Long-term Spectral Variability Research Articles

The optical behaviour of BL Lacertae at its maximum brightness levels: a blend of geometry and energetics

ABSTRACT In 2021 BL Lacertae underwent an extraordinary activity phase, which was intensively followed by the Whole Earth Blazar Telescope (WEBT) Collaboration. We present the WEBT optical data in the BVRI bands acquired at 36 observatories around the world. In mid-2021 the source showed its historical maximum, with R = 11.14. The light curves display many episodes of intraday variability, whose amplitude increases with source brightness, in agreement with a geometrical interpretation of the long-term flux behaviour. This is also supported by the long-term spectral variability, with an almost achromatic trend with brightness. In contrast, short-term variations are found to be strongly chromatic and are ascribed to energetic processes in the jet. We also analyse the optical polarimetric behaviour, finding evidence of a strong correlation between the intrinsic fast variations in flux density and those in polarization degree, with a time delay of about 13 h. This suggests a common physical origin. The overall behaviour of the source can be interpreted as the result of two mechanisms: variability on time-scales greater than several days is likely produced by orientation effects, while either shock waves propagating in the jet, or magnetic reconnection, possibly induced by kink instabilities in the jet, can explain variability on shorter time-scales. The latter scenario could also account for the appearance of quasi-periodic oscillations, with periods from a few days to a few hours, during outbursts, when the jet is more closely aligned with our line of sight and the time-scales are shortened by relativistic effects.

Detecting subparsec supermassive binary black holes: Long-term monitoring perspective

ABSTRACT Here we consider the perspective to detect sub-pc supermassive binary black hole (SMBBH) systems using long-term photometric and spectroscopic monitoring campaigns of active galactic nuclei. This work explores the nature of long-term spectral variability caused by the dynamical effects of SMBBH systems. We describe in great detail a model of SMBBH system which considers that both black holes have their accretion disc and additional line emitting region(s). We simulate the H β spectral band (continuum+broad H β line) for different mass ratios of components and different total masses of the SMBBH systems (106–108 M⊙). We analyse the set of continuum and broad line light curves for several full orbits of SMBBHs with different parameters, to test the possibility to extract the periodicity of the system. We consider different levels of the signal-to-noise ratio, which is added to the simulated spectra. Our analysis showed that the continuum and broad line profiles emitted from an SMBBH system are strongly dependent, not only on the mass ratio of the components but also on the total mass of the system. We found that the mean broad line profile and its rms could indicate the presence of an SMBBH. However, some effects caused by the dynamics of a binary system could be hidden due to a low signal-to-noise ratio. Finally, we can conclude that the long-term AGN monitoring campaigns could be beneficial for the detection of SMBBH candidates.

Variability of Comptonized X-ray spectra of a super-Eddington accretor: Approach using Boltzmann radiation transport

Abstract We investigate the radiation fields around super-Eddington accretion flow, in which multiple inverse-Compton scattering plays a principal role, by using newly developed code describing Boltzmann radiation transfer in the Schwarzschild space-time. We apply this code to post-processing spectral calculations based on general relativistic, radiation magnetohydrodynamic simulation data to obtain X-ray spectra seen from various viewing angles. The radiation fields are distinctively separated into a funnel region with an opening angle of ∼30°, which is full with hot (gas temperature of T > 108 K), tenuous, and high-velocity plasmas, and surrounding cooler (T ∼ 107 K) and optically thick outflow regions. Accordingly, there is a clear tendency that the smaller the viewing angle, the harder the spectra. In particular, hard photons with several tens of keV are observable only by observers at viewing angles less than ∼30°, consistent with past spectral studies based on simulations. Further, we investigate how the spectra are varied by a flare occurring in the innermost region, finding that the variation amplitude grows as the photon energy increases and that the harder photons emerge more quickly than softer photons. The observational implications on long-term spectral variability of ultra-luminous X-ray sources are briefly discussed.

