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Register a new userLong-term optical, UV, and X-ray continuum variations in the changing-look AGN HE 1136-2304
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Wolfram Kollatschny
Publication date
2018
fields
Physics
and research's language is
English
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A strong outburst in the X-ray continuum and a change of its Seyfert spectral type was detected in HE 1136-2304 in 2014. The spectral type changed from nearly Seyfert 2 type (1.95) to Seyfert 1.5 type in comparison to previous observations taken ten to twenty years before. In a subsequent variability campaign we wanted to investigate whether this outburst was a single event or whether the variability pattern following the outburst was similar to those seen in other variable Seyfert galaxies. In addition to a SALT spectral variability campaign, we carried out optical continuum as well as X-ray and UV (Swift) monitoring studies from 2014 to 2017. HE 1136-2304 strongly varied on timescales of days to months from 2014 to 2017. No systematic trends were found in the variability behavior following the outburst in 2014. A general decrease in flux would have been expected for a tidal disruption event. This could not be confirmed. More likely the flux variations are connected to irregular fluctuations in the accretion rate. The strongest variability amplitudes have been found in the X-ray regime: HE 1136-2304 varied by a factor of eight during 2015. The amplitudes of the continuum variability (from the UV to the optical) systematically decreased with wavelength following a power law F_var = a ${times}$ {lambda}^-c with c = 0.84. There is a trend that the B-band continuum shows a delay of three light days with respect to the variable X-ray flux. The Seyfert type 1.5 did not change despite the strong continuum variations for the period between 2014 and 2017.
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We report the detection of high-amplitude X-ray flaring of the AGN HE 1136-2304, which is accompanied by a strong increase in the flux of the broad Balmer lines, changing its Seyfert type from almost type 2 in 1993 down to 1.5 in 2014. HE 1136-2304 was detected by the XMM-Newton slew survey at >10 times the flux it had in the ROSAT all-sky survey, and confirmed with Swift follow-up after increasing in X-ray flux by a factor of 30. Optical spectroscopy with SALT shows that the AGN has changed from a Seyfert 1.95 to a Seyfert 1.5 galaxy, with greatly increased broad line emission and an increase in blue continuum AGN flux by a factor of > 4. The X-ray spectra from XMM-Newton and NuSTAR reveal moderate intrinsic absorption and a high energy cutoff at 100 keV. We consider several different physical scenarios for a flare, such as changes in obscuring material, tidal disruption events, and an increase in the accretion rate. We find that the most likely cause of the increased flux is an increase in the accretion rate, although it could also be due to a change in obscuration.
A strong X-ray outburst was detected in HE1136-2304 in 2014. Accompanying optical spectra revealed that the spectral type has changed from a nearly Seyfert 2 type (1.95), classified by spectra taken 10 and 20 years ago, to a Seyfert 1.5 in our most recent observations. We seek to investigate a detailed spectroscopic campaign on the spectroscopic properties and spectral variability behavior of this changing look AGN and compare this to other variable Seyfert galaxies. We carried out a detailed spectroscopic variability campaign of HE1136-2304 with the 10 m Southern African Large Telescope (SALT) between 2014 December and 2015 July. The broad-line region (BLR) of HE1136-2304 is stratified with respect to the distance of the line-emitting regions. The integrated emission line intensities of Halpha, Hbeta, HeI 5876, and HeII 4686 originate at distances of 15.0 (+4.2,-3.8), 7.5 (+4.6,-5.7), 7.3 (+2.8,-4.4), and 3.0 (+5.3,-3.7) light days with respect to the optical continuum at 4570AA. The variability amplitudes of the integrated emission lines are a function of distance to the ionizing continuum source as well. We derived a central black hole mass of 3.8 (+-3.1) 10exp(7) M_solar based on the line widths and distances of the BLR. The outer line wings of all BLR lines respond much faster to continuum variations indicating a Keplerian disk component for the BLR. The response in the outer wings is about two light days shorter than the response of the adjacent continuum flux with respect to the ionizing continuum flux. The vertical BLR structure in HE1136-2304 confirms a general trend that the emission lines of narrow line active galactic nuclei (AGNs) originate at larger distances from the midplane in comparison to AGNs showing broader emission lines. Otherwise, the variability behavior of this changing look AGN is similar to that of other AGN.
