arXiv subjects

F. A. Harrison

Publications and source records attributed to F. A. Harrison.

At least 37 records · Page 2Linked to original sources

arXiv2019

Discovery and analysis of a ULX nebula in NGC 3521

We present Very Large Telescope/X-shooter and Chandra X-ray observatory/ACIS observations of the ULX [SST2011] J110545.62+000016.2 in the galaxy NGC 3521. The source identified as a candidate near-infrared counterpart to the ULX in our previous study shows an emission line spectrum of numerous recombination and forbidden lines in the visible and near-infrared spectral regime. The emission from the candidate counterpart is spatially extended ($\sim$ 34 pc) and appears to be connected with an adjacent H II region, located $\sim$ 138 pc to the NE. The measured velocities of the emission lines confirm that both the candidate counterpart and H II region reside in NGC 3521. The intensity ratios of the emission lines from the ULX counterpart show that the line emission originates from the combined effect of shock and photoionisation of low metallicity (12 + log (O/H) = 8.19 $\pm$ 0.11) gas. Unfortunately, there is no identifiable spectral signature directly related to the photosphere of the mass-donor star in our spectrum. From the archival Chandra data, we derive the X-ray luminosity of the source in the 0.3-7 keV range to be (1.9 $\pm$ 0.8) $\times$ 10$^{40}$ erg cm$^{-2}$ s$^{-1}$, almost a factor of four higher than what is previously reported.

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arXiv2019

NuSTAR Observations of the Accreting Atolls GX 3+1, 4U 1702-429, 4U 0614+091, and 4U 1746-371

Atoll sources are accreting neutron star (NS) low-mass X-ray binaries. We present a spectral analysis of four persistent atoll sources (GX 3+1, 4U 1702$-$429, 4U 0614+091, and 4U 1746$-$371) observed for $\sim20$ ks each with NuSTAR to determine the extent of the inner accretion disk. These sources range from an apparent luminosity of $0.006-0.11$ of the Eddington limit (assuming the empirical limit of $3.8\times10^{38}$ ergs s$^{-1}$). Broad Fe emission features shaped by Doppler and relativistic effects close to the NS were firmly detected in three of these sources. The position of the disk appears to be close to the innermost stable circular orbit (ISCO) in each case. For GX 3+1, we determine $R_{in}=1.8^{+0.2}_{-0.6}\ R_{\mathrm{ISCO}}$ (90% confidence level) and an inclination of $27^{\circ}-31^{\circ}$. For 4U 1702$-$429, we find a $R_{in}=1.5_{-0.4}^{+1.6}\ R_{\mathrm{ISCO}}$ and inclination of $53^{\circ}-64^{\circ}$. For 4U 0614+091, the disk has a position of $R_{in}=1.3_{-0.2}^{+5.4}\ R_{\mathrm{ISCO}}$ and inclination of $50^{\circ}-62^{\circ}$. If the disk does not extend to the innermost stable circular orbit, we can place conservative limits on the magnetic field strength in these systems in the event that the disk is truncated at the Alfvén radius. This provides the limit at the poles of $B\leq6.7\times10^{8}$ G, $3.3\times10^{8}$ G, and $14.5\times10^{8}$ G for GX 3+1, 4U 1702$-$429, and 4U 0614+091, respectively. For 4U 1746$-$371, we argue that the most plausible explanation for the lack of reflection features is a combination of source geometry and strong Comptonization. We place these sources among the larger sample of NSs that have been observed with NuSTAR.

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arXiv2019

A Low-Flux State in IRAS 00521-7054 seen with NuSTAR and XMM-Newton: Relativistic Reflection and an Ultrafast Outflow

We present results from a deep, coordinated $XMM$-$Newton$+$NuSTAR$ observation of the Seyfert 2 galaxy IRAS 00521-7054. The $NuSTAR$ data provide the first detection of this source in high-energy X-rays ($E > 10$ keV), and the broadband data show this to be a highly complex source which exhibits relativistic reflection from the inner accretion disc, further reprocessing by more distant material, neutral absorption, and evidence for ionised absorption in an extreme, ultrafast outflow ($v_{\rm{out}} \sim 0.4c$). Based on lamppost disc reflection models, we find evidence that the central supermassive black hole is rapidly rotating ($a > 0.77$), consistent with previous estimates from the profile of the relativistic iron line, and that the accretion disc is viewed at a fairly high inclination ($i \sim 59^{\circ}$). Based on extensive simulations, we find the ultrafast outflow is detected at $\sim$4$σ$ significance (or greater). We also estimate that the extreme outflow should be sufficient to power galaxy-scale feedback, and may even dominate the energetics of the total output from the system.