Super-Eddington accretion on to a stellar mass ultraluminous X-ray source NGC 4190 ULX1

ABSTRACT We present the results of high-quality XMM-NEWTON observations of a ultraluminous X-ray source (ULX) in the galaxy NGC 4190. The detection of spectral cutoff in NGC 4190 ULX1 spectra rules out the interpretation of the ULX to be in a standard low/hard canonical accretion state. We report that the high quality EPIC spectra can be better described by broad thermal component, such as a slim disc. In addition we found long-term spectral and flux variability in the source using several XMM-NEWTON and Swift data. A clear anticorrelation between flux and power-law photon index is found which further confirms the unusual spectral state evolution of the ULX. Spectral properties of the ULX suggest that the source is in a broadened disc state with luminosities [≈(3 − 10) × 1039 erg s−1] falling in the ultraluminous regime. The positive luminosity–temperature relation further suggests that the multicolour disc model follows the L ∝ T4 relation that is expected for a blackbody disc emission from a constant area and the slim disc model seems to favour L ∝ T2 relation consistent with an advection-dominated disc emission. From the broadened disc-like spectral feature at such luminosity, we estimated the upper limit of the mass of the central compact object from the inner disc radius and found that the ULX hosts a stellar mass black hole.

A new study of the spectroscopic binary 7 Vul with a Be star primary

We confirmed the binary nature of the Be star 7 Vul, derived a more accurate spectroscopic orbit with an orbital period of 69.d4212±0.d0034, and improved the knowledge of the basic physical elements of the system. Analyzing available photometry and the strength of the Hα emission, we also document the long-term spectral variations of the Be primary. In addition, we confirmed rapid light changes with a period of 0.d5592, which is comparable to the expected rotational period of the Be primary, but note that its amplitude and possibly its period vary with time. We were able to disentangle only the He I 6678 Å line of the secondary, which could support our tentative conclusion that the secondary appears to be a hot subdwarf. A search for this object in high-dispersion far-UV spectra could provide confirmation. Probable masses of the binary components are (6 ± 1) 𝓜⊙N and (0.6 ± 0.1) 𝓜⊙N. If the presence of a hot subdwarf is firmly confirmed, 7 Vul might be identified as a rare object with a B4-B5 primary; all Be + hot subdwarf systems found so far contain B0-B3 primaries.

X-Ray and Optical Monitoring of State Transitions in MAXI J1820+070

Abstract We report results from the X-ray and optical monitoring of the black hole candidate MAXI J1820+070 (=ASSASN-18ey) over the entire period of its outburst from 2018 March to October. In this outburst, the source exhibited two sets of “fast rise and slow decay”-type long-term flux variations. We found that the 1–100 keV luminosities at two peaks were almost the same, although a significant spectral softening was only seen in the second flux rise. This confirms that the state transition from the low/hard state to the high/soft state is not determined by the mass accretion rate alone. The X-ray spectrum was reproduced with the disk blackbody emission and its Comptonization, and the long-term spectral variations seen in this outburst were consistent with a disk truncation model. The Comptonization component, with a photon index of 1.5–1.9 and electron temperature of ≳40 keV, was dominant during the low/hard state periods, and its contribution rapidly decreased (increased) during the spectral softening (hardening). During the high/soft-state period, in which the X-ray spectrum became dominated by the disk blackbody component, the inner disk radius was almost constant, suggesting that the standard disk was present down to the innermost stable circular orbit. The long-term evolution of optical and X-ray luminosities and their correlation suggest that the jets substantially contributed to the optical emission in the low/hard state, while they are quenched and the outer disk emission dominated the optical flux in the intermediate state and the high/soft state.

A short and sudden increase of the magnetic field strength and the accompanying spectral variability in the O9.7 V star HD 54879

Abstract Only 11 O-type stars have been confirmed to possess large-scale organized magnetic fields. The presence of a −600 G longitudinal magnetic field in the O9.7 V star HD 54879 with a lower limit of the dipole strength of ∼2 kG was discovered a few years ago in the framework of the ESO large program ‘B-fields in OB stars’. Our FORS 2 spectropolarimetric observations from 2017 October 4 to 2018 February 21 reveal the presence of short- and long-term spectral variability and a gradual magnetic field decrease from about −300 G down to about −90 G. Different scenarios are discussed in an attempt to interpret our observations. Our FORS 2 radial velocity measurements indicate that HD 54879 is a member of a long-period binary.

