Observations shed new light on Markarian galaxy 817

Using NASA’s Swift spacecraft, an international team of astronomers conducted long-term multi-wavelength monitoring of a nearby active galaxy called Markarian 817. Results of the observation campaign. , published on June 30th on the preprint server arXivyield important insights into the properties of this galaxy.

The active galactic nucleus (AGN) is a compact region at the center of a galaxy, brighter than the light of the surrounding galaxy. AGN is very energetic due to the presence of black holes or star-forming activity at the galaxy’s core.

Seyfert galaxies are a subclass of active galaxies and are classified as nearby, low luminosity, radio-free AGNs stored in spiral or lenticular galaxies. They are further classified as Type 1 or 2, depending on the emission lines shown by their spectra.

Located about 430 million light-years away in the northern constellation of Draco, Markarian 817 (or Mrk 817 for short) is a nearby Seyfert 1 galaxy. The galaxy has an angular size of about 0.513 arc minutes and a radial velocity of 43,761 km/s.

More recently, the Markarian 817 has been the target of the AGN STORM 2 project—a large, coordinated multi-wavelength echo mapping campaign. A team of astronomers led by Edward M. Cackett of Wayne State University in Detroit, Michigan tracked Markarian 817 with Swift for 15 months, obtaining observations of X-rays and six ultraviolet filters. /optics, to shed more light on the structure and properties of this galaxy.

«Neil Gehrels’ Swift Observatory (hereinafter referred to as Swift) monitored Mrk 817 daily, concurrently with HST [Hubble Space Telescope]with 1 ksec observations for ∼15 months from November 22, 2020 to February 24, 2022,» the researchers explained.

The observations show that the 0.3–10 keV X-ray counting rate is on average 6 times weaker than the archival observations. In addition, the X-ray count rate shows suppressed variability beyond a large flare, peaking close to the historical average flux. The X-ray spectrum of the Markarian 817 turned out to be heavily obscured.

Swift UV/optical curves vary widely throughout the entire observational campaign. The astronomers noted that although the X-ray counting rate was significantly weaker, the UV flux remained virtually unchanged when compared to the archived data. They add that the X-ray band seems to correlate poorly with the UV/optical light curves throughout the observation period.

Furthermore, the Swift optical/ultraviolet light curves exhibit alternating continuous hysteresis that increases with increasing wavelength. These light curves show a period at the beginning where the intensity of variations in the continuum is suppressed compared with later periods. The paper’s authors considered different scenarios that could explain such behavior.

«We suggest that this shows a significant contribution to the continuum from the wide-band region gas seen by the absorbed ionization continuum,» the researchers note.

Therefore, Cackett’s group proposes to further extend tracking of Markarian 817 using Swift and ground-based facilities to explain the observed light-curve characteristics of this galaxy.