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arXiv2019

Searching for the Donor Stars of ULX Pulsars

We report on our search for the optical counterparts of two ultraluminous X-ray pulsars with known orbital periods, M82 X-2 and NGC 5907 X-1, in new and archival HST observations, in an effort to characterize the donor stars in these systems. We detect five near-infrared sources consistent with the position of M82 X-2 that are too bright to be single stars. We also detect seven sources in the WFC3/UVIS F336W image whose photometry matches that of 10-15 M$_\odot$ stars turning off the main sequence. Such stars have densities consistent with the properties of the donor star of M82 X-2 as inferred from X-ray timing analysis, although it is also possible that the donor is a lower mass star below our detection limit or that there is a significant contribution from the accretion disc to the optical emission. We detect three candidate counterparts to NGC 5907 X-1 in the near-infrared. All of these are too bright to be the donor star of the ULX, which based on its orbital period is a red giant. The high background at the location of NGC 5907 X-1 precludes us from detecting this expected donor star. The recently discovered NGC 5907 ULX-2 also falls within the field of view of the near-infrared imaging; we detect four sources in the error circle, with photometry that matches AGB stars. The star suggested to be the counterpart of NGC 5907 ULX-2 by Pintore et al. (2018) falls outside our 2-$σ$ error circle.

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arXiv2018

A long hard-X-ray look at the dual active galactic nuclei of M51 with NuSTAR

We present a broadband X-ray spectral analysis of the M51 system, including the dual active galactic nuclei (AGN) and several off-nuclear point sources. Using a deep observation by NuSTAR, new high-resolution coverage of M51b by Chandra, and the latest X-ray torus models, we measure the intrinsic X-ray luminosities of the AGN in these galaxies. The AGN of M51a is found to be Compton thick, and both AGN have very low accretion rates ($λ_{\rm Edd} <10^{-4}$). The latter is surprising considering that the galaxies of M51 are in the process of merging, which is generally predicted to enhance nuclear activity. We find that the covering factor of the obscuring material in M51a is $0.26 \pm 0.03$, consistent with the local AGN obscured fraction at $L_{\rm X}\sim 10^{40}$ erg s$^{-1}$. The substantial obscuring column does not support theories that the torus, presumed responsible for the obscuration, disappears at these low accretion luminosities. However, the obscuration may have resulted from the gas infall driven by the merger rather than the accretion process. We report on several extra-nuclear sources with $L_{\rm X}>10^{39}$ erg s$^{-1}$ and find that a spectral turnover is present below 10 keV in most such sources, in line with recent results on ultraluminous X-ray sources.

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arXiv2018

NuSTAR observations of Mrk 766: distinguishing reflection from absorption

We present two new NuSTAR observations of the narrow line Seyfert 1 (NLS1) galaxy Mrk 766 and give constraints on the two scenarios previously proposed to explain its spectrum and that of other NLS1s: relativistic reflection and partial covering. The NuSTAR spectra show a strong hard (>15 keV) X-ray excess, while simultaneous soft X-ray coverage of one of the observations provided by XMM-Newton constrains the ionised absorption in the source. The pure reflection model requires a black hole of high spin ($a>0.92$) viewed at a moderate inclination ($i=46^{+1}_{-4}$ degrees). The pure partial covering model requires extreme parameters: the cut-off of the primary continuum is very low ($22^{+7}_{-5}$ keV) in one observation and the intrinsic X-ray emission must provide a large fraction (75%) of the bolometric luminosity. Allowing a hybrid model with both partial covering and reflection provides more reasonable absorption parameters and relaxes the constraints on reflection parameters. The fractional variability reduces around the iron K band and at high energies including the Compton hump, suggesting that the reflected emission is less variable than the continuum.

