Ekip»Sci TR-22

2.2 Transient Objects

GRBs in IR:

GRBs are the most luminous events with 1051-1054 ergs energy release in the universe after the Big Bang. These bursts can be characterized as highly dense "gamma - ray flashes" in the range of a few seconds to several minutes duration.

Although there are successful observations and interpretations of GRBs with multi-wavelength observations, internal and external emission mechanisms still remains unclear with big uncertainties. Scientists are still working to understand the nature of the progenitors of GRBs.

The discovery of GRB afterglows in X-rays (Costa et al. 1997), visible (van Paradijs et al. 1997), and radio (Frail et al 1997) has caused a considerable progress in their understanding, however problems still remains. To solve these problems requires continuous monitoring in X-rays, optical and infrared bands so that the important features of the GRB can be derived. These observations are also give information about distance and locations of GRBs.

Approximately half of GRBs that are well localized by X-ray observations have detected optical or IR afterglows. Photometric and spectroscopic observations of GRB afterglows allow us to determine the redshift of GRBs as well as probe their local environments.

4-m class telescopes can allow for GRB afterglow studies with synergy of the other current and/or planned space and ground based facilities. In this part we summarized relevance of 4-m class IR telescope to Gamma Ray Bursts.

Capable of performing rapid, multi-wavelength observations in X-rays, UV, and optical wavelengths, the Swift satellite plays a vital role in identification of GRB counterparts. Observations of GRB afterglows with IR instruments are required for developing an understanding of the GRB-progenitor circumstellar environment, GRB relativistic outflows, and interaction of the GRB jets with the circumburst medium and the properties of the GRB host galaxies (Greiner & Rau 2011). Note that while Swift has an amazingly fast reaction time, Swift UVOT is only a 0.3 m telescope that cannot observe redward of 6000A. Bigger glass is necessary for the detection and long-term (days to weeks) follow-up of many optical afterglows, which quickly decay past UVOTs sensitivity limits. Additionally, high redshift GRBs have optical afterglows that are suppressed in the optical observer frame due to Lyman-alpha forest absorption. Therefore, IR instruments are required to probe rest frame optical afterglows from GRBs at a redshift >~ 6.

Detection of high-redshift GRBs is particularly important. Up to now four GRBs (GRB 000131 at z~4.5; GRB 050904 at z~6; GRB 080913 at z~6.7; GRB 090423 at z~8.2) have been reported with high redshift with systematic near-infrared observations of GRB afterglows it would be possible to catch high-redshift bursts (Greiner & Rau 2011; Andersen et al 2000; Kwai et al. 2006; Greiner et al 2008; Tanvir et al. 2009). With NIR follow-up, GRBs with potentially extreme redshifts can be identified via IR waveband "dropouts" caused by Lyman-alpha forest hydrogen absorption (see Cucchiara et al. 2011 an example of GRB 090429B at z~9.4). Identifying extremely high-z candidates in rapid photometry allows for more time-intensive spectroscopic observations to be focused only on GRBs that have a high probability of being at extreme redshifts. Such observations are critical for investigations of the early universe, including the epoch of reionization.

The other question in the GRB field is observation of "dark GRBs". Dark bursts afterglows are generally detected in X - rays but have no optical or IR counterparts. Observations of these dark bursts with large ground-based telescopes can increase the possibility of optical/IR afterglow detection. Some dark bursts may not be observed at optical wavelengths because of their high redshifts, a 4-m class infrared telescope would help fill in these gaps. Furthermore, it now appears that many of the dark bursts are dark due to dust absorption in their host galaxies (e.g. Perley et al. 2009 for Visible and IR imaging of Orion Nebula). IR capabilities are crucial for probing GRBs that occur in dusty hosts.

(Acknowledgement: Thanks to Dr. Bethany Cobb for her very useful suggestions)

SNe and SNRs in IR:

Supernova Remnant (SNR) studies are important in the theories of interstellar medium (ISM) of matter, and energy into the ISM. However, in spite of the quite large number of Galactic SNRs their observations are impeded by several limitations such as the uncertainty in distances to individual objects and high extinctions along the line of sight in several regions of Galactic plane.

