Mid-infrared Variability Research Articles

The mid-infrared variability of the SDSS optical quasars

Based on the Seventh Data Release (DR7) quasar catalog from the Sloan Digital Sky Survey, we investigate the variability of optical quasars in W1, W2, W3 and W4 bands of the Wide-field Infrared Survey Explorer (WISE) and the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE). Adopting the structure function (SF) method, we calculate the SF (δ t = 1 yr) which shows no obvious correlations with the bolometric luminosity, the black hole mass and the Eddington ratio. The ensemble SFs in W1 and W2 bands show that the SF slopes are steeper than those in previous studies which may be caused by different cadence and observational epoch number. We further investigate the relation of variability amplitude σm between mid-infrared band and optical band, but no obvious correlation is found. No correlation is found between W1–W2 and g – r color. We think that the mid-infrared emission of quasars may be smoothed out by the extended dust distribution, thus leading to no obvious correlation. For the radio-loud quasar sub-sample, we further analyze the relation between the variability amplitude in the mid-infrared band and the radio luminosity at 6 cm, but no obvious correlations are found, which indicate the mid-infrared emission contributed from the synchrotron radiation of the relativistic jet is very weak.

Initial Results from a Systematic Search for Changing-look Active Galactic Nuclei Selected via Mid-infrared Variability

Abstract Changing-look active galactic nuclei (CL AGNs) can yield considerable insight into accretion physics as well as the co-evolution of black holes and their host galaxies. A large sample of these CL AGNs is essential to achieve the latter goal. We propose an effective method to select CL candidates from spectroscopic quasar catalogs using the mid-infrared (MIR) variability information derived from ALLWISE/NEOWISE data releases. Our primary selection criteria include both a large amplitude flux variation and a transition of MIR color from an AGN to a normal galaxy. A pilot spectroscopic follow-up of seven candidates among about 300 candidates selected from Sloan Digital Sky Survey low-redshift (z < 0.5) AGN sample results in three new turn-off CL AGNs and three ambiguous objects for suffering low spectral quality. We expect to obtain hundreds of CL AGNs once full spectroscopic follow-up of the sample is carried out.

WISE Discovery of Mid-infrared Variability in Massive Young Stellar Objects

Abstract We systematically investigate the mid-infrared (MIR; λ > 3 μm) time variability of uniformly selected ∼800 massive young stellar objects (MYSOs) from the Red Midcourse Space Experiment Source survey. Out of the 806 sources, we obtain reliable 9 yr long MIR magnitude variability data of 331 sources at the 3.4 μm (W1) and 4.6 μm (W2) bands by cross-matching the MYSO positions with ALLWISE and NEOWISE catalogs. After applying the variability selections using ALLWISE data, we identify five MIR-variable candidates. The light curves show various classes, with the periodic, plateau-like, and dipper features. Out of the obtained two color–magnitude diagram of W1 and W1−W2, one shows “bluer when brighter and redder when fainter” trends in variability, suggesting change in extinction or accretion rate. Finally, our results show that G335.9960−00.8532 (hereafter, G335) has a periodic light curve, with an ≈690 day cycle. Spectral energy density model fitting results indicate that G335 is a relatively evolved MYSO; thus, we may be witnessing the very early stages of a hyper- or ultra-compact H ii region, a key source for understanding MYSO evolution.