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arXiv2018

A tale of two periods: determination of the orbital ephemeris of the super-Eddington pulsar NGC 7793 P13

We present a timing analysis of multiple XMM-Newton and NuSTAR observations of the ultra-luminous pulsar NGC 7793 P13 spread over its 65d variability period. We use the measured pulse periods to determine the orbital ephemeris, confirm a long orbital period with P_orb = 63.9 (+0.5,-0.6) d, and find an eccentricity of e <= 0.15. The orbital signature is imprinted on top of a secular spin-up, which seems to get faster as the source becomes brighter. We also analyse data from dense monitoring of the source with Swift and find an optical photometric period of 63.9 +/- 0.5 d and an X-ray flux period of 66.8 +/- 0.4 d. The optical period is consistent with the orbital period, while the X-ray flux period is significantly longer. We discuss possible reasons for this discrepancy, which could be due to a super-orbital period caused by a precessing accretion disk or an orbital resonance. We put the orbital period of P13 into context with the orbital periods implied for two other ultra-luminous pulsars, M82 X-2 and NGC 5907 ULX and discuss possible implications for the system parameters.

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arXiv2018

A Potential Cyclotron Resonant Scattering Feature in the ULX Pulsar NGC 300 ULX1 seen by NuSTAR and XMM-Newton

Based on phase-resolved broadband spectroscopy using $XMM$-$Newton$ and $NuSTAR$, we report on a potential cyclotron resonant scattering feature at $E \sim 13$ keV in the pulsed spectrum of the recently discoverd ULX pulsar NGC 300 ULX1. If this interpretation is correct, the implied magnetic field of the central neutron star is $B \sim 10^{12}$ G (assuming scattering off electrons), similar to that estimated from the observed spin-up of the star, and also similar to known Galactic X-ray pulsars. We discuss the implications of this result for the connection between NGC 300 ULX1 and the other known ULX pulsars, particularly in light of the recent discovery of a likely proton Cyclotron line in another ULX, M51 ULX-8.

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arXiv2018

The 1.5 Ms Observing Campaign on IRAS 13224-3809: X-ray Spectral Analysis I

We present a detailed spectral analysis of the recent 1.5\,Ms XMM-Newton observing campaign on the narrow line Seyfert 1 galaxy IRAS~13224$-$3809, taken simultaneously with 500\,ks of NuSTAR data. The X-ray lightcurve shows three flux peaks, registering at about 100 times the minimum flux seen during the campaign, and rapid variability with a time scale of kiloseconds. The spectra are well fit with a primary powerlaw continuum, two relativistic-blurred reflection components from the inner accretion disk with very high iron abundance, and a simple blackbody-shaped model for the remaining soft excess. The spectral variability is dominated by the power law continuum from a corona region within a few gravitational radii from the black hole. Additionally, blueshifted Ne \textsc{x}, Mg \textsc{xii}, Si \textsc{xiv} and S \textsc{xvi} absorption lines are identified in the stacked low-flux spectrum, confirming the presence of a highly ionized outflow with velocity up to $v= 0.267$ and $0.225$\,c. We fit the absorption features with \texttt{xstar} models and find a relatively constant velocity outflow through the whole observation. Finally, we replace the \texttt{bbody} and supersolar abundance reflection models by fitting the soft excess successfully with the extended reflection model \texttt{relxillD}, which allows for higher densities than the standard \texttt{relxill} model. This returns a disk electron density $n_{\rm e}>10^{18.7}$\,cm$^{-3}$ and lowers the iron abundance from $Z_{\rm Fe}=24^{+3}_{-4}Z_\odot$ with $n_{\rm e}\equiv10^{15}$\,cm$^{-3}$ to $Z_{\rm Fe}=6.6^{+0.8}_{-2.1}Z_\odot$.

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arXiv2018

Evidence for Pulsar-like Emission Components in the Broadband ULX Sample

We present broadband X-ray analyses of a sample of bright ultraluminous X-ray sources with the goal of investigating the spectral similarity of this population to the known ULX pulsars, M82 X-2, NGC7793 P13 and NGC5907 ULX. We perform a phase-resolved analysis of the broadband XMM-Newton+NuSTAR dataset of NGC5907 ULX, finding that the pulsed emission from the accretion column in this source exhibits a similar spectral shape to that seen in both M82 X-2 and NGC7793 P13, and that this is responsible for the excess emission observed at the highest energies when the spectra are fit with accretion disk models. We then demonstrate that similar 'hard' excesses are seen in all the ULXs in the broadband sample. Finally, for the ULXs where the nature of the accretor is currently unknown, we test whether the hard excesses are all consistent with being produced by an accretion column similar to those present in M82 X-2, NGC7793 P13 and NGC5907 ULX. Based on the average shape of the pulsed emission, we find that in all cases a similar accretion column can successfully reproduce the observed data, consistent with the hypothesis that this ULX sample may be dominated by neutron star accretors. Compared to the known pulsar ULXs, our spectral fits for the remaining ULXs suggest that the non-pulsed emission from the accretion flow beyond the magnetosphere makes a stronger relative contribution than the component associated with the accretion column. If these sources do also contain neutron star accretors, this may help to explain the lack of detected pulsations.

