arXiv subjects

S. Komossa

Publications and source records attributed to S. Komossa.

At least 127 records · Page 7Linked to original sources

arXiv2015

The 31 Deg$^2$ Release of the Stripe 82 X-ray Survey: The Point Source Catalog

We release the next installment of the Stripe 82 X-ray survey point-source catalog, which currently covers 31.3 deg$^2$ of the Sloan Digital Sky Survey (SDSS) Stripe 82 Legacy field. In total, 6181 unique X-ray sources are significantly detected with {\it XMM-Newton} ($>5σ$) and {\it Chandra} ($>4.5σ$). This catalog release includes data from {\it XMM-Newton} cycle AO 13, which approximately doubled the Stripe 82X survey area. The flux limits of the Stripe 82X survey are $8.7\times10^{-16}$ erg s$^{-1}$ cm$^{-2}$, $4.7\times10^{-15}$ erg s$^{-1}$ cm$^{-2}$, and $2.1\times10^{-15}$ erg s$^{-1}$ cm$^{-2}$ in the soft (0.5-2 keV), hard (2-10 keV), and full bands (0.5-10 keV), respectively, with approximate half-area survey flux limits of $5.4\times10^{-15}$ erg s$^{-1}$ cm$^{-2}$, $2.9\times10^{-14}$ erg s$^{-1}$ cm$^{-2}$, and $1.7\times10^{-14}$ erg s$^{-1}$ cm$^{-2}$. We matched the X-ray source lists to available multi-wavelength catalogs, including updated matches to the previous release of the Stripe 82X survey; 88\% of the sample is matched to a multi-wavelength counterpart. Due to the wide area of Stripe 82X and rich ancillary multi-wavelength data, including coadded SDSS photometry, mid-infrared {\it WISE} coverage, near-infrared coverage from UKIDSS and VHS, ultraviolet coverage from {\it GALEX}, radio coverage from FIRST, and far-infrared coverage from {\it Herschel}, as well as existing $\sim$30\% optical spectroscopic completeness, we are beginning to uncover rare objects, such as obscured high-luminosity AGN at high-redshift. The Stripe 82X point source catalog is a valuable dataset for constraining how this population grows and evolves, as well as for studying how they interact with the galaxies in which they live.

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arXiv2015

Flaring from the supermassive black hole in Mrk 335 studied with Swift and NuSTAR

Monitoring of the narrow line Seyfert 1 galaxy Markarian 335 (Mrk 335) with the Swift satellite discovered an X-ray flare beginning 2014 August 29. At the peak, the 0.5-5keV count rate had increased from that in the low flux state by a factor of 10. A target of opportunity observation was triggered with NuSTAR, catching the decline of the flare on 2014 September 20. We present a joint analysis of Swift and NuSTAR observations to understand the cause of this flare. The X-ray spectrum shows an increase in directly observed continuum flux and the softening of the continuum spectrum to a photon index of 2.49 (-0.07,+0.08) compared to the previous low flux observations. The X-ray spectrum remains well-described by the relativistically blurred reflection of the continuum from the accretion disc whose emissivity profile suggests that it is illuminated by a compact X-ray source, extending at most 5.2rg over the disc. A very low reflection fraction of 0.41 (-0.15,+0.15) is measured, unexpected for such a compact corona. The X-ray flare is, hence, interpreted as arising from the vertical collimation and ejection of the X-ray emitting corona at a mildly relativistic velocity, causing the continuum emission to be beamed away from the disc. As the flare subsides, the base of this jet-like structure collapses into a compact X-ray source that provides the majority of the radiation that illuminates the disc while continuum emission is still detected from energetic particles further out, maintaining the low reflection fraction.

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arXiv2015

Was the soft X-ray flare in NGC 3599 due to an AGN disc instability or a delayed tidal disruption event?

We present unpublished data from a tidal disruption candidate in NGC 3599 which show that the galaxy was already X-ray bright 18 months before the measurement which led to its classification. This removes the possibility that the flare was caused by a classical, fast-rising, short-peaked, tidal disruption event. Recent relativistic simulations indicate that the majority of disruptions will actually take months or years to rise to a peak, which will then be maintained for longer than previously thought. NGC 3599 could be one of the first identified examples of such an event. The optical spectra of NGC 3599 indicate that it is a low-luminosity Seyfert/LINER with L_bol~10^40 ergs/s The flare may alternatively be explained by a thermal instability in the accretion disc, which propagates through the inner region at the sound speed, causing an increase of the disc scale height and local accretion rate. This can explain the <9 years rise time of the flare. If this mechanism is correct then the flare may repeat on a timescale of several decades as the inner disc is emptied and refilled.

