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title: 'Weak Line Quasar_review : meusinger'

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# Weak Line Quasar_review : meusinger
## Introduction
* A tiny fraction of the quasar population shows remarkably **weak emission lines**. Several hypotheses have been developed, but the weak line quasar **(WLQ)** phenomenon still remains puzzling.
* **Broad emission lines (BELs)** are a defining characteristic of **type 1** **Active Galactic Nuclei (AGN)**.The **weakness** or even **absence** of **BELs** is the most remarkable feature of a class of **high-luminosity** **AGNs** called **weak-line quasars (WLQs)**.
* The first discovered **WLQ** was the **radio-quiet** quasar **PG 1407+265** at redshift **z = 0.94** (**McDowell et al. 1995**) with undetectably weak **$H_{\beta}$** and **UV BELs** although the continuum properties are similar to those of normal **radio-quiet quasars**.
* **Fan et al. (1999)** discovered the first **high-z WLQ**, **SDSS** **J153259.96+003944.1 (z = 4.62)** and suggested that it is either the most distant known **BL Lac object** with very **weak radio emission** or a new type of **unbeamed quasars** whose **broad emission line region (BLR)** is very **weak or absent**.
* Based on the **multi-colour** selection of the Sloan Digital Sky Survey **(SDSS; York et al. 2000)**, about **100 high-z WLQs** have been found with **Lyα-N v** rest-frame **EW < 15 Å (Diamond-Stanic et al. 2009; Shemmer et al. 2010; Wu et al. 2012)**.
* Low values of the **equivalent widths** of the BELs can be the result of **abnormally low line fluxes** or of an **unusually strong continuum**.

* **Relativistic beaming** provides an example for dilution of the line strength by a **boosted continuum**. 
* However, such an interpretation of the WLQ phenomenon is widely considered unlikely because many properties of the WLQs e.g. **(radio-loudness, variability, and polarisation)** are different from those of BL Lac objects **(McDowell et al. 1995; Shemmer et al. 2006; Diamond-Stanic et al. 2009; Plotkin et al. 2010; Lane et al. 2011; Wu et al. 2012)**. 
* WLQs are also different from **type 2 quasars** where only the **broad line components** are missed in the **unpolarised spectra**, and the **Eddington ratios ε = L/LEdd** are usually **lower** (**Tran et al. 2003; Shi et al. 2010; Shemmer et al. 2010**). 
* Factors that can mimic WLQ spectra are **line absorption or a strong Fe II pseudo-continuum** (e.g. **Lawrence et al. 1988**). Such explanations may work for some objects but do not explain the WLQ phenomenon in general **(McDowell et al. 1995)**.
* The hypotheses can be roughly grouped into two families :
  1.  **An extraordinary BLR (Broad Line Region)** 
  2.  **Unusual properties of the central ionising source.**

* In particular, the ideas of generally **abnormal** properties of the **BEL** emitting clouds **(Shemmer et al. 2010)**, **a low covering factor** of the **BLR** (i.e. a low fraction of the central source covered by BEL clouds, **Nikołajuk et al. 2012** )
* **Relative shortage** of high-energy **UV/X-ray** photons due to a **shielding gas** with a **high covering factor** that prevents the **X- ray** photons from reaching the **BLR** (**Lane et al. 2011; Wu et al. 2012**). 
* Abnormal properties of the continuum source may include a **high Eddington ratio** as in **PHL 1811 (e.g. Leighly et al. 2007**; but see **Hryniewicz et al. 2010; Shemmer et al. 2010)**, a freshly launched wind from the accretion disk **(Hryniewicz et al. 2010)**, an optically dull AGN **(Comastri et al. 2002; Severgnini et al. 2003)**, or a cold accretion disk around a high-mass **(M ≥ 3 · $10^{9}M_{solar}$)** black hole **(Laor & Davis 2011)**.

### Important Observations in WLQ :
* In a study of the **variability** of the quasars in the **SDSS stripe 82** revealed, it was found that **WLQs** tend to have **lower variability amplitudes (Meusinger et al. 2011)**.
* For the **mean EW** of the comparison, **quasars** they find **(WMg ii,WCiv) = (42±22 Å, 51±52 Å)** compared with **(17±7 Å, 17±18 Å)** for the **rWLQ** sample (**H. Meusinger et al**) .
* Many **WLQs** show a **steeper continuum** than the **SDSS quasar** composite spectrum ( **eg : H. Meusinger et al**) as shown in the figure.
* ![](https://i.imgur.com/1BQtrN2.png)
* The comparison clearly illustrates the **weaker BELs** and the **(i.e. less reddened)** **ultraviolet continuum** for both (a) the **WLQ- EWS** subsample and (b) **the entire WLQ sample.**
* A possible explanation of the **steeper continuum** could be an additional **non-thermal component** in WLQ spectra.
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 ### Quasars are generally classified as : Radio-loud based on the radio-to-optical flux ratio:**   
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  ---------------------------------------------------- **$R$** = **$\frac{F_{5GHZ}}{F_{4400Å}}$**   ---------------------[Kellermann et al. (1989)]
 
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where **$F_{5 GHz}$** and **$F_{4400Å}$** are the flux densities at rest-frame **5 GHz** and **4400Å**, and called quasars with **R greater than 10** as **radio-loud**.
* **R = 10** is commonly used to distinguish **radio-loud** and **radio-quiet** quasars (**e.g. Francis et al. 1993; Urry & Padovani 1995; Ivezi´c et al. 2002; McLure & Jarvis 2004; Richards et al. 2011**).
* Presently followed parameter on which we distinguidh the radio-loud and radio quiet sources : 
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  -------------------------------------**$R$** = **$\frac{F_{6cm}}{F_{2500Å}}$--------------------------------------------- [Jiang et al 2007]

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* It is known from both **multi-epoch** **photometry** and **spectroscopy** of **SDSS quasars** that the **variability** in the **emission line flux** is only ∼ **10%** of the **variability** in the underlying **continuum** (**Wilhite et al. 2005; Meusinger et al. 2011**)
* The above fig shows the **radio-loud WLQ** composite spectrum. It is very **similar** to that of the entire WLQ sample [**H. Meusinger et al**], which is dominated by **radio-quiet quasars**. 
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* ![](https://i.imgur.com/dObvvC6.png)
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**Important Note:**

At the above figure (a), the **continuum** of the **SDSS composite spectrum** between the **Lyα** and the **Mg ii** line is fitted by a simple **multi-temperature black body** **(MTBB)** model with a temperature parameter **T∗ ≈ $8 \times 10^4$ K** that provides a good fit to the observed quasar composite spectra over a much wider wavelength range **(e.g. Meusinger & Weiss 2013)**. **The steeper WLQ composite requires a higher temperature**. Indeed, the fit is considerably improved when we subtract the continuum of the **$8\times 10^4$ K MTBB** from the SDSS composite and add instead the continuum of the $10^5K$ model.
An alternative way to achieve a similar result is shown in above Fig. (b). We simply added a hypothetical further **power-law component $F_λ ∝ λ^{−1.7}$, i.e. $α_ν$ = −0.3,** of approximately the same level as the thermal continuum at 3000 Å. **In both cases, the WLQ composite spectrum is well matched by the modified SDSS quasar composite spectrum.**
* The **enhancement of the continuum flux** **dilutes** the **line flux** and **reduces the EWs of the lines correspondingly**.
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## Luminosity Black Hole Mass Accretion Rate and Eddington Ratio
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