Long-Term Optical and Spectral Variability of FSRQ 3C454.3

3C454.3 is a widely studied and monitored flat spectrum radio quasar. We use the observations from the Small and Moderate Aperture Research Telescope System (SMARTS) to analyze the long-term optical and spectral variabilities of 3C454.3 at B, V, R, J, and K (BVRJK) bands. Based on the relation: Fν∝ν-α, we calculate the multiband spectral indices (α) and analyze the relations between α and Fν, where Fν is the flux density at ν band (ν=B, V, R, J, K). Main results are as follows. (1) The largest variations at BVRJK bands are ΔB = 3.37±0.08 mag, with the timescale ΔTB≈ 1015 days, at B band; ΔV = 3.31±0.07 mag, with the timescale ΔTV≈ 1014 days, at V band; ΔR = 3.62±0.12 mag, with the timescale ΔTR≈ 679 days, at R band; ΔJ = 4.08±0.01 mag, with the timescale ΔTJ≈ 763 days, at J band; and ΔK = 5.03±0.03 mag, with the timescale ΔTK≈ 2715 days, at K band. (2) We analyze the long-term BVRJK lightcurves and obtain the quasiperiodicities: P1=86.92±2.21 days, P2=204±8.1 days, and P3=1.24±0.19 years. (3) Multiband lightcurves show time delays: τVB = 0.15±0.12 days, τRB = 0.37±0.28 days, τJB = 0.58±0.40 days, and τKB = 1.02±0.47 days. (4) The relations between α and Fν show strong correlations, which are typical RWB behaviors; when the source turns to be brighter, the spectral indices turn to be redder.

X-Ray Spectral Properties of Seyfert I Galaxy LEDA 168563

Abstract Interesting among possible mechanisms responsible for X-ray emission from active galactic nuclei (AGNs) is ionized relativistic reflection (IRR). Since it arises close to the central black hole it can test strong gravity. Its characteristic features include a high energy Compton hump, a broad Fe Kα line, and a soft excess. Therefore, using the Swift-Burst Alert Telescope (BAT) catalog we looked for hard X-ray selected AGNs that may exhibit at least one of these characteristic features. Among the possibly interesting targets found is Seyfert I Galaxy LEDA 168563. We obtained a broadband 100 ks Suzaku observation of this source, and careful analysis of the data was carried out. The results support the presence of these IRR distinct features. Moreover, the comparison of the earlier combined XMM-Newton and Swift-BAT and more recent Suzaku data showed interesting long-term spectral variability. The soft excess decreased while higher energy features relatively increased and the power-law component became flatter—the behavior predicted by the IRR model.

In-depth study of long-term variability in the X-ray emission of the Be/X-ray binary system AX J0049.4−7323

AX J0049.4−7323 is a Be/X-ray binary in the Small Magellanic Cloud hosting a ~750 s pulsar which has been observed over the last ~17 years by several X-ray telescopes. Despite numerous observations, little is known about its X-ray behaviour. Therefore, we coherently analysed archival Swift, Chandra, XMM-Newton, RXTE, and INTEGRAL data, and we compared them with already published ASCA data, to study its X-ray long-term spectral and flux variability. AX J0049.4−7323 shows a high X-ray variability, spanning more than three orders of magnitudes, from L ≈ 1.6 × 1037 erg s−1 (0.3−8 keV, d = 62 kpc) down to L ≈ 8 × 1033 erg s−1. RXTE, Chandra, Swift, and ASCA observed, in addition to the expected enhancement of X-ray luminosity at periastron, flux variations by a factor of ~270 with peak luminosities of ≈2.1 × 1036 erg s−1 far from periastron. These properties are difficult to reconcile with the typical long-term variability of Be/XRBs, traditionally interpreted in terms of type I and type II outbursts. The study of AX J0049.4−7323 is complemented with a spectral analysis of Swift, Chandra, and XMM-Newton data which showed a softening trend when the emission becomes fainter, and an analysis of optical/UV data collected by the UVOT telescope on board Swift. In addition, we measured a secular spin-up rate of Ṗ = (−3.00 ± 0.12) × 10−3 s day−1, which suggests that the pulsar has not yet achieved its equilibrium period. Assuming spherical accretion, we estimated an upper limit for the magnetic field strength of the pulsar of ≈3 × 1012 G.