Red supergiant stars in the Large Magellanic Cloud

The characteristics of infrared properties and mid-infrared (MIR) variability of red supergiant (RSG) stars in the Large Magellanic Cloud (LMC) are analyzed based on 12 bands of near-infrared (NIR) to MIR co-added data from 2MASS, Spitzer and WISE, and ∼6.6 yr of MIR time-series data collected by the ALLWISE and NEOWISE-R projects. 773 RSGs candidates were compiled from the literature and verified by using the color-magnitude diagram (CMD), spectral energy distribution (SED) and MIR variability. About 15% of valid targets in the IRAC1–IRAC2/IRAC2–IRAC3 diagram may show polycyclic aromatic hydrocarbon (PAH) emission. We show that arbitrary dereddening Q parameters related to the IRAC4, S9W, WISE3, WISE4, and MIPS24 bands could be constructed based on a precise measurement of MIR interstellar extinction law. Several peculiar outliers in our sample are discussed, in which one outlier might be a RSG right before the explosion or an extreme asymptotic giant branch (AGB) star in the very late evolutionary stage based on the MIR spectrum and photometry. There are 744 identified RSGs in the final sample having both the WISE1- and WISE2-band time-series data. The results show that the MIR variability is increasing along with the increasing of brightness. There is a relatively tight correlation between the MIR variability, mass loss rate (MLR; in terms of KS–WISE3 color), and the warm dust or continuum (in terms of WISE4 magnitude/flux), where the MIR variability is evident for the targets with KS–WISE3 > 1.0 mag and WISE4 < 6.5 mag, while the rest of the targets show much smaller MIR variability. The MIR variability is also correlated with the MLR for which targets with larger variability also show larger MLR with an approximate upper limit of −6.1 M⊙ yr−1. Both the variability and the luminosity may be important for the MLR since the WISE4-band flux is increasing exponentially along with the degeneracy of luminosity and variability. The identified RSG sample has been compared with the theoretical evolutionary models and shown that the discrepancy between observation and evolutionary models can be mitigated by considering both variability and extinction.

Intraday Mid-infrared Variability of CTA 102 During Its 2016 Giant Outburst

Abstract We report the discovery in archival data from Wide-field Infrared Survey Explorer (WISE) of intraday mid-infrared (mid-IR) variability of CTA 102 during its 2016 giant outburst. The mid-IR emission of CTA 102 reached its peak around 2016 November and exhibited prominent minor flares within one day. Mid-IR color shows a bluer-when-brighter trend like the optical band during the flaring period. These characteristics likely suggest that CTA 102 was experiencing a transition from flat-spectrum radio quasar to BL Lac, whose synchrotron emission from the relativistic jet was then boosted to totally dominate optical and infrared emission.

Discovery of 21 New Changing-look AGNs in the Northern Sky

Abstract The rare case of changing-look (CL) active galactic nuclei (AGNs), with the appearance or disappearance of broad Balmer emission lines within a few years, challenges our understanding of the AGN unified model. We present a sample of 21 new CL AGNs at 0.08 < z < 0.58 , which doubles the number of such objects known to date. These new CL AGNs were discovered in various ways, from (1) repeat spectra in the SDSS, (2) repeat spectra in the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) and SDSS, and (3) photometric variability and new spectroscopic observations. We use the photometric data from surveys, including the SDSS imaging survey, the Pan-STARRS1, the DESI Legacy imaging survey, the Wide-field Infrared Survey Explorer (WISE), the Catalina Real-time Transient Survey, and the Palomar Transient Factory. The estimated upper limits of the transition timescale of the CL AGNs in this sample spans from 0.9 to 13 years in the rest-frame. The continuum flux in the optical and mid-infrared becomes brighter when the CL AGNs turn on, or vice versa. Variations of more than 0.2 mag in the W1 band were detected in 15 CL AGNs during the transition. The optical and mid-infrared variability is not consistent with the scenario of variable obscuration in 10 CL AGNs at more than the 3σ confidence level. We confirm a bluer-when-brighter trend in the optical. However, the mid-infrared WISE colors W1–W2 become redder when the objects become brighter in the W1 band, possibly due to a stronger hot dust contribution in the W2 band when the AGN activity becomes stronger. The physical mechanism of type transition is important for understanding the evolution of AGNs.

Time-resolved WISE/NEOWISE Coadds

Abstract We have used the first ∼3 years of 3.4 μm (W1) and 4.6 μm (W2) observations from the WISE and NEOWISE missions to create a full-sky set of time-resolved coadds. As a result of the WISE survey strategy, a typical sky location is visited every six months and is observed during ≳12 exposures per visit, with these exposures spanning a ∼1 day time interval. We have stacked the exposures within such ∼1 day intervals to produce one coadd per band per visit—that is, one coadd every six months at a given position on the sky in each of W1 and W2. For most parts of the sky, we have generated six epochal coadds per band, with one visit during the fully cryogenic WISE mission, one visit during NEOWISE, and then, after a 33-month gap, four more visits during the NEOWISE-Reactivation mission phase. These coadds are suitable for studying long-timescale mid-infrared variability and measuring motions to ∼1.3 mag fainter than the single-exposure detection limit. In most sky regions, our coadds span a 5.5-year time period and therefore provide a >10× enhancement in time baseline relative to that available for the AllWISE catalog’s apparent motion measurements. As such, the signature application of these new coadds is expected to be motion-based identification of relatively faint brown dwarfs, especially those cold enough to remain undetected by Gaia.