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arXiv2015

Identification of a New $γ$-ray-emitting narrow-line Seyfert 1 Galaxy, at Redshift $\sim1$

We report on the identification of a new $γ$-ray-emitting narrow-line Seyfert 1 (NLS1) galaxy, SDSS J122222.55+041315.7, which increases the number of known objects of this remarkable but rare type of active galactic nuclei (AGN) to seven. Its optical spectrum, obtained in the Sloan Digital Sky Survey-Baryon Oscillation Spectroscopic Survey, reveals a broad H $β$ emission line with a width (FWHM) of 1734$\pm$104 km s$^{-1}$. This, along with strong optical Fe II multiplets [$R_{4570}=0.9$] and a weak [O III] $λ5007$ emission line, makes the object a typical NLS1. On the other hand, the source exhibits a high radio brightness temperature, rapid infrared variability, and a flat X-ray spectrum extending up to $\sim$200 keV. It is associated with a luminous $γ$-ray source detected significantly with {\it Fermi}/LAT. Correlated variability with other wavebands has not yet been tested. The spectral energy distribution can be well modelled by a one-zone leptonic jet model. This new member is by far the most distant $γ$-ray-emitting NLS1, at a redshift of $z=0.966$.

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arXiv2015

The Radio Properties of Radio-Loud Narrow-Line Seyfert 1 Galaxies on Parsec Scales

We present the detection of compact radio structures of fourteen radio-loud narrow line Seyfert 1 (NLS1) galaxies from Very Long Baseline Array observations at 5 GHz, which were performed in 2013. While 50\% of the sources of our sample show a compact core only, the remaining 50\% exhibit a core-jet structure. The measured brightness temperatures of the cores range from $10^{8.4}$ to $10^{11.4}$ K with a median value of $10^{10.1}$ K, indicating that the radio emission is from non-thermal jets, and that, likely, most sources are not strongly beamed, then implying a lower jet speed in these radio-loud NLS1 galaxies. In combination with archival data taken at multiple frequencies, we find that seven sources show flat or even inverted radio spectra, while steep spectra are revealed in the remaining seven objects. Although all these sources are very radio-loud with $R > 100$, their jet properties are diverse, in terms of their milli-arcsecond (mas) scale (pc scale) morphology and their overall radio spectral shape. The evidence for slow jet speeds (i.e., less relativistic jets), in combination with the low kinetic/radio power, may offer an explanation for the compact VLBA radio structure in most sources. The mildly relativistic jets in these high accretion rate systems are consistent with a scenario, where jets are accelerated from the hot corona above the disk by the magnetic field and the radiation force of the accretion disk. Alternatively, a low jet bulk velocity can be explained by low spin in the Blandford-Znajek mechanism.

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arXiv2015

An Ultrasoft X-ray Flare from 3XMM J152130.7+074916: a Tidal Disruption Event Candidate

We report on the discovery of an ultrasoft X-ray transient source, 3XMM J152130.7+074916. It was serendipitously detected in an XMM-Newton observation on 2000 August 23, and its location is consistent with the center of the galaxy SDSS J152130.72+074916.5 (z=0.17901 and d_L=866 Mpc). The high-quality X-ray spectrum can be fitted with a thermal disk with an apparent inner disk temperature of 0.17 keV and a rest-frame 0.24-11.8 keV unabsorbed luminosity of ~5e43 erg/s, subject to a fast-moving warm absorber. Short-term variability was also clearly observed, with the spectrum being softer at lower flux. The source was covered but not detected in a Chandra observation on 2000 April 3, a Swift observation on 2005 September 10, and a second XMM-Newton observation on 2014 January 19, implying a large variability (>260) of the X-ray flux. The optical spectrum of the candidate host galaxy, taken ~11 yrs after the XMM-Newton detection, shows no sign of nuclear activity. This, combined with its transient and ultrasoft properties, leads us to explain the source as tidal disruption of a star by the supermassive black hole in the galactic center. We attribute the fast-moving warm absorber detected in the first XMM-Newton observation to the super-Eddington outflow associated with the event and the short-term variability to a disk instability that caused fast change of the inner disk radius at a constant mass accretion rate.