Einstein Probe’s scientific opportunities in the field of active galactic nuclei

Active galactic nuclei (AGNs) are accreting supermassive black holes (SMBHs) that reside at the centers of galaxies and emit excessive electromagnetic radiation, whose ultimate energy source is the release of the gravitational potential energy of the accreted matter in the form of thermal and radiation energy. Multi-timescale and multi-band variability is a characteristic observational feature of AGNs. In particular, X-ray variability is most worth noting because of being intense and rapid, as well as carrying rich physical information of the innermost region of the accretion disk. Therefore, AGN X-ray variability has long been used as a probe for studying SMBHs. However, at present, we still lack a very fundamental understanding of AGN X-ray variability, whose origin and mechanisms remain largely unclear. The Einstein Probe (EP) satellite has an unprecedented soft X-ray survey capability, with a grasp (i.e., the product of detector effective area and field of view) being one to two orders of magnitude larger than previous similar satellites. EP is able to monitor hundreds of bright AGNs in the whole sky with sampling intervals ranging from 100 s to years, and will thereby build an exceptional AGN variability database. Accordingly, we propose to carry out AGN investigations mainly in the following four aspects: comprehensive measurement of the soft X-ray power spectral density for a large sample of bright AGNs; systematic monitoring and study of the rare large-amplitude AGN X-ray variability and flaring phenomenon; examination of long-term spectral variability for a large sample of AGNs and monitoring of their outbursts; and AGN/quasar survey. These unprecedented data will facilitate our further understanding of many scientific questions, such as the intense AGN X-ray variability and flaring phenomenon and its mechanisms, the physical conditions, structures, dynamics, and radiation processes of the AGN accretion disk, jet, and corona, as well as the AGN cosmological evolution. Furthermore, given EP’s capability of exploring a brand-new discovery space (large sky-area, long-timescale, and systematic soft X-ray monitoring), we are likely to discover some black hole accretion phenomena never seen before—the complexity and diversity of the Universe are always beyond human imagination.

A search for the presence of magnetic fields in the two supergiant fast X-ray transients, IGR J08408−4503 and IGR J11215−5952

Abstract A significant fraction of high-mass X-ray binaries are supergiant fast X-ray transients (SFXTs). The prime model for the physics governing their X-ray behaviour suggests that the winds of donor OB supergiants are magnetized. To investigate if magnetic fields are indeed present in the optical counterparts of such systems, we acquired low-resolution spectropolarimetric observations of the two optically brightest SFXTs, IGR J08408−4503 and IGR J11215−5952, with the ESO FORS 2 instrument during two different observing runs. No field detection at a significance level of 3σ was achieved for IGR J08408−4503. For IGR J11215−5952, we obtain 3.2σ and 3.8σ detections (⟨Bz⟩hydr = −978 ± 308 G and ⟨Bz⟩hydr = 416 ± 110 G) on two different nights in 2016. These results indicate that the model involving the interaction of a magnetized stellar wind with the neutron star magnetosphere can indeed be considered to characterize the behaviour of SFXTs. We detected long-term spectral variability in IGR J11215−5952, whereas for IGR J08408−4503, we find an indication of the presence of short-term variability on a time-scale of minutes.

Long-term Spectral Variability of the Ultraluminous X-Ray Source Holmberg IX X–1

Abstract We investigate the long-term spectral variability in the ultraluminous X-ray source Holmberg IX X–1. By analyzing the data from 8 Suzaku and 13 XMM-Newton observations conducted between 2001 and 2015, we perform a detailed spectral modeling for all spectra with simple models and complex physical models. We find that the spectra can be well explained by a disk-plus-thermal-Comptonization model. Applying this model, we unveil correlations between the X-ray luminosity ( L X ) and the spectral parameters. One particular correlation is the statistically significant positive correlation between L X and the photon index (Γ), while at the high luminosities of > 2 × 10 40 erg s − 1 , the source becomes marginally hard, which results in a change in the slope of the Γ – L X correlation. Similar variability behavior is observed in the optical depth of the source around L X ∼ 2 × 10 40 erg s − 1 as the source becomes more optically thick. We consider the scenario that a corona covers the inner part of the disk, and the correlations can be explained as being driven by the variability of seed photons from the disk input into the corona. On the basis of the disk-corona model, we discuss the physical processes that are possibly indicated by the variability of the spectral parameters. Our analysis reveals the complex variability behavior of Holmberg IX X–1 and the variability mechanism is likely related to the geometry of the X-ray-emitting regions.