Ground-based and spaced-based observations have allowed many supernova and SNRs in galaxies to be resolved, showing various different morphologies in radio, X-ray, and optical wavelengths. Also with Infrared (IR) observations of supernova and SNRs primarily reveal the thermal continuum emission of shock-heated dust. Although the dust content of SNRs is not directly observable in other wavelength regime, it is possible reveal its fine-structure lines from neutral to moderately ionized species of atoms from carbon to nickel in infrared regime. The distinct advantage of IR observations is that extinction is much lower at IR than optical or UV wavelengths (Arendt et al 1999).

In order to determine the role of SNe in the production of dust in universe the mid-IR observations are critical. Also, the near- and mid-IR observations are useful tools for studying physics of SNRs, such as composition, temperature, distributions and density of the ejecta, radiative shocks of SNRs and shock interactions between SNRs and nearby molecular clouds (Bouchet et al 2006, Lee et al 2011 and reference therein).

Galactic Black Holes in IR

Jet emission from Galactic Black Hole Systems

Radio observations of black holes revealed the presence of jets through their synchrotron emission properties. Technically, having a compact object in a binary system is enough to observe jets, they are observed in neutron star systems (Migliari, S. and Fender, R.P., 2006), even in white dwarfs (Kording, E. et al., 2008). However, the jets from black holes are much more powerful than the jets in neutron stars and white dwarfs. Since the jets observed in Galactic solar mass black holes have similar properties to those observed in AGN, these systems are also called microquasars. Only for a few of microquasars the jets are resolved in radio, and for the majority of the systems the presence of jets are inferred through properties of radio and near infared emission.

Spectral properties of radio emission indicates that there are two types of jets in microquasars. The jets observed during transitions from the hard spectral state to soft or very high state are usually very powerful. These are transient jets and they are so powerful that they could distrupt the inner part of the disk and the corona. For these type of jets, the radio spectrum is a power law with flux decreasing with increasing frequency.

On the other hand, if the radio spectrum is inverted (flux increases, or stays constant with increasing frequency) then the system hosts a compact jet. As the name implies, the compact jets are geometrically small and therefore optically thick close to the black hole. The optically thick part creates the inverted spectrum up to the near infrared band (Blandford, R. D. and Konigl, A., 1979). Sufficiently away from the black hole the jet is optically thin, and the spectral energy distribution (SED) shows a break around the near infrared bands (typically the H band, see Fig. 1.). These jets are only observed in the hard spectral state.

Fig-2.2-1. The origin of emission from black holes.

For some sources it is claimed that the syncrotron radiation from the optically thin part of the jet extends all the way to X-rays and contribute to X-ray emission (Markoff, S. et al., 2001; Markoff, S. and Nowak, M, A., 2004). It was even claimed that the entire X-ray emission from XTE J1550-564 in 2000 at very low X-ray luminosities is originated in the jet (Russell, D. M. et al., 2010).

The jets emit in near-infrared, moreover, the near infrared band corresponds to the break of the SED which provides important information on the size and power of the jet. Therefore microquasars are observed frequently in near-infrared when they are in outburst, especially during the rise and the decay of the outbursts where the system is in the hard state. First detailed investigation of microquasars jets in near infrared was done for XTE J1550-564 (Jain, RAJ, K. et al., 2001), and subsequent work by Homan, J. et al. (2005); Russell, D. M. et al. (2006); Buxton, M., Bailyn, C. D. (2004); Kalemci, E., et al. (2005) detailed information about the relation between near infrared emission, jets and X-ray spectral and temporal properties. As seen in Fig. 1, the near infrared can be originated in the jet, but there can also be contributions from the secondary star and outer parts of the accretion disk. Therefore detailed investigation of light curves in near infrared, optical and X-rays (Buxton, M. et al., 2012) as well as SEDs must be done to fully account for each component (Dincer, T. et al., 2012).

