arXiv subjects

D. Porquet

Publications and source records attributed to D. Porquet.

At least 55 records · Page 3Linked to original sources

arXiv2011

Rapid Intrinsic Variability of Sgr A* at Radio Wavelengths

Sgr A* exhibits flares in radio, millimeter and submm wavelengths with durations of $\sim 1$ hour. Using structure function, power spectrum and autocorrelation function analysis, we investigate the variability of Sgr A* on time scales ranging from a few seconds to several hours and find evidence for sub-minute time scale variability at radio wavelengths. These measurements suggest a strong case for continuous variability from sub-minute to hourly time scales. This short time scale variability constrains the size of the emitting region to be less than 0.1\,AU. Assuming that the minute time scale fluctuations of the emission at 7\,mm arise through the expansion of regions of optically thick synchrotron-emitting plasma, this suggests the presence of explosive, energetic expansion events at speeds close to $c$. The required rate of mass processing and energy loss of this component are estimated to be $\ga 6\times 10^{-10} \msol$ yr$^{-1}$ and 400\,$L_\odot$ respectively. The inferred scale length corresponding to one-minute light travel time is comparable to the time averaged spatially resolved 0.1AU scale observed at 1.3mm emission of Sgr A*. This steady component from Sgr A* is interpreted mainly as an ensemble average of numerous weak and overlapping flares that are detected on short time scales. The nature of such short time scale variable emission or quiescent variability is not understood but could result from fluctuations in the accretion flow of Sgr A* that feed the base of an outflow or jet.

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arXiv2010

The two states of Sgr A* in the near-infrared: bright episodic flares on top of low-level continuous variability

In this paper we examine properties of the variable source Sgr A* in the near-infrared (NIR) using a very extensive Ks-band data set from NACO/VLT observations taken 2004 to 2009. We investigate the variability of Sgr A* with two different photometric methods and analyze its flux distribution. We find Sgr A* is continuously emitting and continuously variable in the near-infrared, with some variability occurring on timescales as long as weeks. The flux distribution can be described by a lognormal distribution at low intrinsic fluxes (<~5 mJy, dereddened with A_{Ks}=2.5). The lognormal distribution has a median flux of approximately 1.1 mJy, but above 5 mJy the flux distribution is significantly flatter (high flux events are more common) than expected for the extrapolation of the lognormal distribution to high fluxes. We make a general identification of the low level emission above 5 mJy as flaring emission and of the low level emission as the quiescent state. We also report here the brightest Ks-band flare ever observed (from August 5th, 2008) which reached an intrinsic Ks-band flux of 27.5 mJy (m_{Ks}=13.5). This flare was a factor 27 increase over the median flux of Sgr A*, close to double the brightness of the star S2, and 40% brighter than the next brightest flare ever observed from Sgr~A*.

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arXiv2010

Iron line profiles in Suzaku spectra of bare Seyfert galaxies

We methodically model the broad-band Suzaku spectra of a small sample of six 'bare' Seyfert galaxies: Ark 120, Fairall 9, MCG-02-14-009, Mrk 335, NGC 7469 and SWIFT J2127.4+5654. The analysis of bare Seyferts allows a consistent and physical modelling of AGN due to a weak amount of any intrinsic warm absorption, removing the degeneracy between the spectral curvature due to warm absorption and the red-wing of the Fe K region. Through effective modelling of the broad-band spectrum and investigating the presence of narrow neutral or ionized emission lines and reflection from distant material, we obtain an accurate and detailed description of the Fe K line region using models such as laor, kerrdisk and kerrconv. Results suggest that ionized emission lines at 6.7 keV and 6.97 keV (particularly Fe XXVI) are relatively common and the inclusion of these lines can greatly affect the parameters obtained with relativistic models i.e. spin, emissivity, inner radius of emission and inclination. Moderately broad components are found in all objects, but typically the emission originates from tens of Rg, rather than within <6Rg of the black hole. Results obtained with kerrdisk line profiles suggest an average emissivity of q~2.3 at intermediate spin values with all objects ruling out the presence of a maximally spinning black hole at the 90% confidence level. We also present new spin constraints for Mrk 335 and NGC 7469 with intermediate values of a=0.70(+0.12,-0.01) and a=0.69(+0.09,-0.09) respectively.

