Abstract
The Seyfert-1 galaxy NGC 3516 has undergone major spectral changes in recent years. In 2017 we obtained Chandra, NuSTAR, and Swift observations during its new low-flux state. Using these ...observations, we model the spectral energy distribution (SED) and the intrinsic X-ray absorption, and compare the results with those from historical observations taken in 2006. We thereby investigate the effects of the changing-look phenomenon on the accretion-powered radiation and the ionized outflows. Compared to its normal high-flux state in 2006, the intrinsic bolometric luminosity of NGC 3516 was lower by a factor of 4–8 during 2017. Our SED modeling shows a significant decline in the luminosity of all the continuum components from the accretion disk and the X-ray source. As a consequence, the reprocessed X-ray emission lines have also become fainter. The Swift monitoring of NGC 3516 shows remarkable X-ray spectral variability on short (weeks) and long (years) timescales. We investigate whether this variability is driven by obscuration or the intrinsic continuum. We find that the new low-flux spectrum of NGC 3516, and its variability, do not require any new or variable obscuration, and instead can be explained by changes in the ionizing SED that result in the lowering of the ionization of the warm-absorber outflows. This in turn induces enhanced X-ray absorption by the warm-absorber outflows, mimicking the presence of new obscuring gas. Using the response of the ionized regions to the SED changes, we place constraints on their densities and locations.
X-ray polarimetry has been suggested as a prominent tool for investigating the geometrical and physical properties of the emissions from active galactic nuclei (AGN). The successful launch of the ...Imaging X-ray Polarimetry Explorer (IXPE) on 9 December 2021 has expanded the previously restricted scope of polarimetry into the X-ray domain, enabling X-ray polarimetric studies of AGN. Over a span of two years, IXPE has observed various AGN populations, including blazars and radio-quiet AGN. In this paper, we summarize the remarkable discoveries achieved thanks to the opening of the new window of X-ray polarimetry of AGN through IXPE observations. We will delve into two primary areas of interest: first, the magnetic field geometry and particle acceleration mechanisms in the jets of radio-loud AGN, such as blazars, where the relativistic acceleration process dominates the spectral energy distribution; and second, the geometry of the hot corona in radio-quiet AGN. Thus far, the IXPE results from blazars favor the energy-stratified shock acceleration model, and they provide evidence of helical magnetic fields inside the jet. Concerning the corona geometry, the IXPE results are consistent with a disk-originated slab-like or wedge-like shape, as could result from Comptonization around the accretion disk.
Multi-wavelength campaign on NGC 7469 Behar, Ehud; Peretz, Uria; Kriss, Gerard A. ...
Astronomy and astrophysics (Berlin),
05/2017, Volume:
601
Journal Article
Peer reviewed
Open access
Aims. Outflows in active galaxies (AGNs) are common, although their launching mechanism, location, and physical impact on the host galaxy remain controversial. We conducted a multi-wavelength ...six-month campaign to observe the nearby Seyfert galaxy NGC 7469 with several observatories in order to better understand and quantify the outflow in this AGN. Methods. We report on the time-integrated line-resolved X-ray spectrum of NGC 7469 obtained with the Reflection Grating Spectrometer (RGS) on board XMM-Newton. We used the RGS spectrum to discern the many AGN outflow components and applied a global fit to obtain their physical parameters. Results. We find that the AGN wind can be well described by three narrow velocity components at ~–650, –950, and –2050 km s-1. The RGS clearly resolves the –2050 km s-1 component in C5+ Ly α, while the –650 km s-1 and –950 km s-1 velocities are blended. Similar velocities (±200 km s-1) are resolved in the UV. The H-equivalent column densities of these components are, respectively, NH ~ 7 × 1020, 2.2 × 1021, and 1020 cm-2, for a total of ~3 × 1021 cm-2, which was also measured in 2004, indicating the absorber did not significantly change. The –650 km s-1 component shows a broad ionization distribution (−1 ≲ log ξ ≲ 2,ξ being the ionization parameter in erg s-1 cm). We identify a photo-ionized emission component blue-shifted by ~–450 km s-1, somewhat broad (FWHM = 1400 km s-1), and with −1 ≲ log ξ ≲ 1 erg s-1 cm, which we ascribe to the same outflow that produces the absorption lines. We also find a collisionally ionized component at kT = 0.35 keV that we associate with the circum-nuclear star-formation activity of NGC 7469, as it follows the LFIR/LX ≈ 104 relation found in star forming galaxies. The elemental abundance ratios of C, N, Ne, S, and Fe to O in the outflow tend to be between one and two times solar. Preliminary estimates of the absorber distance from the AGN center suggest it is at least a few pc away from the center, but more advanced methods need to be applied in order to obtain better constraints. Conclusions. The complex X-ray spectrum of NGC 7469 demonstrates the richness of high energy phenomena taking place in AGN cores. The subtle spectroscopic differences between the various components require deep, high-resolution observations, such as the present RGS spectrum, if one is to resolve them and perform quantitative plasma diagnostics.