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arXiv2015

Variable Reddening and Broad Absorption Lines in the Narrow-line Seyfert 1 Galaxy WPVS 007: an Origin in the Torus

We report the discovery of an occultation event in the low-luminosity narrow-line Seyfert 1 galaxy WPVS 007 in 2015 February and March. In concert with longer timescale variability, these observations place strong constraints on the nature and location of the absorbing material. Swift monitoring has revealed a secular decrease since ~2010 accompanied by flattening of the optical and UV photometry that suggests variable reddening. Analysis of four Hubble Space Telescope COS observations since 2010, including a Director's Discretionary time observation during the occultation, shows that the broad-absorption-line velocity offset and the CIV emission-line width both decrease as the reddening increases. The occultation dynamical timescale, the BAL variability dynamical timescale, and the density of the BAL gas show that both the reddening material and the broad-absorption-line gas are consistent with an origin in the torus. These observations can be explained by a scenario in which the torus is clumpy with variable scale height, and the BAL gas is blown from the torus material like spray from the crest of a wave. As the obscuring material passes into our line of sight, we alternately see high-velocity broad absorption lines and a clear view to the central engine, or low-velocity broad absorption lines and strong reddening. WPVS 007 has a small black hole mass, and correspondingly short timescales, and so we may be observing behavior that is common in BALQSOs, but is not typically observable.

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arXiv2015

Total eclipse of the heart: The AM CVn Gaia14aae / ASSASN-14cn

We report the discovery and characterisation of a deeply eclipsing AM CVn-system, Gaia14aae (= ASSASN-14cn). Gaia14aae was identified independently by the All-Sky Automated Survey for Supernovae (ASAS-SN; Shappee et al. 2014) and by the Gaia Science Alerts project, during two separate outbursts. A third outburst is seen in archival Pan-STARRS-1 (PS1; Schlafly et al. 2012; Tonry et al. 2012; Magnier et al. 2013) and ASAS-SN data. Spectroscopy reveals a hot, hydrogen-deficient spectrum with clear double-peaked emission lines, consistent with an accreting double degenerate classification. We use follow-up photometry to constrain the orbital parameters of the system. We find an orbital period of 49.71 min, which places Gaia14aae at the long period extremum of the outbursting AM CVn period distribution. Gaia14aae is dominated by the light from its accreting white dwarf. Assuming an orbital inclination of 90 degrees for the binary system, the contact phases of the white dwarf lead to lower limits of 0.78 M solar and 0.015 M solar on the masses of the accretor and donor respectively and a lower limit on the mass ratio of 0.019. Gaia14aae is only the third eclipsing AM CVn star known, and the first in which the WD is totally eclipsed. Using a helium WD model, we estimate the accretor's effective temperature to be 12900+-200 K. The three out-burst events occurred within 4 months of each other, while no other outburst activity is seen in the previous 8 years of Catalina Real-time Transient Survey (CRTS; Drake et al. 2009), Pan-STARRS-1 and ASAS-SN data. This suggests that these events might be rebrightenings of the first outburst rather than individual events.

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arXiv2015

The Gamma-Ray Detected Narrow-Line Seyfert 1 Galaxy 1H 0323+342: Swift Monitoring and Suzaku Spectroscopy

As a radio-loud narrow-line Seyfert 1 galaxy (NLS1) detected by Fermi/LAT in GeV $γ$-rays, 1H 0323+342 is a remarkable Active Galactic Nucleus (AGN) showing properties characteristic of both NLS1s and blazars. Here we present results of simultaneous X-ray and UV/optical monitoring observations on 1H 0323+342 taken with the UV/Optical Telescope (UVOT) and X-ray Telescope (XRT) onboard the Swift satellite over six years from 2006. Overall, the object showed statistically correlated variations in both the UV and X-ray bands on timescales of years as well as on timescales of days. A deep Suzaku observation reveals X-ray variability on timescales as short as a few tens of thousand seconds, and an X-ray spectrum typical of Seyfert galaxies. The broad-band spectral energy distribution, for which the data of UV and X-ray observations taken on 2009 July 26-27 were used, can be well modeled with a simple one-zone leptonic jet model plus accretion disk/corona emission. The latter is predominantly responsible for the UV/optical and X-ray (0.3-10 keV) emission and their observed variations. The correlated UV-X-ray variability on the timescale of days is consistent with reprocessing of the X-ray radiation by the accretion disk. The shortest timescale and large normalized excess variance of the X-ray variability detected with Suzaku suggest a relatively small black hole mass of the order of $10^7M_{\odot}$, consistent with the estimation based on the broad H$β$ line in the optical.