Finally, there is a correlation between the near infrared flux and X-ray flux in these sources (Corbel, S., et al., 2000; Russell, D. M. et al., 2006; Coriat, M., et al., 2009). This correlation may mean a common emission mechanism (synchrotron), but it may also mean a common reservoir for the accretion and ejection. There is another mechanism that relates X-rays to jet launch: synchrotron self Compton (SSC). In SSC, the synchrotron radiation created in the jet is Compton upscattered by the electrons that create the synchrotron radiation. In this case the origin of soft photons that are upscattered and created the variability in these system may originate in the jet (Markoff, S., et al., 2003).

In the mid-infrared band usually satellites are required to conduct observations and this reduces the number of studies on microquasars in mid-infrared. An important study at mid infrared was done by Migliari, S., et al. (2007) using VLA in radio, SPITZER in mid infrared, SMARTS in near infrared and optical and RXTE for X-rays. This work showed that for GRO J1655-40 synchrotron+SSC can explain the entire SED in the hard state. A recent work by Gandhi, P. et al. (2011) used WISE data to constrain the break frequency in the infrared. But both cases utilized single pointed observations, and could not take into account day to day variability and could not constrain the properties of jet launch.

X-ray timing studies to understand the relation between power spectra and jet launch

For XTE J1550-564 and XTE J1752-223 it was claimed that direct synchrotron emission dominates the X-ray emission in the hard state at low luminosities. If this is the case there should be a manifestation of this change in X-ray timing properties. Tolga Dinçer and Emrah Kalemci have been investigating this for many microquasars for which there is good coverage in near infrared that lets determining the time of jet launch. In Fig. 2. an example sequence of power spextra is shown for XTE J1550-564. The sequence shows that there is no drastic change in the power spectra as the jet turns on, however the disappearance of the quasi-periodic oscillation is interesting

Fig-2.2-2. XTE J1550-564 2000 outburst decay, evolution of power spectra and the near infrared flux indicating jet launch. Turqoise circles are near infrared background, and the purple circles are the near infrared fluxes on top of the background.
Fast timing studies in optical and infrared

Two very important fast timing studies in optical and infrared were done for the microquasar GX 339-4. In both studies ms scale timing in optical (Gandhi, P. et al., 2008) and infrared (Casella, P. et al., 2010) data were used to create power spectra and compared to simultaneous X-ray power spectra using RXTE. In the hard state the infrared and the X-ray power spectra are very similar (see Fig. 3). With ms capability it is also possible to do autocorrelation studies. This kind of studies require large infrared telescopes with appropriate CCDs or special instruments and software pipeline to analyze the data.

Fig-2.2-3. Left: Simultaneous X-ray-K band light curve of microquasar GX 339-4 taken with VLT. Right: Power spectra from these light curves. Both from Casella, P. et al., 2010].
Jet launch timescale

The jet signatures in the near infrared are always observed a few weeks after transition in X-ray timing (Kalemci, E., et al., 2005; Kalemci, E., et al., 2006), when the X-ray spectra are hardest and the disk flux is less than 1% of the total X-ray flux in 3-25 keV band. This is shown in Fig-2.2-4.

The jet formation timescale is probably related to formation of corona[33]. The formation of the corona may also be important in terms of carrying magnetic flux inwards for jet launch (Beckwith, K et al., 2009).

Microquasars and DAG

Most microquasars are transient systems and the jets are observed during hard X-ray spectral states. This means that infrared observations must be target of opportunity, and must be coordinated with X-ray and radio facilities. Even though this places a strain on telescope scheduling, it is a well established fact that the true nature of astrophysical sources are often investigated only with multi-wavelength observations.

To track all state transitions and to obtain jet reactivation time the near infrared and optical observations must be conducted daily after the X-ray spectral hardness is confirmed. The photometric observations in J, H, K, I, B, V should be conducted for 10 to 20 days to evaluate the entire evolution.