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arXiv2010

FERO: Finding Extreme Relativistic Objects. I. Statistics of Relativistic Fe Kalpha lines in Radio-Quiet Type 1 AGN

Accretion models predict that fluorescence lines broadened by relativistic effects should arise from reflection of X-ray emission onto the inner region of the accretion disc surrounding the central black hole of active galactic nuclei (AGN). The theory behind the origin of relativistic lines is well established, and observational evidence from a moderate number of sources seems to support the existence of these lines. The aim of this work is to establish the fraction of AGN with relativistic Fe Kalpha lines, and study possible correlations with source physical properties. An XMM-Newton collection of 149 radio-quiet Type 1 AGN has been systematically and uniformly analyzed in order to search for significant evidence of a relativistically broadened Fe Kalpha line. To enable statistical studies, an almost complete, flux-limited subsample of 31 sources has been defined. The 2-10 keV spectra of the FERO sources have been compared with a complex model including most of the physical components observed in the X-ray spectra of Seyfert galaxies: a power law primary continuum modified by non-relativistic Compton reflection and warm absorption, plus a series of narrow Fe line reflection features. The observed fraction of sources in the flux-limited sample that show significant evidence of a relativistic Fe Kalpha line is 36%. The average line Equivalent Width (EW) is of the order of 100 eV, while the average disc inclination angle is 28+/-5 deg and the average power-law index of the radial disc emissivity law is 2.4+/-0.4. The spin value is well constrained only in 2 cases (MCG-6-30-15 and MRK509), and in the rest of the cases, whenever a constraint can be placed, it always implies the rejection of the static black hole solution. The Fe Kalpha line EW does not correlate with disc parameters or with system physical properties, such as black hole mass, accretion rate and hard X-ray luminosity.

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arXiv2010

Evidence for a Truncated Accretion Disc in the Low Luminosity Seyfert Galaxy, NGC 7213?

We present the broad-band 0.6-150 keV Suzaku and Swift BAT spectra of the low luminosity Seyfert galaxy, NGC 7213. The time-averaged continuum emission is well fitted by a single powerlaw of photon index Gamma = 1.75 and from consideration of the Fermi flux limit we constrain the high energy cutoff to be 350 keV < E < 25 MeV. Line emission from both near-neutral iron K_alpha at 6.39 keV and highly ionised iron, from Fe_(xxv) and Fe_(xxvi), is strongly detected in the Suzaku spectrum, further confirming the results of previous observations with Chandra and XMM-Newton. We find the centroid energies for the Fe_(xxv) and Fe_(xxvi) emission to be 6.60 keV and 6.95 keV respectively, with the latter appearing to be resolved in the Suzaku spectrum. We show that the Fe_(xxv) and Fe_(xxvi) emission can result from a highly photo-ionised plasma of column density N_(H) ~ 3 x 10^(23) cm^(-2). A Compton reflection component, e.g., originating from an optically-thick accretion disc or a Compton-thick torus, appears either very weak or absent in this AGN, subtending < 1 sr to the X-ray source, consistent with previous findings. Indeed the absence of either neutral or ionised Compton reflection coupled with the lack of any relativistic Fe K signatures in the spectrum suggests that an inner, optically-thick accretion disc is absent in this source. Instead, the accretion disc could be truncated with the inner regions perhaps replaced by a Compton-thin Radiatively Inefficient Accretion Flow. Thus, the Fe_(xxv) and Fe_(xxvi) emission could both originate in ionised material perhaps at the transition region between the hot, inner flow and the cold, truncated accretion disc on the order of 10^(3) - 10^(4) gravitational radii from the black hole. The origin for the unresolved neutral Fe K_alpha emission is then likely to be further out, perhaps originating in the optical BLR or a Compton-thin pc-scale torus.

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arXiv2010

X-ray and multiwavelength view of NGC 4278. A LINER-Seyfert connection?