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arXiv2015

Tidal disruption of stars by supermassive black holes: Status of observations

Stars in the immediate vicinity of supermassive black holes (SMBHs) can be ripped apart by the tidal forces of the black hole. The subsequent accretion of the stellar material causes a spectacular flare of electromagnetic radiation. Here, we provide a review of the observations of tidal disruption events (TDEs), with an emphasis on the important contributions of Swift to this field. TDEs represent a new probe of matter under strong gravity, and have opened up a new window into studying accretion physics under extreme conditions. The events probe relativistic effects, provide a new means of measuring black hole spin, and represent signposts of intermediate-mass BHs, binary BHs and recoiling BHs. Luminous, high-amplitude X-ray flares, matching key predictions of the tidal disruption scenario, have first been discovered with ROSAT, and more recently with other missions and in other wavebands. The Swift discovery of two gamma-ray emitting, jetted TDEs, never seen before, has provided us with a unique probe of the early phases of jet formation and evolution, and SwiftJ1644+75 has the best covered lightcurve of any TDE to date. Further, Swift has made important contributions in providing well-covered lightcurves of TDEs discovered with other instruments, setting constraints on the physics that govern the TDE evolution, and including the discovery of the first candidate binary SMBH identified from a TDE lightcurve.

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arXiv2015

IC 3599 did it again: A second outburst of the X-ray transient Seyfert 1.9 Galaxy

We report on the Swift discovery of a second high-amplitude (factor 100) outburst of the Seyfert 1.9 galaxy IC 3599, and discuss implications for outburst scenarios. Swift detected this active galactic nucleus (AGN) again in February 2010 in X-rays at a level of (1.50\plm0.11)$\times 10^{36}$ W (0.2-2.0 keV), which is nearly as luminous as the first outburst detected with ROSAT in 1990. Optical data from the Catalina sky survey show that the optical emission was already bright two years before the Swift X-ray high-state. Our new Swift observations performed between 2013 and 2015 show that IC 3599 is currently again in a very low X-ray flux state. This repeat optical and X-ray outburst, and the long optical duration, suggest that IC 3599 is likely not a tidal disruption event (TDE). Instead, variants of AGN-related variability are explored. The data are consistent with an accretion disk instability around a black hole of mass on the order 10$^6$--10$^7$ M$_{\odot}$; a value estimated using several different methods.

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arXiv2015

Seyfert galaxies with Swift: giant flares, rapid drops, and other surprises

Swift has initiated a new era of understanding the extremes of active galactic nuclei (AGN) variability, their drivers and underlying physics. This is based on its rapid response, high sensitivity, good spatial resolution, and its ability to collect simultaneously X--ray-to-optical SEDs. Here, we present results from our recent monitoring campaigns with Swift of highly variable AGN, including outbursts, deep low states, and unusual long-term trends in several Seyfert galaxies including Mrk 335, WPVS007, and RXJ2314.9+2243. We also report detection of a new X-ray and optical outburst of IC 3599 and our Swift follow-ups. IC 3599 was previously known as one of the AGN with the highest-amplitude outbursts. We briefly discuss implications of this second outburst of IC 3599 for emission scenarios including accretion-disk variability, repeat tidal disruption events, and the presence of a binary supermassive black hole.

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arXiv2015

Finding AGN in Deep X-ray Flux States with Swift

We report on our ongoing project of finding Active Galactic Nuclei (AGN) that go into deep X-ray flux states detected by Swift. Swift is performing an extensive study on the flux and spectral variability of AGN using Guest Investigator and team fill-in programs followed by triggering XMM_Newton for deeper follow-up observations. So far this program has been very successful and has led to a number of XMM-Newton follow up observations, including Mkn 335, PG 0844+349, and RX J2340.8-5329. Recent analysis of new Swift AGN observations reveal several AGN went into a very low X-ray flux state, particularly Narrow-Line Seyfert 1 galaxies. One of these is RX J2317-4422, which dropped by a factor of about 60 when compared to the ROSAT All-Sky Survey.

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arXiv2015

SKA as a powerful hunter of jetted Tidal Disruption Events

Observational consequences of the tidal disruption of stars by supermassive black holes (SMBHs) can enable us to discover quiescent SMBHs and constrain their mass function. Moreover, observing jetted TDEs (from previously non-active galaxies) provides us with a new means of studying the early phases of jet formation and evolution in an otherwise "pristine" environment. Although several (tens) TDEs have been discovered since 1999, only two jetted TDEs have been recently discovered in hard X-rays, and only one, Swift J1644+57, has a precise localization which further supports the TDE interpretation. These events alone are not sufficient to address those science issues, which require a substantial increase of the current sample. Despite the way they were discovered, the highest discovery potential for {\em jetted} TDEs is not held by current and up-coming X-ray instruments, which will yield only a few to a few tens events per year. In fact, the best strategy is to use the Square Kilometer Array to detect TDEs and trigger multi-wavelength follow-ups, yielding hundreds candidates per year, up to $z \sim 2$. Radio and X-ray synergy, however, can in principle constrain important quantities such as the absolute rate of jetted TDEs, their jet power, bulk Lorentz factor, the black hole mass function, and perhaps discover massive black holes (MBH) with $<10^{5} M_{\odot}$. Finally, when comparing SKA results with information from optical surveys like LSST, one can more directly constrain the efficiency of jet production.