Mid-infrared variability of blazars: a view from NEOWISE survey

We studied mid-infrared (MIR) variability of a large sample of blazars detected by LAT/Fermi (called Fermi-detected blazars) and those not detected by LAT/Fermi (called non-Fermi-detected blazars) in order to investigate any difference in the variability characteristics between them as well as between FSRQs and BL Lacs. Making use of NEOWISE archival data from October 2013 to December 2017, we constructed the long-term 3.4 μm and 4.6 μm light curves of 2,573 blazars and computed their intrinsic variability amplitudes, $\varPsi $ . As a result, we found that (1) Fermi-detected blazars show higher $\varPsi $ than non-Fermi-detected blazars; (2) Fermi-detected FSRQs show higher $\varPsi $ than non-Fermi-detected FSRQs; (3) Fermi-detected BL Lacs show higher $\varPsi $ than non-Fermi-detected BL Lacs; (4) FSRQs show higher $\varPsi $ than BL Lacs. By comparing their distributions of “ $\mathit{var}\_\mathit{flg}$ ” in the ALLWISE database, we also found that Fermi-detected blazars/FSRQs/BL Lacs tend to be more variable than non-Fermi-detected blazars/FSRQs/BL Lacs. For the Fermi-detected blazars, there are highly significant correlations between the flux densities and spectral indices in the MIR and gamma-ray bands. These results imply that the activity in the two bands is connected. Our results are consistent with several earlier results on the differences between Fermi-detected blazars and non-Fermi-detected blazars. We also give some possible explanations about the statistical results.

Mid-infrared Variability of the Neutrino Source Blazar TXS 0506+056

The IceCube instrument detected a high-energy cosmic neutrino event on 2017 September 22 (IceCube_170922A, IceCube Collaboration 2018), which the electromagnetic follow-up campaigns associated with the flaring $\gamma$-ray blazar TXS 0506$+$056 (e.g., Padovani et al., 2018). We investigated the mid-infrared variability of the source by using the available single exposure data of the WISE satellite at $3.4$ and $4.6\mu$m. TXS 0506$+$056 experienced a $\sim 30$% brightening in both of these bands a few days prior to the neutrino event. Additional intraday infrared variability can be detected in 2010. Similar behaviour seen previously in $\gamma$-ray bright radio-loud AGN has been explained by their jet emission (e.g., Jiang et al. 2012).

YSOVAR: Mid-infrared Variability among YSOs in the Star Formation Region Serpens South

Abstract We present a time-variability study of young stellar objects (YSOs) in the Serpens South cluster performed at 3.6 and 4.5 μm with the Spitzer Space Telescope; this study is part of the Young Stellar Object VARiability project. We have collected light curves for more than 1500 sources, including 85 cluster members, over 38 days. This includes 44 class I sources, 19 sources with flat spectral energy distributions (SEDs), 17 class II sources, and five diskless YSO candidates. We find a high variability fraction among embedded cluster members of ∼70%, whereas young stars without a detectable disk display no variability. We detect periodic variability for 32 sources with periods primarily in the range of 0.2–14 days and a subset of fast rotators thought to be field binaries. The timescale for brightness changes are shortest for stars with the most photospheric SEDs and longest for those with flat or rising SEDs. While most variable YSOs become redder when fainter, as would be expected from variable extinction, about 10% get bluer as they get fainter. One source, SSTYSV J183006.13−020108.0, exhibits “cyclical” color changes.

Mid-infrared Variability of Changing-look AGNs

Abstract It is known that some active galactic nuclei (AGNs) transit from Type 1 to Type 2 or vice versa. There are two explanations for the so-called changing-look AGNs: one is the dramatic change of the obscuration along the line of sight, and the other is the variation of accretion rate. In this Letter, we report the detection of large amplitude variations in the mid-infrared luminosity during the transitions in 10 changing-look AGNs using the Wide-field Infrared Survey Explorer (WISE) and newly released Near-Earth Object WISE Reactivation data. The mid-infrared light curves of 10 objects echo the variability in the optical band with a time lag expected for dust reprocessing. The large variability amplitude is inconsistent with the scenario of varying obscuration, rather it supports the scheme of dramatic change in the accretion rate.