It is unfortunate that most of the microquasars can only be observed from the South Hemisphere, on the other hand for a handful of systems DAG would be suitably located, especially during summer.

It is absolutely essential to have CCDs, or special readout systems for fast timing. The size of the DAG would be enough for bright systems. Especially if LOFT is selected for the next medium size explorer at ESA, simultaneous observations with LOFT and DAG would be very beneficial. Without LOFT it may be possible to use Indian ASTROSAT for fast timing in X-rays along with DAG.

Fig-2.2-4. The evolution of X-ray spectral index (black), near infrared fluxes (red) and radio (blue).

Cataclysmic Variables (Novae, Dwarf Novae, Magnetic CVs)

Cataclysmic variables (CVs) are compact binaries hosting a white dwarf (WD) primary star accreting material from the Hydrogen rich companion. The accretion occurs through an accretion disc in cases where magnetic field of the WD is weak (B< 0.01 MG) and such systems are referred as nonmagnetic CVs.

A subset of Cataclysmic Variable systems gives rise to classical/recurrent novae (CNe/RNe) in which an outburst on the surface of the white dwarf is due to a thermonuclear runaway (TNR) in the accreted material and results in the ejection of about 10$-4-10-7 solar masses of material at several thousand kilometers per second (Warner 1995, Starrfield 2001, Bode & Evans 2008, Balman 2012). RNe repeat their outbursts with a cycle of few tens of years. CNe/RNe are the third most violent of the stellar explosions that can occur in a galaxy after Gamma-ray bursts and Supernovae. CNe/RNe also participate in the cycle of Galactic chemical evolution by ejecting grains and metal enriched gas as a source of heavy elements for the interstellar medium (Gehrz et al. 1998). CNe/RNe are related to Supersoft X-ray sources (SSS) which are the probable progenitors of SN Ia explosions. During a CN outburst the stellar remnant and the ejecta can be probed in NIR and Mid-IR wavelengths. A quasi-photospheric stellar spectrum with weak atomic lines and some P-Cygni profiles changes to one showing a wide range of deep absorption features indicating a cool extended atmosphere (H2O, CO, AlO, SiO, TiO, SO2, OH, SH, Si) with possible circumstellar dust shell (Lynch et al. 2004, Evans et al. 2007). Following a classical/recurrent nova eruption as the early phases subsides the ejecta/shell will start to show IR emission lines. The ejecta is then believed to be synthesizing molecular grains and dust. Eventually, as the central WD enter UV and the X-ray rejime, the ejecta goes into nebular and coronal line emission phase where one finds hydrogen recombination lines, free-free continuum and IR coronal lines. One can use CLOUDY software to physically interpret the IR data (Hayward et al. 1996, Evans et al. 2007a, Lynch et al. 2008). In this way, we can study the spectroscopy of the ejecta, its elemental abundances, dust grain minerology, dust formation and dust distruption. Using spectroscopy we can model the inhomogeneous dust shell and the interaction of this shell with the interstellar medium (Lynch et al. 2004). Magnetic CV Studies The magnetic CVs (mCVs) constitute about 25% of the entire population of CVs. It has two subclasses depending on magnetic field strength. Polars (AM Her type systems) have B>10 MG where this strong field causes the accretion flow to directly channel onto the magnetic pole of the WD inhibiting the creation of an accretion disk (see Cropper 2002; Warner 1995). Polars show strong circular an linear polarization modulated at Porb and the WD rotation Pspin is synchronized with the binary orbit. Only about 1% of the Polars are found to show asynchronicity. The second class of mCVs are Intermediate Polars (DQ Her type systems) that have less field strength compared with Polars of about 1-10 MG. In this case, the accretion takes place via a truncated disk to the magnetic poles through accretion curtains. Near the surface of the WD, the accreting material forms a strong shock where the post shock region heats up to 10-20 keV and then cools via thermal Bremsstrahlung (Patterson 1994; Hellier 1996; de Martino et al. 2008; Brunschweiger et al. 2009). IPs are asynchronous systems where orbital period of the system Porb is larger than the spin period of the white dwarf P$_{\rm{spin}}\$. Majority of the IPs have Pspin/Porb <0.1, theoretically in the 0.01-0.6 range (Norton, Wynn & Somerscales 2004; Norton et al. 2008; Scaringi et al. 2010).