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arXiv2018

New Spectral Model for Constraining Torus Covering Factors from Broadband X-ray Spectra of Active Galactic Nuclei

The basic unified model of active galactic nuclei (AGN) invokes an anisotropic obscuring structure, usually referred to as a torus, to explain AGN obscuration as an angle-dependent effect. We present a new grid of X-ray spectral templates based on radiative transfer calculations in neutral gas in an approximately toroidal geometry, appropriate for CCD-resolution X-ray spectra (FWHM > 130 eV). Fitting the templates to broadband X-ray spectra of AGN provides constraints on two important geometrical parameters of the gas distribution around the supermassive black hole: the average column density and the covering factor. Compared to the currently available spectral templates, our model is more flexible, and capable of providing constraints on the main torus parameters in a wider range of AGN. We demonstrate the application of this model using hard X-ray spectra from NuSTAR (3-79 keV) for four AGN covering a variety of classifications: 3C 390.3, NGC 2110, IC 5063, and NGC 7582. This small set of examples was chosen to illustrate the range of possible torus configurations, from disk-like to sphere-like geometries with column densities below, as well as above, the Compton-thick threshold. This diversity of torus properties challenges the simple assumption of a standard geometrically and optically thick toroidal structure commonly invoked in the basic form of the unified model of AGN. Finding broad consistency between the our constraints and those from infrared modeling, we discuss how the approach from the X-ray band complements similar measurements of AGN structures at other wavelengths.

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arXiv2018

The NuSTAR Extragalactic Surveys: X-ray spectroscopic analysis of the bright hard-band selected sample

We discuss the spectral analysis of a sample of 63 Active Galactic Nuclei (AGN) detected above a limiting flux of $S(8-24$ keV$)=7\times10^{-14}$ erg/s/cm$^2$ in the multi-tiered NuSTAR Extragalactic Survey program. The sources span a redshift range z=0-2.1 (median $\langle$z$\rangle=$0.58). The spectral analysis is performed over the broad 0.5-24 keV energy range, combining NuSTAR with Chandra and/or XMM-Newton data and employing empirical and physically motivated models. This constitutes the largest sample of AGN selected at $ >10$ keV to be homogeneously spectrally analyzed at these flux levels. We study the distribution of spectral parameters such as photon index, column density ($N_{\rm H}$), reflection parameter ($R$) and 10-40 keV luminosity ($L_{X}$). Heavily obscured ($\log [N_{\rm H}/cm^{-2}]\ge23$) and Compton Thick (CT; $\log [N_{\rm H}/cm^{-2}]\ge24$) AGN constitute $\sim$25% (15-17 sources) and $\sim$2-3% ( 1-2 sources) of the sample, respectively. The observed $N_{\rm H}$ distribution fairly agrees with predictions of Cosmic X-ray Background population synthesis models (CXBPSM). We estimate the intrinsic fraction of AGN as a function of $N_{H}$, accounting for the bias against obscured AGN in a flux-selected sample. The fraction of CT AGN relative to $\log [N_{\rm H}/cm^{-2}]=20-24$ AGN is poorly constrainted, formally in the range 2-56% (90% upper limit of 66%). We derived a fraction ($f_{abs}$) of obscured AGN ($\log [N_{\rm H}/cm^{-2}]=22-24$) as a function of $L_{X}$ in agreement with CXBPSM and previous z < 1 X-ray determinations. Furthermore $f_{abs}$ at z=0.1-0.5 and $\log (L_{X}/erg$ $s^{-1})\approx43.6-44.3$ agrees with observational measurements/trends obtained over larger redshift intervals. We report a significant anti-correlation of $R$ with $L_{X}$ (confirmed by our companion paper on stacked spectra) with considerable scatter around the median $R$ values.