(Abridged) Based on UV to X-ray and radio to UV flux ratios, some argue that low ionization emission line regions (LINERs) and low luminosity AGN (LLAGN) are a scaled-down version of their more luminous predecessors Seyfert galaxies. Others, based on the lack of X-ray short (hours) time-scale variability, the non-detection of an iron line at 6.4 keV, and the faint UV emission, suggest the truncation of the classical thin accretion disk in the inner regions of the AGN where a radiatively inefficient accretion flow (RIAF) structure forms. We investigate the LINER-Seyfert connection by studying the unabsorbed, AGN powered, LINER galaxy NGC 4278. We analyzed one XMM-Newton and seven Chandra X-ray observations of NGC 4278 spread over a three year period. We detected a flux increase by a factor of ~3 on a ~3 months time-scale and by a factor of 5 between the faintest and the brightest observation separated by ~3 years. During only the XMM-Newton observation, where the highest flux level is detected, we found a 10% flux increase on a ~1 hour time-scale. A combination of an absorbed power law (N(H)~10^20 cm^-2, Gamma~2.2) plus a thermal component (kT~0.6 keV) were able to fit the Chandra spectra. The XMM-Newton spectra, where the highest X-ray flux is detected, are well fitted with a single absorbed power-law. No Fe K(alpha) emission line is detected at 6.4 keV. We constructed SEDs based on simultaneous or quasi simultaneous observations and compared them to LINER, radio-loud, and radio-quiet quasar SEDs. We find that at a low X-ray flux the NGC 4278 SED resembles that of typical LINER sources where the radio to X-ray emission can be considered as originating from a jet and/or RIAF, whereas at a high X-ray flux, NGC 4278 SED is more like a low luminosity Seyfert SED. Consequently, NGC 4278 could exhibit both LINER and Seyfert nuclear activity depending on the strength of its X-ray emission.

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arXiv2010

Multi-Wavelength Study of Sgr A*: The Short Time Scale Variability

To understand the correlation and the radiation mechanism of flare emission in different wavelength bands, we have coordinated a number of telescopes to observe SgrA* simultaneously. We focus only on one aspect of the preliminary results of our multi-wavelength observing campaigns, namely, the short time scale variability of emission from SgrA* in near-IR, X-ray and radio wavelengths. The structure function analysis indicate most of the power spectral density is detected on hourly time scales in all wavelength bands. We also report minute time scale variability at 7 and 13mm placing a strong constraint on the nature of the variable emission. The hourly time scale variability can be explained in the context of a model in which the peak frequency of emission shifts toward lower frequencies as a self-absorbed synchrotron source expands adiabatically near the acceleration site. The short time scale variability, on the other hand, places a strong constraint on the size of the emitting region. Assuming that rapid minute time scale fluctuations of the emission is optically thick in radio wavelength, light travel arguments requires relativistic particle energy, thus suggesting the presence of outflow from SgrA*.

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arXiv2010

Flares from Sgr A* and their emission mechanism

We summarize recent observations and modeling of the brightest Sgr A* flare to be observed simultaneously in (near)-infrared and X-rays to date. Trying to explain the spectral characteristics of this flare through inverse Compton mechanisms implies physical parameters that are unrealistic for Sgr A*. Instead, a "cooling break" synchrotron model provides a more feasible explanation for the X-ray emission. In a magnetic field of about 5-30 Gauss the X-ray emitting electrons cool very quickly on the typical dynamical timescale while the NIR-emitting electrons cool more slowly. This produces a spectral break in the model between NIR and X-ray wavelengths that can explain the differences in the observed spectral indices.

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arXiv2009

Soft gamma-ray constraints on a bright flare from the Galactic Center supermassive black hole

Sagittarius A* (Sgr A*) is the supermassive black hole residing at the center of the Milky Way. It has been the main target of an extensive multiwavelength campaign we carried out in April 2007. Herein, we report the detection of a bright flare from the vicinity of the horizon, observed simultaneously in X-rays (XMM/EPIC) and near infrared (VLT/NACO) on April 4th for 1-2 h. For the first time, such an event also benefitted from a soft gamma-rays (INTEGRAL/ISGRI) and mid infrared (VLT/VISIR) coverage, which enabled us to derive upper limits at both ends of the flare spectral energy distribution (SED). We discuss the physical implications of the contemporaneous light curves as well as the SED, in terms of synchrotron, synchrotron self-Compton and external Compton emission processes.