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arXiv2015

LOFT as a discovery machine for jetted Tidal Disruption Events

This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of jetted tidal disruption events. For a summary, we refer to the paper.

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arXiv2015

Radio jet emission from GeV-emitting narrow-line Seyfert 1 galaxies

We studied the radio emission from four radio-loud and gamma-ray-loud narrow-line Seyfert 1 galaxies. The goal was to investigate whether a relativistic jet is operating at the source, and quantify its characteristics. We relied on the most systematic monitoring of such system in the cm and mm radio bands which is conducted with the Effelsberg 100 m and IRAM 30 m telescopes and covers the longest time-baselines and the most radio frequencies to date. We extract variability parameters and compute variability brightness temperatures and Doppler factors. The jet powers were computed from the light curves to estimate the energy output. The dynamics of radio spectral energy distributions were examined to understand the mechanism causing the variability. All the sources display intensive variability that occurs at a pace faster than what is commonly seen in blazars. The flaring events show intensive spectral evolution indicative of shock evolution. The brightness temperatures and Doppler factors are moderate, implying a mildly relativistic jet. The computed jet powers show very energetic flows. The radio polarisation in one case clearly implies a quiescent jet underlying the recursive flaring activity. Despite the generally lower flux densities, the sources appear to show all typical characteristics seen in blazars that are powered by relativistic jets.

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arXiv2014

Unshifted Metastable He I* Mini-Broad Absorption Line System in the Narrow Line Type 1 Quasar SDSS J080248.18$+$551328.9

We report the identification of an unusual absorption line system in the quasar SDSS J080248.18$+$551328.9 and present a detailed study of the system, incorporating follow-up optical and NIR spectroscopy. A few tens of absorption lines are detected, including He I*, Fe II* and Ni II* that arise from metastable or excited levels, as well as resonant lines in Mg I, Mg II, Fe II, Mn II, and Ca II. All of the isolated absorption lines show the same profile of width $Δv\sim 1,500$km s$^{-1}$ centered at a common redshift as that of the quasar emission lines, such as [O II], [S II], and hydrogen Paschen and Balmer series. With narrow Balmer lines, strong optical Fe II multiplets, and weak [O III] doublets, its emission line spectrum is typical for that of a narrow-line Seyfert 1 galaxy (NLS1). We have derived reliable measurements of the gas-phase column densities of the absorbing ions/levels. Photoionization modeling indicates that the absorber has a density of $n_{\rm H} \sim (1.0-2.5)\times 10^5~ {\rm cm}^{-3}$ and a column density of $N_{\rm H} \sim (1.0-3.2)\times 10^{21} \sim {\rm cm}^{-2}$, and is located at $R\sim100-250$ pc from the central super-massive black hole. The location of the absorber, the symmetric profile of the absorption lines, and the coincidence of the absorption and emission line centroid jointly suggest that the absorption gas is originated from the host galaxy and is plausibly accelerated by stellar processes, such as stellar winds \zhy{and/or} supernova explosions. The implications for the detection of such a peculiar absorption line system in an NLS1 are discussed in the context of co-evolution between super-massive black hole growth and host galaxy build-up.

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arXiv2014

An Unobscured type II quasar candidate: SDSS J012032.19-005501.9

We report the finding of an unobscured type II Active Galactic Nuclei (AGN) candidate, SDSS J012032.19-005501.9 at a relatively high redshift of 0.601,which shows a number of unusual properties. It varies significantly on timescales of years as typical type I AGNs and marginally on timescales of weeks. The color-magnitude relation and the structure function are also consistent with that of type I AGNs, which imply that its variability likely originates from the black hole accretion system .However, no broad emission line is detected in the SDSS spectrum, and the upper limit of the equivalent width of the H$\rm β$ broad emission line is much less than that of type I AGNs. These properties suggest that SDSS J012032.19-005501.9 may be an unobscured quasar without broad emission lines intrinsically, namely an unobscured type II AGN or "true" type II AGN. Furthermore, its continuum luminosity is at least one order of magnitude fainter than the average value of thepast century expected from the [OIII] emission line. It indicates that SDSS J012032.19-005501.9 may be switching off. Additional possible scenarios to explain this intriguing source are also discussed. Future deep observations at multi-wavelengths are needed to reveal the nature of this peculiar and intriguing AGN.

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