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arXiv2018

The NuSTAR Extragalactic Surveys: source catalog and the Compton-thick fraction in the UDS field

We present the results and the source catalog of the NuSTAR survey in the UKIDSS Ultra Deep Survey (UDS) field, bridging the gap in depth and area between NuSTAR's ECDFS and COSMOS surveys. The survey covers a $\sim 0.6$ deg$^2$ area of the field for a total observing time of $\sim$ 1.75 Ms, to a half-area depth of $\sim$ 155 ks corrected for vignetting at $3-24$ keV, and reaching sensitivity limits at half-area in the full ($3-24$ keV), soft ($3-8$ keV) and hard ($8-24$ keV) bands of $2.2 \times 10^{-14}$ erg cm$^{-2}$ s$^{-1}$, $1.0 \times 10^{-14}$ erg cm$^{-2}$ s$^{-1}$, and $2.7 \times 10^{-14}$ erg cm$^{-2}$ s$^{-1}$, respectively. A total of 67 sources are detected in at least one of the three bands, 56 of which have a robust optical redshift with a median of $\langle z\rangle \sim 1.1$. Through a broadband ($0.5-24$ keV) spectral analysis of the whole sample combined with the NuSTAR hardness ratios, we compute the observed Compton-thick (CT; $N_{\rm H} > 10^{24}$ cm$^{-2}$) fraction. Taking into account the uncertainties on each $N_{\rm H}$ measurement, the final number of CT sources is $6.8\pm1.2$. This corresponds to an observed CT fraction of $(11.5\pm2.0)\%$, providing a robust lower limit to the intrinsic fraction of CT AGN and placing constraints on cosmic X-ray background synthesis models.

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arXiv2017

Super-Eddington Accretion onto the Neutron Star NGC 7793 P13: Broadband X-ray Spectroscopy and Ultraluminous X-ray Sources

We present a detailed, broadband X-ray spectral analysis of the ULX pulsar NGC 7793 P13, a known super-Eddington source, utilizing data from the $XMM$-$Newton$, $NuSTAR$ and $Chandra$ observatories. The broadband $XMM$-$Newton+NuSTAR$ spectrum of P13 is qualitatively similar to the rest of the ULX sample with broadband coverage, suggesting that additional ULXs in the known population may host neutron star accretors. Through time-averaged, phase-resolved and multi-epoch studies, we find that two non-pulsed thermal blackbody components with temperatures $\sim$0.5 and $\sim$1.5 keV are required to fit the data below 10 keV, in addition to a third continuum component which extends to higher energies and is associated with the pulsed emission from the accretion column. The characteristic radii of the thermal components appear to be similar, and are too large to be associated with the neutron star itself, so the need for two components likely indicates the accretion flow outside the magnetosphere is complex. We suggest a scenario in which the thick inner disc expected for super-Eddington accretion begins to form, but is terminated by the neutron star's magnetic field soon after its onset, implying a limit of $B \lesssim 6 \times 10^{12}$ G for the dipolar component of the central neutron star's magnetic field. Evidence of similar termination of the disc in other sources may offer a further means of identifying additional neutron star ULXs. Finally, we examine the spectrum exhibited by P13 during one of its unusual 'off' states. These data require both a hard powerlaw component, suggesting residual accretion onto the neutron star, and emission from a thermal plasma, which we argue is likely associated with the P13 system.

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arXiv2017

Disentangling the Complex Broadband X-ray Spectrum of IRAS 13197-1627 with NuSTAR, XMM-Newton and Suzaku

We present results from a coordinated $XMM$-$Newton$+$NuSTAR$ observation of the type 1.8 Seyfert galaxy IRAS 13197-1627. This is a highly complex source, with strong contributions from relativistic reflection from the inner accretion disk, neutral absorption and further reprocessing by more distant material, and ionised absorption from an outflow. We undertake a detailed spectral analysis combining the broadband coverage provided by $XMM$-$Newton$+$NuSTAR$ with a multi-epoch approach incorporating archival observations performed by $XMM$-$Newton$ and $Suzaku$. Our focus is on characterising the reflection from the inner accretion disk, which previous works have suggested may dominate the AGN emission, and constraining the black hole spin. Using lamppost disk reflection models, we find that the results for the inner disk are largely insensitive to assumptions regarding the geometry of the distant reprocessor and the precise form of the illuminating X-ray continuum. However, these results do depend on the treatment of the iron abundance of the distant absorber/reprocessor. The multi-epoch data favour a scenario in which the AGN is chemically homogeneous, and we find that a rapidly rotating black hole is preferred, with $a^* \geq 0.7$, but a slowly-rotating black hole is not strongly excluded. In addition to the results for the inner disk, we also find that both the neutral and ionised absorbers vary from epoch to epoch, implying that both have some degree of inhomogeneity in their structure.