Observations have shown that UV/optical variation amplitude of quasars depend on several physi- cal parameters including luminosity, Eddington ratio, and likely also black hole mass. Identifying new factors which correlate with the variation is essential to probe the underlying physical processes. Combining ~ten years long quasar light curves from SDSS stripe 82 and X-ray data from Stripe 82X, we build a sample of X-ray detected quasars to investigate the relation between UV/optical variation amplitude ($sigma_{rms}$) and X-ray loudness. We find that quasars with more intense X-ray radiation (com- pared to bolometric luminosity) are more variable in UV/optical. Such correlation remains highly significant after excluding the effect of other parameters including luminosity, black hole mass, Ed- dington ratio, redshift, rest-frame wavelength (i.e., through partial correlation analyses). We further find the intrinsic link between X-ray loudness and UV/optical variation is gradually more prominent on longer timescales (up to 10 years in the observed frame), but tends to disappear at timescales < 100 days. This suggests a slow and long-term underlying physical process. The X-ray reprocessing paradigm, in which UV/optical variation is produced by a variable central X-ray emission illuminating the accretion disk, is thus disfavored. The discovery points to an interesting scheme that both the X-ray corona heating and UV/optical variation is quasars are closely associated with magnetic disc turbulence, and the innermost disc turbulence (where corona heating occurs) correlates with the slow turbulence at larger radii (where UV/optical emission is produced).
We analyze the X-ray, optical, and mid-infrared data of a changing-look Seyfert galaxy sdssj15 at $zsimeq0.086$. Over a period of one decade (2009 - 2018), its broad H$alpha$ line intensity increased by a factor of $sim$4. Meanwhile, the X-ray emission in 2014 as observed by chandra was about five times brighter than that in 2010 by {it Suzaku}, and the corresponding emissions in V-band, mid-infrared W1 band brighten by $sim$ 0.18, 0.32 mag, respectively. Moreover, the absorption in X-rays is moderate and stable, i.e. ${rm N_{H}}sim 10^{21} {rm cm^{-2}}$, but the X-ray spectrum becomes harder in the 2014 chandra bright state (i.e. photon index $Gamma = 1.52^{+0.06}_{-0.06}$) than that of the 2010 suzaku low state ($Gamma=2.03^{+0.22}_{-0.21}$). With an Eddington ratio being lower than a few percent, the inner region of the accretion disk in sdssj15 is likely a hot accretion flow. We then compile from literature the X-ray data of changing-look AGNs, and find that they generally follow the well-established V-shaped correlation in AGNs, that is, above a critical turn-over luminosity the X-ray spectra soften with the increasing luminosity, and below that luminosity the trend is reversed in a way of harder when brighter. This presents a direct evidence that CL-AGNs have distinctive changes in not only the optical spectral type, but also the X-ray spectral shape. The similarity in the X-ray spectral evolution between CL-AGNs and black hole X-ray binaries indicates that the observed CL-AGNs phenomena may relate to the state transition in accretion physics.
We detected a very strong X-ray decline in the galaxy IRAS23226-3843 within the XMM-Newton slew survey in 2017. Subsequently, we carried out multi-band follow-up studies to investigate this fading galaxy in more detail. We took deep follow-up Swift, XMM-Newton, and NuSTAR observations in combination with optical SALT spectra of IRAS23226-3843 in 2017. In addition, we reinspected optical, UV, and X-ray data that were taken in the past. IRAS23226-3843 decreased in X-rays by a factor of more than 30 with respect to ROSAT and Swift data taken 10 to 27 years before. The broadband XMM-Newton/NuSTAR spectrum is power-law dominated, with a contribution from photoionized emission from cold gas, likely the outer accretion disk or torus. The optical continuum decreased by 60 percent and the Balmer line intensities decreased by 50 percent between 1999 and 2017. The optical Seyfert spectral type changed simultaneously with the X-ray flux from a clear broad-line Seyfert 1 type in 1999 to a Seyfert 1.9 type in 2017. The Balmer line profiles in IRAS23226-3843 are extremely broad. The profiles during the minimum state indicate that they originate in an accretion disk. The unusual flat Balmer decrement Ha/Hb with a value of 2 indicates a very high hydrogen density of n_(H) > 10 exp(11) cm^(-3) at the center of the accretion disk. IRAS23226-3843 shows unusually strong FeII blends with respect to the broad line widths, in contrast to what is known from Eigenvector 1 studies.
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