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arXiv2009

Simultaneous Multi-Wavelength Observations of Sgr A* during 2007 April 1-11

We report the detection of variable emission from Sgr A* in almost all wavelength bands (i.e. centimeter, millimeter, submillimeter, near-IR and X-rays) during a multi-wavelength observing campaign. Three new moderate flares are detected simultaneously in both near-IR and X-ray bands. The ratio of X-ray to near-IR flux in the flares is consistent with inverse Compton scattering of near-IR photons by submillimeter emitting relativistic particles which follow scaling relations obtained from size measurements of Sgr A*. We also find that the flare statistics in near-IR wavelengths is consistent with the probability of flare emission being inversely proportional to the flux. At millimeter wavelengths, the presence of flare emission at 43 GHz (7mm) using VLBA with milli-arcsecond spatial resolution indicates the first direct evidence that hourly time scale flares are localized within the inner 30$\times$70 Schwarzschild radii of Sgr A*. We also show several cross correlation plots between near-IR, millimeter and submillimeter light curves that collectively demonstrate the presence of time delays between the peaks of emission up to three hours. The evidence for time delays at millimeter and submillimeter wavelengths are consistent with the source of emission being optically thick initially followed by a transition to an optically thin regime. In particular, there is an intriguing correlation between the optically thin near-IR and X-ray flare and optically thick radio flare at 43 GHz that occurred on 2007 April 4. This would be the first evidence of a radio flare emission at 43 GHz delayed with respect to the near-IR and X-ray flare emission.

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arXiv2009

Bursting behavior of the Galactic Center faint X-ray transient GRS 1741.9-2853

The neutron star low-mass X-ray binary GRS 1741.9-2853 is a known type-I burster of the Galactic Center. It is transient, faint, and located in a very crowded region, only 10 arcmin from the supermassive black hole Sgr A*. Therefore, its bursting behavior has been poorly studied so far. In particular, its persistent emission has rarely been detected between consecutive bursts, due to lack of sensitivity or confusion. This is what made GRS 1741.9-2853 one of the nine "burst-only sources" identified by BeppoSAX a few years ago. The physical properties of GRS 1741.9-2853 bursts are yet of great interest since we know very little about the nuclear regimes at stake in low accretion rate bursters. We examine here for the first time several bursts in relation with the persistent emission of the source, using INTEGRAL, XMM-Newton, and Swift observations. We investigate the source flux variability and bursting behavior during its 2005 and 2007 long outbursts. The persistent luminosity of GRS 1741.9-2853 varied between ~1.7 and 10.5 10^36 erg s^-1, i.e. 0.9-5.3% of the Eddington luminosity. The shape of the spectrum as described by an absorbed power-law remained with a photon index Gamma ~ 2 and a column density $N_{\rm H} ~ 12 10^22 cm^-2 throughout the outbursts. We discovered 11 type-I bursts with INTEGRAL, and inspected 4 additional bursts: 2 recorded by XMM-Newton and 2 by Swift. From the brigthest burst, we derive an upper limit on the source distance of ~7 kpc. The observed bursts characteristics and source accretion rate suggest pure helium explosions igniting at column depths y_{ign} ~ 0.8-4.8 10^8 g cm^-1, for typical energy releases of ~1.2-7.4 10^39 erg.

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arXiv2009

Evidence for X-ray synchrotron emission from simultaneous mid-IR to X-ray observations of a strong Sgr A* flare

This paper reports measurements of Sgr A* made with NACO in L' -band (3.80 um), Ks-band (2.12 um) and H-band (1.66 um) and with VISIR in N-band (11.88 um) at the ESO VLT, as well as with XMM-Newton at X-ray (2-10 keV) wavelengths. On 4 April, 2007, a very bright flare was observed from Sgr A* simultaneously at L'-band and X-ray wavelengths. No emission was detected using VISIR. The resulting SED has a blue slope (beta > 0 for nuL_nu ~ nu^beta, consistent with nuL_nu ~ nu^0.4) between 12 micron and 3.8 micron. For the first time our high quality data allow a detailed comparison of infrared and X-ray light curves with a resolution of a few minutes. The IR and X-ray flares are simultaneous to within 3 minutes. However the IR flare lasts significantly longer than the X-ray flare (both before and after the X-ray peak) and prominent substructures in the 3.8 micron light curve are clearly not seen in the X-ray data. From the shortest timescale variations in the L'-band lightcurve we find that the flaring region must be no more than 1.2 R_S in size. The high X-ray to infrared flux ratio, blue nuL_nu slope MIR to L' -band, and the soft nuL_nu spectral index of the X-ray flare together place strong constraints on possible flare emission mechanisms. We find that it is quantitatively difficult to explain this bright X-ray flare with inverse Compton processes. A synchrotron emission scenario from an electron distribution with a cooling break is a more viable scenario.