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arXiv2017

Swift and NuSTAR observations of GW170817: detection of a blue kilonova

With the first direct detection of merging black holes in 2015, the era of gravitational wave (GW) astrophysics began. A complete picture of compact object mergers, however, requires the detection of an electromagnetic (EM) counterpart. We report ultraviolet (UV) and X-ray observations by Swift and the Nuclear Spectroscopic Telescope ARray (NuSTAR) of the EM counterpart of the binary neutron star merger GW170817. The bright, rapidly fading ultraviolet emission indicates a high mass ($\approx0.03$ solar masses) wind-driven outflow with moderate electron fraction ($Y_{e}\approx0.27$). Combined with the X-ray limits, we favor an observer viewing angle of $\approx 30^{\circ}$ away from the orbital rotation axis, which avoids both obscuration from the heaviest elements in the orbital plane and a direct view of any ultra-relativistic, highly collimated ejecta (a gamma-ray burst afterglow).

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arXiv2017

Investigating the evolution of the dual AGN system ESO~509-IG066

We analyze the evolution of the dual AGN in ESO 509-IG066, a galaxy pair located at z=0.034 whose nuclei are separated by 11 kpc. Previous observations with XMM-Newton on this dual AGN found evidence for two moderately obscured ($N_H\sim10^{22}$ cm$^{-2}$) X-ray luminous ($L_X\sim10^{43}$ erg/s) nuclear sources. We present an analysis of subsequent Chandra, NuSTAR and Swift/XRT observations that show one source has dropped in flux by a factor of 10 between 2004 and 2011, which could be explained by either an increase in the absorbing column or an intrinsic fading of the central engine possibly due to a decrease in mass accretion. Both of these scenarios are predicted by galaxy merger simulations. The source which has dropped in flux is not detected by NuSTAR, which argues against absorption, unless it is extreme. However, new Keck/LRIS optical spectroscopy reveals a previously unreported broad H-alpha line which is highly unlikely to be visible under the extreme absorption scenario. We therefore conclude that the black hole in this nucleus has undergone a dramatic drop in accretion rate. From AO-assisted near-infrared integral-field spectroscopy of the other nucleus, we find evidence that the galaxy merger is having a direct effect on the kinematics of the gas close to the nucleus of the galaxy, providing a direct observational link between the galaxy merger and the mass accretion rate on to the black hole.

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arXiv2017

The NuSTAR Extragalactic Survey: Average broad-band X-ray spectral properties of the NuSTAR detected AGN

We present a study of the average X-ray spectral properties of the sources detected by the NuSTAR extragalactic survey, comprising observations of the E-CDFS, EGS and COSMOS fields. The sample includes 182 NuSTAR sources (64 detected at 8-24 keV), with 3-24 keV fluxes ranging between $f_{\rm 3-24 keV}\approx10^{-14}$ and $6\times10^{-13}$ erg/cm$^2$/s ($f_{\rm 8-24 keV}\approx3\times10^{-14}-3\times10^{-13}$ erg/cm$^2$/s) and redshifts of $z=0.04-3.21$. We produce composite spectra from the Chandra+NuSTAR data ($E\approx2-40$ keV, rest frame) for all the sources with redshift identifications (95%) and investigate the intrinsic, average spectra of the sources, divided into broad-line (BL) and narrow-line (NL) AGN, and also in different bins of X-ray column density and luminosity. The average power-law photon index for the whole sample is $Γ=1.65_{-0.03}^{+0.03}$, flatter than $Γ\approx1.8$ typically found for AGN. While the spectral slope of BL and X-ray unabsorbed AGN is consistent with typical values ($Γ=1.79_{-0.01}^{+0.01}$), a significant flattening is seen in NL AGN and heavily-absorbed sources ($Γ=1.60_{-0.05}^{+0.08}$ and $Γ=1.38_{-0.12}^{+0.12}$, respectively), likely due to the effect of absorption and to the contribution from Compton reflection to the high-energy flux (E>10 keV). We find that the typical reflection fraction in our spectra is $R\approx0.5$ (for $Γ=1.8$), with a tentative indication of an increase of the reflection strength with column density. While there is no significant evidence for a dependence of the photon index with X-ray luminosity in our sample, we find that $R$ decreases with luminosity, with relatively high levels of reflection ($R\approx1.2$) for $L_{\rm 10-40 keV}<10^{44}$ erg/s and $R\approx0.3$ for $L_{\rm 10-40 keV}>10^{44}$ erg/s AGN, assuming $Γ=1.8$.

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