Long-Term Spectral Variability of the Spotted Star in Com

High-resolution (R = 20000) spectral observations over many years of the spotted star IN Com in the regions of the Hα, Hβ, and He I 5876 A lines are reported. The distinctive feature of the spectrum of this star is extended two-component emission in the Hα line over limits of ±400 km/s. This emission manifests variability caused by rotational modulation at the star’s period and variability on a time scale of many years. Similar emission is observed in the Hα and He I 5876 A lines. These data show that the observed emission profile is formed in optically thin hot gas which forms a circumstellar disk around IN Com with a radius no greater than a few times the star’s radius. Matter is supplied to the disk by stellar wind. The observed variability of the Hα emission parameters is closely coupled to the star’s photopolarimetric activity, so that we can relate the multiyear spectral variability to the activity cycles of IN Com.

Illuminating the past 8 billion years of cold gas towards two gravitationally lensed quasars

Using the Boolardy Engineering Test Array of the Australian Square Kilometre Array Pathfinder (ASKAP BETA), we have carried out the first $z = 0 - 1$ survey for HI and OH absorption towards the gravitationally lensed quasars PKSB1830$-$211 and MGJ0414$+$0534. Although we detected all previously reported intervening systems towards PKSB1830$-$211, in the case of MGJ0414+0534 three systems were not found, indicating that the original identifications may have been confused with radio frequency interference. Given the sensitivity of our data, we find that our detection yield is consistent with the expected frequency of intervening HI systems estimated from previous surveys for 21-cm emission in nearby galaxies and $z \sim 3$ damped Lyman $\alpha$ absorbers. We find spectral variability in the $z = 0.886$ face-on spiral galaxy towards PKSB1830$-$211, from observations undertaken with the Westerbork Synthesis Radio Telescope in 1997/1998 and ASKAP BETA in 2014/2015. The HI equivalent width varies by a few per cent over approximately yearly time-scales. This long-term spectral variability is correlated between the north-east and south-west images of the core, and with the total flux density of the source, implying that it is observationally coupled to intrinsic changes in the quasar. The absence of any detectable variability in the ratio of HI associated with the two core images is in stark contrast to the behaviour previously seen in the molecular lines. We therefore infer that coherent opaque HI structures in this galaxy are larger than the parsec-scale molecular clouds found at mm-wavelengths.

EVIDENCE FOR PERIODICITY IN 43 YEAR-LONG MONITORING OF NGC 5548

ABSTRACT We present an analysis of 43 years (1972 to 2015) of spectroscopic observations of the Seyfert 1 galaxy NGC 5548. This includes 12 years of new unpublished observations (2003 to 2015). We compiled about 1600 Hβ spectra and analyzed the long-term spectral variations of the 5100 Å continuum and the Hβ line. Our analysis is based on standard procedures, including the Lomb–Scargle method, which is known to be rather limited to such heterogeneous data sets, and a new method developed specifically for this project that is more robust and reveals a ∼5700 day periodicity in the continuum light curve, the Hβ light curve, and the radial velocity curve of the red wing of the Hβ line. The data are consistent with orbital motion inside the broad emission line region of the source. We discuss several possible mechanisms that can explain this periodicity, including orbiting dusty and dust-free clouds, a binary black hole system, tidal disruption events, and the effect of an orbiting star periodically passing through an accretion disk.

The origin of seed photons for Comptonization in the black hole binary Swift J1753.5–0127