Mid-infrared interferometric variability of DG Tauri: Implications for the inner-disk structure

Context. DG Tau is a low-mass pre-main sequence star, whose strongly accreting protoplanetary disk exhibits a so-far enigmatic behavior: its mid-infrared thermal emission is strongly time-variable, even turning the 10 $\mu$m silicate feature from emission to absorption temporarily. Aims. We look for the reason for the spectral variability at high spatial resolution and at multiple epochs. Methods. We study the temporal variability of the mid-infrared interferometric signal, observed with the VLTI/MIDI instrument at six epochs between 2011 and 2014. We fit a geometric disk model to the observed interferometric signal to obtain spatial information about the disk. We also model the mid-infrared spectra by template fitting to characterize the profile and time dependence of the silicate emission. We use physically motivated radiative transfer modeling to interpret the mid-infrared interferometric spectra. Results. The inner disk (r<1-3 au) spectra exhibit a 10 $\mu$m absorption feature related to amorphous silicate grains. The outer disk (r>1-3 au) spectra show a crystalline silicate feature in emission, similar to the spectra of comet Hale-Bopp. The striking difference between the inner and outer disk spectral feature is highly unusual among T Tauri stars. The mid-infrared variability is dominated by the outer disk. The strength of the silicate feature changed by more than a factor of two. Between 2011 and 2014 the half-light radius of the mid-infrared-emitting region decreased from 1.15 to 0.7 au. Conclusions. For the origin of the absorption we discuss four possible explanations: a cold obscuring envelope, an accretion heated inner disk, a temperature inversion on the disk surface and a misaligned inner geometry. The silicate emission in the outer disk can be explained by dusty material high above the disk plane, whose mass can change with time, possibly due to turbulence in the disk.

First Detection of Mid-infrared Variability from an Ultraluminous X-Ray Source Holmberg II X-1

Abstract We present mid-infrared (IR) light curves of the Ultraluminous X-ray Source (ULX) Holmberg II X-1 from observations taken between 2014 January 13 and 2017 January 5 with the Spitzer Space Telescope at 3.6 and 4.5 μm in the Spitzer Infrared Intensive Transients Survey. The mid-IR light curves, which reveal the first detection of mid-IR variability from a ULX, is determined to arise primarily from dust emission rather than from a jet or an accretion disk outflow. We derived the evolution of the dust temperature ( ), IR luminosity ( ), mass ( ), and equilibrium temperature radius ( ). A comparison of X-1 with a sample of spectroscopically identified massive stars in the Large Magellanic Cloud on a mid-IR color–magnitude diagram suggests that the mass donor in X-1 is a supergiant (sg) B[e]-star. The sgB[e]-interpretation is consistent with the derived dust properties and the presence of the [Fe ii] ( ) emission line revealed from previous near-IR studies of X-1. We attribute the mid-IR variability of X-1 to the increased heating of dust located in a circumbinary torus. It is unclear what physical processes are responsible for the increased dust heating; however, it does not appear to be associated with the X-ray flux from the ULX, given the constant X-ray luminosities provided by serendipitous, near-contemporaneous X-ray observations around the first mid-IR variability event in 2014. Our results highlight the importance of mid-IR observations of luminous X-ray sources traditionally studied at X-ray and radio wavelengths.