Context.
The X-ray polarization degree of the molecular clouds surrounding Sgr A* is expected to be greatly reduced due to the mixing of the polarized reflection emission with the unpolarized thermal ...emission that pervades the Galactic center region. This makes this type of observation a challenging experiment for the upcoming Imaging X-ray Polarimeter Explorer (IXPE), whose launch is expected in 2021.
Aims.
We aim to determine the detectability of four molecular clouds of the Sgr A complex (MC2, Bridge B2, Bridge E, and G0.11-0.11) in a realistic IXPE pointing of the Sgr A field of view. We assess the minimum increase in the detectable polarization when a molecular cloud is off-axis. We provide two different strategies for reconstructing the intrinsic cloud polarization once the data is available.
Methods.
We used the Monte Carlo tool ixpeobssim to simulate IXPE observations of the Sgr A molecular cloud complex. We used
Chandra
maps and spectra to model the diffuse emission in the Galactic center region along with a realistic model of the instrumental and diffuse background. We created synthetic polarization products of the unpolarized emission and combined them with a test data set obtained from a simulation of a 2 Ms long IXPE observation to retrieve the intrinsic polarization degree of the molecular clouds.
Results.
We find that for the molecular clouds considered here, the minimum detectable polarization (MDP) increases by ~1–15% with respect to the case in which a cloud is observed on-axis. We successfully retrieve the intrinsic polarization degree in the 4.0–8.0 keV band and line-of-sight distance of one of them taken as an example, namely, G0.11-0.11, by correcting the observed (i.e., for a 2 Ms-long simulation) polarization degree map using either a synthetic dilution map or a Stokes intensity map of the unpolarized emission. With both methods, the position of the cloud along the line-of-sight is derived from the reconstructed polarization degree with an uncertainty of 7 and 4 pc, respectively.
Conclusions.
We confirm the results of previous studies, finding that G0.11-0.11 is the most promising target. For the Sgr A molecular complex region, we propose an observation strategy that may permit detection up to three clouds in the 4.0–8.0 keV band, depending on their true line-of-sight position. We demonstrate that by using simulated data products of the unpolarized components, it is possible to clean up the observed polarization maps from the environmental contamination. The methods we present here are potentially useful for the analysis of X-ray polarimetric data of any extended source that is affected by environmental dilution of the polarized signal. To accurately measure (i.e, with uncertainties of the order of a few parsec) the distance of the cloud along the line-of-sight, a high-quality spectrum and image of the clouds quasi simultaneous to the IXPE pointing are needed.