In magnetic cataclysmic variables the material that has channeled onto the field lines will emit cyclotron radiation as it spirals in. This emission will reveal itself as humps in the spectra and one calculate the magnetic fields of CVs in such systems. If the field strength is B<40 MG all magnetic WDs will emit cyclotron radiation in the IR rejime, this includes low field Polars and Intermediate polar systems (Schwope, Schwartz, Greiner, 1999, Johnson, J.J. 2005).

IR rejime is one of the most important bands where the outer accretion disk is studied in CVs (largely-time variability).

As part of Cataclysmic Variable studies, in Classical Nova outbursts, Intermediate Polars and Polars, polarization studies can be conducted in IR wavelenths. In CN polarization occurs due to electron scattering, polarization in resonance lines or Mie scattering from dust grains in the IR (Evans et al. 2002). We can study the geometry of the ejected nova shell using polarization (see IR polarization applications Jones, T.J. 2011).

In Mid IR wavelenths SPITZER telescope has been able identify excretion disks of dust outside the Cataclysmic Binaries and hot WDs. SPITZER has identified this excess emission on top of the binary/star emssion in the IR and the accretion disk component. It is possible to try to disentengle dust disks and study them in the NIR and Mid-IR with longer exposures using a ground-based observatory (Brinkworth et al. 2012, 2009, Hoard et al. 2009, 2007).

Ultraluminous X-ray Sources (ULXs)

Ultraluminous X-ray sources are the brightest extragalactic X-ray point sources located away from the nucleus of their host galaxy. It is widely believed that ULXs are formed from some extreme structure of accreting X-ray binary containing a black hole. Their X-ray luminosities greatly exceed the Eddington luminosity (LEd) for a 20 M black hole. They are characterized by extremely high luminosities in the range of 1039-1042 erg s-1 (Colbert & Mushotzky 1999, Makishima et al. 2000). These high luminosities could be as a result of accreting rates onto intermediate-mass black holes (IMBHs) with masses ~102-104 M if they radiate isotropically at sub-Eddington levels (0.01-0.1 of LEd) as observed in stellar black holes (Colbert & Mushotzky, 1999). The high luminosity may also be explained by particular radiation mechanisms from accretion either onto normal stellar-mass black holes (~10 M) with strong beaming effects (King et al. 2001) or massive stellar black holes (≤ 100 M) with super-Eddington emission (Begelman 2002). ULXs are ideal targets to study accretion physics under extreme conditions, and to search for IMBHs which are important to investigate stellar evolution and formation of super massive black holes (Feng & Soria 2011).

The nature of ULXs is still uncertain. Therefore, multiwavelength observations with ground- and space-based telescopes are valuable tool to understand physics and nature of the ULXs. These observations are also important to compare characteristics of ULXs with active galactic nucleus (AGN) and Galactic stellar-mass black holes. Radio observations could give important hint about their geometry, energetics, and lifetime (Miller et al. 2005). Optical/infrared observations are considerable to study the properties of putative ULX binary systems, and these observations also provide relatively direct indications of their masses of the components. For example, a sample of ULXs are associated with extended optical emission nebulae which often show both low and high ionization emission lines. The morphologies and emission line fluxes of these nebulae could then be used to investigate properties of the emitting gas, which provides important information (such as the intrinsic luminosity) about the energizing source (Pakull & Mirioni 2002, Russell et al 2011 and reference therein).