Aims. The black hole binary SWIFT J1753.5-0127 is providing a unique data set to study accretion flows. Various investigations of this system and of other black holes have not, however, led to an agreement on the accretion flow geometry or on the seed photon source for Comptonization during different stages of X-ray outbursts. We place constraints on these accretion flow properties by studying long-term spectral variations of this source. Methods. We performed phenomenological and self-consistent broad band spectral modeling of Swift J1753.5-0127 using quasi-simultaneous archived data from INTEGRAL/ISGRI, Swift/UVOT/XRT/BAT, RXTE/PCA/HEXTE and MAXI/GSC instruments. Results. We identify a critical flux limit, F \sim 1.5 \times 10^{-8} erg/cm^2/s, and show that the spectral properties of SWIFT J1753.5-0127 are markedly different above and below this value. Above the limit, during the outburst peak, the hot medium seems to intercept roughly 50 percent of the disk emission. Below it, in the outburst tail, the contribution of the disk photons reduces significantly and the entire spectrum from the optical to X-rays can be produced by a synchrotron-self-Compton mechanism. The long-term variations in the hard X-ray spectra are caused by erratic changes of the electron temperatures in the hot medium. Thermal Comptonization models indicate unreasonably low hot medium optical depths during the short incursions into the soft state after 2010, suggesting that non-thermal electrons produce the Comptonized tail in this state. The soft X-ray excess, likely produced by the accretion disk, shows peculiarly stable temperatures for over an order of magnitude changes in flux. Conclusions. The long-term spectral trends of SWIFT J1753.5-0127 are likely set by variations of the truncation radius and a formation of a hot, quasi-spherical inner flow in the vicinity of the black hole. (abridged)

Swift J174540.7−290015: a new accreting binary in the Galactic Centre

We report on the identification of the new Galactic Center (GC) transient Swift J174540.7-290015 as a likely low mass X-ray binary (LMXB) located at only 16 arcsec from Sgr A*. This transient was detected on 2016 February 6th during the Swift GC monitoring, and it showed long-term spectral variations compatible with a hard to soft state transition. We observed the field with XMM-Newton on February 26th for 35 ks, detecting the source in the soft state, characterised by a low level of variability and a soft X-ray thermal spectrum with a high energy tail (detected by INTEGRAL up to ~50 keV), typical of either accreting neutron stars or black holes. We observed: i) a high column density of neutral absorbing material, suggesting that Swift J174540.7-290015 is located near or beyond the GC and; ii) a sub-Solar Iron abundance, therefore we argue that Iron is depleted into dust grains. The lack of detection of FeK absorption lines, eclipses or dipping suggests that the accretion disc is observed at a low inclination angle. Radio (VLA) observations did not detect any radio counterpart to Swift J174540.7-290015. No evidence for X-ray or radio periodicity is found. The location of the transient was observed also in the near-IR with GROND at MPG/ESO La Silla 2.2m telescope and VLT/NaCo pre- and post-outburst. Within the Chandra error region we find multiple objects that display no significant variations.

HARD X-RAY TAIL DISCOVERED IN THE CLOCKED BURSTER GS 1826–238

ABSTRACT The low-mass X-ray binary (LMXB) neutron star (NS) GS 1826–238 was discovered by Ginga in 1988 September. Due to the presence of quasi-periodicity in the type I X-ray burst rate, the source has been a frequent target of X-ray observations for almost 30 years. Though the bursts were too soft to be detected by INTEGRAL/SPI, the persistent emission from GS 1826–238 was detected over 150 keV during the ∼10 years of observations. Spectral analysis found a significant high-energy excess above a Comptonization model that is well fit by a power law, indicating an additional spectral component. Most previously reported spectra with hard tails in LMXB NS have had an electron temperature of a few keV and a hard tail dominating above ∼50 keV with an index of Γ ∼ 2–3. GS 1826–238 was found to have a markedly different spectrum with kT e ∼ 20 keV and a hard tail dominating above ∼150 keV with an index of Γ ∼ 1.8, more similar to black hole X-ray binaries. We report on our search for long-term spectral variability over the 25–370 keV energy range and on a comparison of the GS 1826–238 average spectrum to the spectra of other LMXB NSs with hard tails.

The stellar wind as a key to the understanding of the spectral activity of IN Com

We present long-term spectral observations (R = 20000) of IN Com in the region of the Hα, Hβ, and He I 5876 lines. One distinguishing characteristic of the stellar spectrum is the presence in the Hα line of an extended two-component emission with limits up to ±400 km/s. Emission parameters show the rotation modulation with the stellar rotation period and a significant variability on the long-term scale. Similar emissions are also observed in the Hβ and He I 5876 lines. Our results allow us to conclude that observational emission profiles are formed in an optically thin hot gas. This is a result of the presence of a circumstellar gas disk around IN Com. Its size does not exceed several stellar radii. The material for the disk is supported by the stellar wind from IN Com. The detected variability of Hα-emission parameters shows a clear connection with the photopolarimetric activity of the star. This fact allows us to associate the long-term spectral variability with cycles of stellar activity of IN Com.