The relationship between warm absorber (WA) outflows of active galactic nuclei and nuclear obscuration activities caused by optically thick clouds (obscurers) crossing the line of sight is still ...unclear. NGC 3227 is a suitable target for studying the properties of both WAs and obscurers because it matches the following selection criteria: WAs in both ultraviolet (UV) and X-rays, suitably variable, bright in UV and X-rays, and adequate archival spectra for making comparisons with the obscured spectra. In the aim of investigating WAs and obscurers of NGC 3227 in detail, we used a broadband spectral-energy-distribution model that is built in findings of the first paper in our series together with the photoionization code of
SPEX
software to fit the archival observational data taken by
XMM-Newton
and
NuSTAR
in 2006 and 2016. Using unobscured observations, we find four WA components with different ionization states (log
ξ
erg cm s
−1
∼ −1.0, 2.0, 2.5, 3.0). The highest-ionization WA component has a much higher hydrogen column density (∼10
22
cm
−2
) than the other three components (∼10
21
cm
−2
). The outflow velocities of these WAs range from 100 to 1300 km s
−1
, and show a positive correlation with the ionization parameter. These WA components are estimated to be distributed from the outer region of the broad line region (BLR) to the narrow line region. It is worth noting that we find an X-ray obscuration event in the beginning of the 2006 observation, which was missed by previous studies. We find that it can be explained by a single obscurer component. We also study the previously published obscuration event captured in one observation in 2016, which needs two obscurer components to fit the spectrum. A high-ionization obscurer component (log
ξ
∼ 2.80; covering factor
C
f
∼ 30%) only appears in the 2016 observation, which has a high column density (∼10
23
cm
−2
). A low-ionization obscurer component (log
ξ
∼ 1.0 − 1.9;
C
f
∼ 20%−50%) exists in both 2006 and 2016 observations, which has a lower column density (∼10
22
cm
−2
). These obscurer components are estimated to reside within the BLR by their crossing time of transverse motions. The obscurers of NGC 3227 are closer to the center and have larger number densities than the WAs, which indicate that the WAs and obscurers might have different origins.
We report the discovery of X-ray polarization from the X-ray-bright filament G0.13−0.11 in the Galactic center (GC) region. This filament features a bright, hard X-ray source that is most plausibly a ...pulsar wind nebula (PWN) and an extended and structured diffuse component. Combining the polarization signal from IXPE with the imaging/spectroscopic data from Chandra , we find that X-ray emission of G0.13−0.11 is highly polarized PD = 57(±18)% in the 3−6 keV band, while the polarization angle is PA = 21 ° ( ± 9 ° ). This high degree of polarization proves the synchrotron origin of the X-ray emission from G0.13−0.11. In turn, the measured polarization angle implies that the X-ray emission is polarized approximately perpendicular to a sequence of nonthermal radio filaments that may be part of the GC Radio Arc. The magnetic field on the order of 100 μG appears to be preferentially ordered along the filaments. The above field strength is the fiducial value that makes our model self-consistent, while the other conclusions are largely model independent.
Multi-wavelength campaign on NGC 7469 Behar, Ehud; Peretz, Uria; Kriss, Gerard A. ...
Astronomy and astrophysics (Berlin),
05/2017, Volume:
601
Journal Article
Peer reviewed
Open access
Aims. Outflows in active galaxies (AGNs) are common, although their launching mechanism, location, and physical impact on the host galaxy remain controversial. We conducted a multi-wavelength ...six-month campaign to observe the nearby Seyfert galaxy NGC 7469 with several observatories in order to better understand and quantify the outflow in this AGN.Methods. We report on the time-integrated line-resolved X-ray spectrum of NGC 7469 obtained with the Reflection Grating Spectrometer (RGS) on board XMM-Newton. We used the RGS spectrum to discern the many AGN outflow components and applied a global fit to obtain their physical parameters. Results. We find that the AGN wind can be well described by three narrow velocity components at ~–650, –950, and –2050 km s-1. The RGS clearly resolves the –2050 km s-1 component in C5+ Ly α, while the –650 km s-1 and –950 km s-1 velocities are blended. Similar velocities (±200 km s-1) are resolved in the UV. The H-equivalent column densities of these components are, respectively, NH ~ 7 × 1020, 2.2 × 1021, and 1020 cm-2, for a total of ~3 × 1021 cm-2, which was also measured in 2004, indicating the absorber did not significantly change. The –650 km s-1 component shows a broad ionization distribution (−1 ≲ log ξ ≲ 2,ξ being the ionization parameter in erg s-1 cm). We identify a photo-ionized emission component blue-shifted by ~–450 km s-1, somewhat broad (FWHM = 1400 km s-1), and with −1 ≲ log ξ ≲ 1 erg s-1 cm, which we ascribe to the same outflow that produces the absorption lines. We also find a collisionally ionized component at kT = 0.35 keV that we associate with the circum-nuclear star-formation activity of NGC 7469, as it follows the LFIR/LX ≈ 104 relation found in star forming galaxies. The elemental abundance ratios of C, N, Ne, S, and Fe to O in the outflow tend to be between one and two times solar. Preliminary estimates of the absorber distance from the AGN center suggest it is at least a few pc away from the center, but more advanced methods need to be applied in order to obtain better constraints.Conclusions. The complex X-ray spectrum of NGC 7469 demonstrates the richness of high energy phenomena taking place in AGN cores. The subtle spectroscopic differences between the various components require deep, high-resolution observations, such as the present RGS spectrum, if one is to resolve them and perform quantitative plasma diagnostics.Key words: galaxies: active / quasars: absorption lines / galaxies: Seyfert / galaxies: individual: NGC 7469
Context.