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arXiv2008

X-ray hiccups from SgrA* observed by XMM-Newton. The second brightest flare and three moderate flares caught in half a day

[truncated] In Spring 2007, we observed SgrA* with XMM with a total exposure of ~230ks. We have performed timing and spectral analysis of the new X-ray flares detected during this campaign. To study the range of flare spectral properties, in a consistent manner, we have also reprocessed, using the same analysis procedure and the latest calibration, archived XMM data of previously reported rapid flares. The dust scattering was taken into account during the spectral fitting. We also used Chandra archived observations of the quiescent state of SgrA* for comparison. On April 4, 2007, we observed for the first time within a time interval of ~1/2 day, an enhanced incidence rate of X-ray flaring, with a bright flare followed by three flares of more moderate amplitude. The former event represents the second brightest X-ray flare from Sgr A* on record. This new bright flare exhibits similar light-curve shape (nearly symmetrical), duration (~3ks) and spectral characteristics to the very bright flare observed in October 3, 2002. The measured spectral parameters of the new bright flare, assuming an absorbed power law model taken into account dust scattering effect, are N_H=12.3(+2.1,-1.8)e22 cm-2 and Gamma~2.3+/-0.3 calculated at the 90% c.l. The spectral parameter fits of the sum of the three following moderate flares, while lower, are compatible within the error bars with those of the bright flares. The column density found, for a power-law, during the flares is at least two times higher than the value expected from the (dust) visual extinction toward SgrA* (AV~25 mag). However, our fitting of the SgrA* quiescent spectra obtained with Chandra shows that an excess of column density is already present during the non-flaring phase. The two brightest X-ray flares observed so far from SgrA* exhibited similar soft spectra.

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arXiv2007

Relativistic Iron K Emission and absorption in the Seyfert 1.9 galaxy MCG-5-23-16

We present the results of the simultaneous deep XMM and Chandra observations of the bright Seyfert 1.9 galaxy MCG-5-23-16, which is thought to have one of the best known examples of a relativistically broadened iron K-alpha line. The time averaged spectral analysis shows that the iron K-shell complex is best modeled with an unresolved narrow emission component (FWHM < 5000 km/s, EW ~ 60 eV) plus a broad component. This latter component has FWHM ~ 44000 km/s and EW ~ 50 eV. Its profile is well described by an emission line originating from an accretion disk viewed with an inclination angle ~ 40^\circ and with the emission arising from within a few tens of gravitational radii of the central black hole. The time-resolved spectral analysis of the XMM EPIC-pn spectrum shows that both the narrow and broad components of the Fe K emission line appear to be constant in time within the errors. We detected a narrow sporadic absorption line at 7.7 keV which appears to be variable on a time-scale of 20 ksec. If associated with Fe XXVI Ly-alpha this absorption is indicative of a possibly variable, high ionization, high velocity outflow. The variability of this absorption feature appears to rule out a local (z=0) origin. The analysis of the XMM RGS spectrum reveals that the soft X-ray emission of MCG-5-23-16 is likely dominated by several emission lines superimposed on an unabsorbed scattered power-law continuum. The lack of strong Fe L shell emission together with the detection of a strong forbidden line in the O VII triplet is consistent with a scenario where the soft X-ray emission lines are produced in a plasma photoionized by the nuclear emission.