PHOTOMETRIC MONITORING OF THE COLDEST KNOWN BROWN DWARF WITH THE SPITZER SPACE TELESCOPE*

ABSTRACT Because WISE J085510.83−071442.5 (hereafter WISE 0855-0714) is the coldest known brown dwarf (∼250 K) and one of the Sun’s closest neighbors (2.2 pc), it offers a unique opportunity to study a planet-like atmosphere in an unexplored regime of temperature. To detect and characterize inhomogeneities in its atmosphere (e.g., patchy clouds, hot spots), we have performed time-series photometric monitoring of WISE 0855-0714 at 3.6 and 4.5 μm with the Spitzer Space Telescope during two 23 hr periods that were separated by several months. For both bands, we have detected variability with peak-to-peak amplitudes of 4%–5% and 3%–4% in the first and second epochs, respectively. The light curves are semiperiodic in the first epoch for both bands, but they are more irregular in the second epoch. Models of patchy clouds have predicted a large increase in mid-infrared (mid-IR) variability amplitudes (for a given cloud covering fraction) with the appearance of water ice clouds at T eff &lt; 375 K, so if such clouds are responsible for the variability of WISE 0855-0714, then its small amplitudes of variability indicate a very small deviation in cloud coverage between hemispheres. Alternatively, the similarity in mid-IR variability amplitudes between WISE 0855-0714 and somewhat warmer T and Y dwarfs may suggest that they share a common origin for their variability (i.e., not water clouds). In addition to our variability data, we have examined other constraints on the presence of water ice clouds in the atmosphere of WISE 0855-0714, including the recent mid-IR spectrum from Skemer et al. (2016). We find that robust evidence of such clouds is not yet available.

YSOVAR: MID-INFRARED VARIABILITY IN NGC 1333

As part of the Young Stellar Object VARiability (YSOVAR) program, we monitored NGC 1333 for ~35 days at 3.6 and 4.5 um using the Spitzer Space Telescope. We report here on the mid-infrared variability of the point sources in the ~ 10 X ~ 20 area centered on 03:29:06, +31:19:30 (J2000). Out of 701 light curves in either channel, we find 78 variables over the YSOVAR campaign. About half of the members are variable. The variable fraction for the most embedded SEDs (Class I, flat) is higher than that for less embedded SEDs (Class II), which is in turn higher than the star-like SEDs (Class III). A few objects have amplitudes (10-90th percentile brightness) in [3.6] or [4.5]>0.2 mag; a more typical amplitude is 0.1-0.15 mag. The largest color change is >0.2 mag. There are 24 periodic objects, with 40% of them being flat SED class. This may mean that the periodic signal is primarily from the disk, not the photosphere, in those cases. We find 9 variables likely to be 'dippers', where texture in the disk occults the central star, and 11 likely to be 'bursters', where accretion instabilities create brightness bursts. There are 39 objects that have significant trends in [3.6]-[4.5] color over the campaign, about evenly divided between redder-when-fainter (consistent with extinction variations) and bluer-when-fainter. About a third of the 17 Class 0 and/or jet-driving sources from the literature are variable over the YSOVAR campaign, and a larger fraction (~half) are variable between the YSOVAR campaign and the cryogenic-era Spitzer observations (6-7 years), perhaps because it takes time for the envelope to respond to changes in the central source. The NGC 1333 brown dwarfs do not stand out from the stellar light curves in any way except there is a much larger fraction of periodic objects (~60% of variable brown dwarfs are periodic, compared to ~30% of the variables overall).

Mid-infrared Variability and Accretion in NGC 2264 Protostars

AbstractVariable mass accretion is thought to be an important aspect of protostar formation. Mid-infrared wavelength observations trace variations in accretion luminosity and thus can probe mass accretion on sub-AU scales. We present results from the Spitzer YSOVAR campaign towards Class I protostars in NGC 2264. The precise (0.02 mag) medium-cadence light curves at 3.6 and 4.5 microns show that young star variability is ubiquitous, with a variety of morphologies and time scales. A structure function analysis shows the light curves, on average, have a power-law behavior up to 30 days. The trend continues to longer timescales (years) for protostars (Class I), in contrast with the smaller brightness changes displayed by T Tauri stars (Class II). The power-law behavior suggests a stochastic process, such as turbulent mass accretion, drives the variability.

Selection of AGN candidates in the GOODS-South Field through Spitzer/MIPS 24 μm variability