NGC 5548 is an X-ray bright Seyfert 1 active galaxy. It exhibits a variety of spectroscopic features in the soft X-ray band, in particular including the absorption by the active galactic ...nucleus (AGN) outflows of a broad range of ionization states, with column densities up to 10
27
m
−2
, and having speeds up to several thousand kilometers per second. The known emission features are in broad agreement with photoionized X-ray narrow and broad emission line models.
Aims.
We report on an X-ray spectroscopic study using 1.1 Ms
XMM-Newton
and 0.9 Ms
Chandra
grating observations of NGC 5548 spanning two decades. The aim is to search and characterize any potential spectroscopic features in addition to the known primary spectral components that are already modeled in high precision.
Methods.
For each observation, we modeled the data using a global fit including an intrinsic spectral energy distribution of the AGNs and the known distant X-ray absorbers and emitters. We utilized as much knowledge from previous studies as possible. The fit residuals were stacked and scanned for possible secondary features.
Results.
We detect a weak unidentified excess emission feature at ∼18.4 Å (18.1 Å in the restframe). The feature is seen at > 5
σ
statistical significance taking the look-elsewhere effect into account. No known instrumental issues, atomic transitions, or astrophysical effects can explain this excess. The observed intensity of the possible feature seems to anticorrelate in time with the hardness ratio of the source. However, even though the variability might not be intrinsic, it might be caused by the time-variable obscuration by the outflows. An intriguing possibility is the line emission from charge exchange between a partially ionized outflow and a neutral layer in the same outflow, or in the close environment. Other possibilities, such as emission from a highly ionized component with high outflowing speed, cannot be fully ruled out.
X-ray polarimetry has been suggested as a prominent tool for investigating the geometrical and physical properties of the emissions from active galactic nuclei (AGN). The successful launch of the ...Imaging X-ray Polarimetry Explorer (IXPE) on 9 December 2021 has expanded the previously restricted scope of polarimetry into the X-ray domain, enabling X-ray polarimetric studies of AGN. Over a span of two years, IXPE has observed various AGN populations, including blazars and radio-quiet AGN. In this paper, we summarize the remarkable discoveries achieved thanks to the opening of the new window of X-ray polarimetry of AGN through IXPE observations. We will delve into two primary areas of interest: first, the magnetic field geometry and particle acceleration mechanisms in the jets of radio-loud AGN, such as blazars, where the relativistic acceleration process dominates the spectral energy distribution; and second, the geometry of the hot corona in radio-quiet AGN. Thus far, the IXPE results from blazars favor the energy-stratified shock acceleration model, and they provide evidence of helical magnetic fields inside the jet. Concerning the corona geometry, the IXPE results are consistent with a disk-originated slab-like or wedge-like shape, as could result from Comptonization around the accretion disk.
Abstract
We describe IXPE polarization observations of the pulsar wind nebula (PWN) MSH 15−5
2
, the “Cosmic Hand.” We find X-ray polarization across the PWN, with
B
-field vectors generally aligned ...with filamentary X-ray structures. High-significance polarization is seen in arcs surrounding the pulsar and toward the end of the “jet,” with polarization degree PD > 70%, thus approaching the maximum allowed synchrotron value. In contrast, the base of the jet has lower polarization, indicating a complex magnetic field at significant angle to the jet axis. We also detect significant polarization from PSR B1509−58 itself. Although only the central pulse phase bin of the pulse has high individual significance, flanking bins provide lower-significance detections and, in conjunction with the X-ray image and radio polarization, can be used to constrain rotating vector model solutions for the pulsar geometry.
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