The high-excitation lines observed in nebulae, such as the optical HII recombination line (λ=4686) used to characterize the the intrinsic X-ray luminosites of the ULX (Kaaret et al 2004). Similar work have been done in the infrared band, where some contributions found by the ULX to the emission lines like (:cell PQA(PSS(SiII):) 34.82μm, (:cell PQA(PSS(SIII):) 33.48μm, (:cell PQA(PSS(OIV):) 25.89μm, (:cell PQA(PSS(NeV):) 24.32μm, and (:cell PQA(PSS(NeIII):) 15.56 μm. Especially the infrared (:cell PQA(PSS(OIV):) emission line (together with (:cell PQA(PSS(NeV):)) is a well-established signature of high excitation, usually associated with actively accreting black holes and active galactic nuclei. These high ionization lines used to constrain the Spectral Energy Distribution (SED) of the ULX and its companion. The photoionization modelling of the infrared lines of the ULXs can also be perform to constrain the bolometric luminosity of the ULX and for better understanding the morphology and origin of the emission lines in some starbust systems containing black hole binaries (Berghea et al 2010a, 2010b).

Optical observations showed that ULXs have point-like counterparts with correspond to early-type donors. However, the optical emission from the counterpart may have a strong contribution from the accretion disk (direct and irradiated). This could make it harder to determine the mass and spectral type of the donor star from the luminosity and colors of the optical counterpart. Therefore, obtaining a full SED of the ULX from the far-UV to the near-infrared could be helpful in order to separate the contributions coming from the accretion disk versus the donor star (Feng & Soria 2011, Grise et al. 2012).

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Updates: 20.11.2012 (Emrah, Şölen, Aybüke, İlham)

What can be done with DAG

DAG could be utilized along with X-ray and radio faicilities to conduct target of opportunity observations of galactic black hole transients with jets. Close to jet formation (confirmed by X-ray analysis), photometric observations in J, H, K, I, B, V should be conducted for 10 to 20 days to evaluate the entire evolution. Having a 4-4.5 m telescope has its advantages for faint sources and sources in crowded areas. It would fare much better than current telescopes like SMARTS in terms of such sources.

It is absolutely essential to have CCDs, or special readout systems for fast timing. The size of the DAG would be enough for relatively bright systems. Especially if LOFT is selected for the next medium size explorer at ESA, simultaneous observations with LOFT and DAG would be very beneficial. Without LOFT it may be possible to use Indian ASTROSAT for fast timing in X-rays along with DAG. The only example we have for such studies in done by Casella et al. at VLT with 8m telescope. The time resolution of 60ms was more than enough to do simultaneous timing observations in the low hard state. For very bright sources going down in time resolution may result in observations at the beginning of hard state or in the very high state in which the variability is faster.

If DAG is extended to mid-infrared, it will be possible to characterize the break in the jet SED much better. The WISE data for GX 339-4 indicates that the break may be at mid-infrared for the rise hard state.

In order to conduct the Cataclysmic Variables reserach in IR the 3.5-4 m size of the telescope should suffice. The CCDs should have high QE and sensitivity. A good echelle and high and low resolution providing spectrograph is also essential. High time resolution for fast timing will be important to study the outer disk and eclipse studies in CVs to derive structure of the outer disk or the accretion shock at the poles but should be done with a different CCD then the main CCD. An instrument to conduct polarimetry should be optimized (see Jones 2011)

Rapid follow-up of GRBs in the near-IR is essential in order to detect and localise both the high-redshift candidates and "dark" GRBs. An instrument covering an optical and all near-IR bands at the same time (i.e. GROND for imaging or XSHOOTER for spectroscopy) is ideal as it will help avoiding the complication due to the shape of the light-curve of the GRB afterglows.

To search SNe, SNRs and ULXs in near and mid -infrared region could be possible with ~4m telescope and high resolution spectrograph. The near- and mid-IR emission lines ((:cell PQA(PSS(Fe II):), H 2, etc.) are useful tools for studying evolution of SNRs and determining role of SNe in the production of dust in universe. Observing high-excitation lines spectrally and obtaining a full SED of the ULX in infrared could be helpful to separate the contributions coming from the accretion disk versus the donor star in the ULX systems.