We present a study of galaxies showing mid-infrared variability in data taken in the deepest Spitzer/MIPS 24 $\mu$m surveys in the GOODS-South field. We divide the dataset in epochs and subepochs to study the long-term (months-years) and the short-term (days) variability. We use a $\chi^2$-statistics method to select AGN candidates with a probability $\leq$ 1% that the observed variability is due to statistical errors alone. We find 39 (1.7% of the parent sample) sources that show long-term variability and 55 (2.2% of the parent sample) showing short-term variability. That is, 0.03 sources $\times$ arcmin$^{-2}$ for both, long-term and short-term variable sources. After removing the expected number of false positives inherent to the method, the estimated percentages are 1.0% and 1.4% of the parent sample for the long-term and short-term respectively. We compare our candidates with AGN selected in the X-ray and radio bands, and AGN candidates selected by their IR emission. Approximately, 50% of the MIPS 24 $\mu$m variable sources would be identified as AGN with these other methods. Therefore, MIPS 24 $\mu$m variability is a new method to identify AGN candidates, possibly dust obscured and low luminosity AGN, that might be missed by other methods. However, the contribution of the MIPS 24 $\mu$m variable identified AGN to the general AGN population is small ($\leq$ 13%) in GOODS-South.

Spectra and fast multi-wavelength variability of compact jets powered by internal shocks

AbstractThe emission of steady compact jets observed in the hard spectral state of X-ray binaries is likely to be powered by internal shocks caused by fluctuations of the outflow velocity. The dynamics of the internal shocks and the resulting spectral energy distribution (SED) of the jet is very sensitive to the shape of the Power Spectral Density (PSD) of the fluctuations of the jet Lorentz factor. It turns out that Lorentz factor fluctuations injected at the base of the jet with a flicker noise power spectrum (i.e. P(f) ∝1/f) naturally produce the canonical flat SED observed from radio to IR band in X-ray binary systems in the hard state. This model also predicts a strong, wavelength dependent, variability that resembles the observed one. In particular, strong sub-second variability is predicted in the infrared and optical bands. The assumed fluctuations of the jet Lorentz factor are likely to be triggered by the variability of the accretion flow which is best traced by the X-ray emission. In the case of GX339-4 for which high quality and simultaneous multi-wavelength data are available, we performed simulations assuming that the fluctuation of the jet Lorentz factor have the same PSD as the observed X-ray PSD. The synthetic SED calculated under this assumption provides a remarkable match to the observed radio to IR SED. In this case the model also produces strong mid-infrared spectral variability that is comparable to that reported in this source.

Mid-infrared Variability of the Low Mass Stellar Binary TWA 30 A and B

Mid-infrared Variability of the Low Mass Stellar Binary TWA 30 A and B

Potential Drivers of Mid-Infrared Variability in Young Stars: Testing Physical Models with Multiepoch Near-Infrared Spectra of YSOs in ρ Oph

ABSTRACTRecent studies have identified several young stellar objects (YSOs) which exhibit significant mid-infrared (mid-IR) variability. A wide range of physical mechanisms may be responsible for these variations, including changes in a YSO’s accretion rate or in the extinction or emission from the inner disk. We have obtained and analyzed multiepoch near-infrared (NIR) spectra for five actively accreting YSOs in the ρ Oph star-forming region along with contemporaneous mid-IR light curves obtained as part of the YSOVAR Spitzer/IRAC survey. Four of the five YSOs exhibit mid-IR light curves with modest (∼0.2–0.4 mag) but statistically significant variations over our 40-day observation window. Measuring the strengths of prominent photospheric absorption lines and accretion sensitive H I and He I lines in each NIR spectrum, we derive estimates of each YSO’s spectral type, effective temperature (Teff), and H-band extinction (AH), and analyze the time evolution of their NIR veiling (rH and rK) and mass accretion rates (). Defining a YSO’s evolutionary stage such that heavily veiled, high accretion rate objects are less evolved than those with lower levels of veiling and ongoing accretion, we infer that GY 314 is the most evolved YSO in our sample, with GY 308 and GY 292 at progressively earlier evolutionary stages. Leveraging our multiepoch, multiwavelength dataset, we detect significant variations in mass accretion rates over timescales of days to weeks, but find that extinction levels in these YSOs remain relatively constant. We find no correlation between these YSO mid-IR light curves and time-resolved veiling or mass accretion rates, such that we are unable to link their mid-IR variability with physical processes localized near the inner edge of the circumstellar disk or within regions which are directly responsive to mass accretion. We do find, however, that redshifted He I λ10830 emission, where present in our spectra, shows both quantitative and qualitative temporal correlations with accretion-sensitive H I emission lines. Blueshifted He I absorption, on the other hand, does not demonstrate a similar correlation, although the time-averaged strength of this blueshifted absorption is correlated with the time-averaged accretion rate in our sample of YSOs.