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arXiv2006

An XMM-Newton view of the X-ray flat radio-quiet quasar PG 1416-129

(abridged) The radio-quiet quasar PG1416-129 (z=0.129) exhibits atypical optical and X-ray properties. Between 1990 and 2000, in response to its optical continuum decrease, the ``classical'' broad component of Hbeta almost completely disappeared, with a factor of 10 decrease in the line flux. In the X-ray band, this object was observed by Ginga in 1988 to have the hardest quasar photon index, with Gamma=1.1+/-0.1. We present an XMM/EPIC observation of PG1416-129 performed in July 2004. We analyze the time-averaged pn spectrum of this quasar, as well as perform time-resolved spectroscopy. We find that during the present XMM observation, PG1416-129 still has a rather hard photon index, both in the soft and hard energy ranges, compared to radio-quiet quasars but compatible with the photon index value found for radio-loud quasars. This object also shows long-term luminosity variability over 16 years by a factor of three with a variation of photon index from ~1.2 to ~1.8. In the soft energy band (0.2-2keV), we found a very weak soft X-ray excess compared to other RQ quasars. The whole time averaged spectrum is fit very well either by X-ray ionized reflection from the accretion disk surface, by a warm absorber-emitter plus power-law, or by a smeared absorption/emission from a relativistic outflow. While no constant narrow FeK line at 6.4keV is observed, we find the possible presence of two non-simultaneous transient iron lines: a redshifted narrow iron line at about 5.5keV (96.4% confidence level according to multi-trial Monte-Carlo simulations) at the beginning of this observation and the appearance of a line at 6.3-6.4keV (99.1% c.l.) at the end of the observation. These variable lines could be generated by discrete hot-spots on the accretion disk surface.

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arXiv2006

Variable iron-line emission near the black hole of Markarian 766

We investigate the link between ionised Fe X-ray line emission and continuum emission in the bright nearby AGN, Mrk 766. A new long (433 ks) XMM-Newton observation is analysed, together with archival data from 2000 and 2001. The contribution from ionised line emission is measured and its time variations on short (5-20 ks) timescales are correlated with the continuum emission. The ionised line flux is found to be highly variable and to be strongly correlated with the continuum flux, demonstrating an origin for the ionised line emission that is co-located with the continuum emission. Most likely the emission is ionised reflection from the accretion disc within a few A.U. of the central black hole, and its detection marks the first time that such an origin has been identified other than by fitting to spectral line profiles. Future observations may be able to measure a time lag and hence achieve reverberation mapping of AGN at X-ray energies.

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arXiv2005

Discovery of X-ray eclipses from the transient source CXOGC J174540.0-290031 with XMM-Newton

We present the XMM-Newton observations obtained during four revolutions in Spring and Summer 2004 of CXOGC J174540.0-290031, a moderately bright transient X-ray source, located at only 2.9" from SgrA*. We report the discovery of sharp and deep X-ray eclipses, with a period of 27,961+/-5 s and a duration of about 1,100+/-100 s, observed during the two consecutive XMM revolutions from August 31 to September 2. No deep eclipses were present during the two consecutive XMM revolutions from March 28 to April 1, 2004. The spectra during all four observations are well described with an absorbed power law continuum. While our fits on the power law index over the four observations yield values that are consistent with Gamma=1.6-2.0, there appears to be a significant increase in the column density during the Summer 2004 observations, i.e. the period during which the eclipses are detected. The intrinsic luminosity in the 2-10 keV energy range is almost constant with 1.8-2.3 x 10^34 (d_8kpc)^2 erg/s over the four observations. In the framework of eclipsing semidetached binary systems, we show that the eclipse period constrains the mass of the assumed main-sequence secondary star to less than 1.0 M_odot. Therefore, we deduce that CXOGC J174540.0-290031 is a low-mass X-ray binary (LMXB). Moreover the eclipse duration constrains the mass of the compact object to less than about 60 M_odot, which is consistent with a stellar mass black hole or a neutron star. The absence of deep X-ray eclipses during the Spring 2004 observations could be explained if the centroid of the X-ray emitting region moves from a position on the orbital plane to a point above the compact object, possibly coincident with the base of the jet which was detected in radio at this epoch. [Abstract truncated].

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arXiv2005

A changing inner radius in the accretion disc of Q0056-363?

Q0056-363 is the most powerful X-ray quasar known to exhibit a broad, likely relativistic iron line (Porquet & Reeves 2003). It has been observed twice by XMM-$Newton$, three and half years apart (July 2000 and December 2003). In the second observation, the UV and soft X-ray fluxes were fainter, the hard X-ray power law flatter, and the iron line equivalent width (EW) smaller than in the 2000 observation. These variations can all be explained, at least qualitatively, if the disc is truncated in the second observation. We report also on the possible detection of a transient, redshifted iron absorption line during the 2003 observation.

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