at the Galactic value, even though we note that Mrk 335 can be well fit with a partial covering model most or all of the time. On the same date, we measured with the UVOT the following magnitudes in the VEGA system (where values in brackets are corrected for Galactic extinction): V = 14.90 +/- 0.05 (14.78 ), B = 15.33 +/- 0.05 (15.18), U = 14.35 +/- 0.05 (14.16), W1 = 14.11 +/- 0.04 (13.87), M2 = 14.22 +/- 0.04 (13.88), and W2 = 14.20 +/- 0.04 (13.91). The optical-UV emission is not tightly correlated with the X-ray emission. Since September, X-ray levels have remained well above the lowest state and continue to vary between 0.1 and 0.5 cts/s in our weekly monitoring. For countrates between 0.2 and 0.5 cts/s, the hardness ratio HR has remained nearly constant within its error bars, consistent with the earlier finding (Komossa et al. 2020) that the HR changes strongly at lowest (<0.1 cts/s) countrates, changes in opposite direction at low-intermediate (0.1 - 0.2 cts/s) countrate levels, and remains essentially constant at higher countrates (their Fig. 4). During our latest observation on December 10, we measured an X-ray countrate of 0.197+/-0.017 counts/s, HR = 0.11+/-0.08, and UVOT magnitudes (uncorrected for extinction) of V= 14.99 +/- 0.05 (14.87), B = 15.41 +/- 0.05 (15.26), U = 14.47 +/- 0.05 (14.28), W1 = 14.23 +/- 0.04 (13.99), M2 = 14.41 +/- 0.05 (14.07), and W2 = 14.32 +/- 0.04 (14.03).

Now, in its intermediate phase of activity, re-covering slowly from the deep low-state but not yet back to its brightest state, is an ideal time to carry out multiple follow-up observations of Mrk 335 including (re-)measuring time delays between multiple continuum bands at high cadence, performing reverberation mapping including the important HeII4686A emission line, and obtaining deep X-ray spectroscopy in order to measure if the X-ray spectral complexity observed during the slow multi-year switch-off phase is the same as during the switch-on phase.

We would like to thank Brad Cenko for approving our current monitoring campaigns and the continuous support by the Swift team for our project. We would also like to acknowledge the use of the 'Build an XRT Product' page at the University of Leicester (https://www.swift.ac.uk/user_objects/). Its service has been instrumental for our research.